NASA Technical Reports Server (NTRS) 20120010503: Optical Polarization and Spectral Variability in the M87 Jet

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OPTICAL POLARlZATION AND SPECTRAL VARIABILITY IN THE M87 JET
ERIC S. PERLMAN 1, STEVEN C. ADAMS2,3, 11IHAI CARA1,4, MATTHEW BOURQUE1,3, D.E. HARRIS5, JUAN P. MADRID6,
l 7 8 9 ll
RAYMOND C. SIMONS , ERIC CLAUSEN-BROWN , C. C. CHEUNG , LUKASZ STAWARZ ,1O, M.\RKOS GEORGANOPOULOS ,
WILLIAM B. SPARKS12, JOHN A. BIRETTA 12
ABSTRACT
During the last decade, M87's jet has been the site of an extraordinary variability event, with one
knot (HST-1) increasing by over a factor 100 in brightness. Variability was also seen on timescales
of months in the nuclear flux. Here we discuss the optical-UV polarization and spectral variability of
these components, which show vastly different behavior. HST -1 shows a highly significant correlation
between flux and polarization, with P increasing from 20% at minimum to > 40% at maximum,
f'V
while the orientation of its electric vector stayed constant. HST-l's optical-UV spectrum is very hard
(auv -0 ~ 0.5, Fv ()( v-a), and displays "hard lags" during epochs 2004.9-2005.5, including the peak
of the flare, with soft lags at later epochs. We interpret the behavior of HST-1 as enhanced particle
acceleration in a shock, with cooling from both particle aging and the relaxation of the compression.
We set 20" upper limits of 0.58 parsecs and 1.02c on the size and advance speed ofthe flaring region.
The slight deviation of the electric vector orientation from the jet PA, makes it likely that on smaller
scales the flaring region has either a double or twisted structure. By contrast, the nucleus displays
much more rapid variability, vnth a highly variable electric vector orientation and 'looping' in the
(I, P) plane. The nucleus has a much steeper spectrum (auv-o ~ 1.5) but does not show UV-optical
spectral variability. Its behavior can be interpreted as either a helical distortion to a steady jet or a
shock propagating through a helical jet.
Subject headings: galaxies: individual (M87) -galaxies: active - galaxies: jets; nuclei
1. INTRODUCTION I reporting our first results (Harris et al. 2003), Paper II
which focused on the HST data (Perlman et al. 2003),
M87's jet was one of the first manifestations observed
Paper III which was mainly on the X-ray lightcurve of
of the active galactic nucleus (AGN) phenomenon (Cur
HST-1 which delineated the massive 2005 flare (Harris
tis 1918), and has been the target of myriad observations
due to its brightness and also proximity (d = 16 Mpc, et al. 2006), Paper IV which focussed on the VLBA
results, showing superluminal proper motions in HST-l
Tonry 1991). During the last decade, M8Ts jet has been
(Cheung et al. 2007), and Paper V (Harris et al. 2009)
the site of an extraordinary variability event, with knot
which focused on a more detailed analysis of the variabil
HST- 1 increasing in optical/BY brightness by a factor of
ity timescales of HST-1 and the nucleus. Madrid (2009)
more than 100 between 2000 and its peak in 2005. The
has also added a complete analysis of the UV light-curve
flare in knot HST -1 has been the target of several moni
of HST-1 and the nucleus between 2000-2006.
toring efforts using the Hubble Space Telescope (hereafter
In this paper (VI of the series), we discuss two addi
HST), Chandra X-ray Observatory, VLA and other tele
. scopes. Previous papers from this project include Paper tional aspects of the monitoring campaign, namely the
· evolution of the polarization and spectral index in the
...>... Electronic address: [email protected] optical-UV. We concentrate on the nucleus and HST-1,
1 Department of Physics and Space Sciences, 150 W. University as they are the main variable components in the jet. A
~ Blvd., Florida Institute of Technolo~-, Melbourne, FL 32901 future paper will combine these observations to produce
~ At2h eDnesp, aGrtAm 3en06t 0o5f Physics an.d Ast:ronomy, University of Georgia, a new polarization map of the entire jet and discuss any
3 Southeastern Association for Research in Astronomy (SARA) changes over the decade between the data of Perlman et
NSF-REU Summer Intern al. (1999) and this paper. Section 2 provides a detailed
4 Current address: Physics Department, Case Western Reserve explanation of the observations and data reduction pro
University, 10900 Euclid Ave., Cleveland, OH 44106-7079
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden cess. Then, in Section 3, we will discuss the techniques
Street, Cambridge, MA 02138 used in reducing the data. In section 4, we will discuss
6 Center for Astrophysics and Supercomputing, Swinburne Uni results. Finally, in Section 5, we discuss implications for
versity of Technology, Hawthorn, VIO 3122, Australia jet variability models and close in Section 6 with a sum
7 Department of Physics, Purdue University, West Lafayette, IN
47907 mary.
8 National Research Council Research Associate, National
Academy of Sciences, Washington, DC 20001, resident at Naval 2. OBSERVATIONS
Research Laboratory, Washington, DC 20375
As has been well documented (e.g., Paper I, Waters
9 Institute of Space and Astronautical Science, JAXA, 3-1-1
Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan & Zepf 2005), the flare of knot HST-1 began sometime
10 Astronomical Observatory, Jagiellonian University, uI. Orla during 2000. While the jet of M87 Was a regular target
171 3D-244 Krak6w, Poland for HST and Chandra almost from the start, intensive
Ii Department of Physics, University of Maryland - Baltimore
monitoring by these telescopes began in 2002 (Papers
County, 1000 Hilltop Circle, Baltimore,. MD 21250
12 Space Telescope Science Institute, 3700 San Martin Drive, I, II, III, Madrid 2009). Here we review these observa
Baltimore, MD, 21218 tions, concentrating on the optical polarimetric part of
2 Perlman et a1.
the campaign. that can cause sources that. are far away from one's re
Optical polarimetry was obtained on a somewhat dif gion of interest to shift apparently as compared to other
ferent schedule than the UV imaging discussed in Madrid sources in the field. The procedure was also checked by
(2009). Of the eighteen observations obtained between using only the nucleus as canonical, thus allowing for
2002 December and 2007 November, fourteen occurred the possibility of motions in HST-L Unfortunately with
on the same schedule as the imaging observations during the short exposure times (typically only 600s per polar
November 2004-December 2006. The other four obser izer) there were few or no globular clusters that could
vations were at roughly yearly intervals before and after be used for all frames, especially as the chip orientation
this period. Polarimetry VIas done in two bands, F330W changed from epoch to epoch. Extensive testing gives us
and F506W, with F606W observations being done much confidence, however, that our procedure successfully and
more often. Table 1 details the scheduling of these obser repeatably aligns the images to ±0.2 pixels. Following
vations, along with other information about the nucleus l\lULTIDRIZZLE, the orientation was set so that the y
to
and HST-1 which will be described later. axis corresponds north. The final result is a cosmic
The High-Resolution Channel (HRC) of the Advanced ray rejected and geometrically corrected image for each
Camera for Surveys (ACS) was used to ob.tain the po polarizer at each epoch (Fruchter and Sosey 2009).
larime~ry data for 17 of the 18 epochs. The ACS For the F606W data, it was necessary to subtract
HRC i3 a single-chip CCD camera, with a plate scaie galaxy emission before performing photometry and po
of 0.028xO.025 arcsec/pixel, corresponding to a field of larimetry. We first created a composite image of the
view of about 28x 25 arcseconds and yielding diffrac galaxy by drizzle-combining all epochs together, to im
tion limited resolution of ",,0" .06 for the F606W obser prove the S /N on the host galaxy. We then modeled the
vations, and a Nyquist-limited resolution of ~ 0" .05 for galaxy emission using the ELLIPSE and BMODEL tasks
the F330W observations (see Maybhate et a1. 2010). On in IRAF. After the model image was computed, it was
ACS the polarizing set is comprised of either the POLOV, split into 3 models to correspond to each polarizer, and
POL60V and POL120V filters (used for F606W) or the subtracted, using the IRAF command IMCALC, from
POLOUV, POL60UV, and POL120UV filters (used for the corresponding drizzled image for each polarizer.
F330W). As the three polarization filters can be selected
individually Maybhate et a1. (2010), no change in posi 3.1. Polarization Images
tion o!' chip was necessary between them. All observa
Next, the drizzled images from each polarizer were used
tions were CR-SPLIT to mitigate the effects of cosmic
to create images for Stokes I, Q, and U, along with their
rays, but dithering was typically not done.
errors. For the ACS data, we followed the procedure in
For the final epoch, which occurred after ACS went
the ACS data handbook (Maybhate et al. 2010). For
offiine due to an electrical short, the Wide Field Plan
the WFPC2 data, the Stokes images v:ere computed by
etary Camera 2 (WFPC2) was used for polarimetry.
On WFPC2 the polarizing filter set is comprised of the using the WFPC2 Polarization Calibrator tool 18 This
produces the coefficients needed to cbmpute the Stokes
POLQ quad, which has filters at angles of 0, 45 and 90
images by using Mueller matrices that describe the pick
degreees. The WFPC2 is a chevron-shaped, four-CCD
off mirror, the polarizer filter, and the various rotations
camera, with three wide-field chips (WF2, WF3, and
between the optical elements and the reference frames.
WF4) and a fine-resolution one (PC1). It. setup is less
This tool is accurate to ""1-2% (Biretta & McMaster
flexible for polarimetry, as the POLQ quad can only be
1997). Both of these procedures yield Stokes U, Q and
rotated through 51 degrees (Biretta & McMaster 1997).
I images that are combined in a standard way to pro
For this reason, we used the three WF chips. These chips
duce emission weighted fractional polarization (defined
fhianvael rae spollautteio snc a~le 0 o.2f" 0s.0e9t 9b6y5 tahrec sNeyc/qpuiixset lt, hleeoardeimng. to a as P = (Q2 + U2)'/2/I) and electric vector position an
gle (defined as EVPA = 1/2 x tan-l (U/Q)) images.
3. DATA REDUCTION After the Stokes images are found, we accounted for
We obtained the data for these observations from the well-known Rician bias in P (Serkowski 1962) using
the HST archive. All data were recalibrated with up a Python code adapted from the STECF !RAF pack
dated flat field files and image distortion correction age (Hook et a1. 2000). This code debiases the P im
(IDC) tables, obtained from from the STScI Calibra age following Wardle & Kronberg (1974), and calcu
tion Database System. Standard techniques were used lates the error in polarization PA, accounting for the
to recalibrate the data. These methods are described in non-Gaussian nature of its distribution (see Naghizadeh
detail in the instrument handbooks for ACS and WFPC2 Khouei & Clarke 1993). In performing this caicula
(Maybhate et a1. 2010; Mobasher et al. 2002). CTE tion, pixels with signal to noise (S/N) < 0.1 were ex
corrections were computed using the data found in the cluded outright, and since the debiasing is done with a
ACS Instrument Handbook (Maybhate et a1. 2010). "most-probable value" estimator, pixels where the most
After the data were retrieved, l\!uLTIDRIZZLE was probable vaiue of P was negative, or above the Stokes I
used to combine and cosmic-ray reject the images. The value (i.e. P> 100%) were blanked. This code was first
alignment of images was refined, assuming the positions used in Perlman et a1. (2006).
of the core and HST- 1 as canonical and using TWEAK
SHIFTS in'IRAF to shift the images to a common frame 3.2. Aperture Photometry and Polarimetry
of refe::-ence and correct for geometric distortions using To obtain fluxes in the Stokes parameters, we per
the models in the IDe. This was done because experience formed aperture photometry. For the ACS data, we used
with the polarizing filters on both ACS and WFPC-2 has
shown that there can be small irregularities in the filters 13 http://www.stsci.edu/hst/wfpc2/software/wfpc2_poLcalib.html
Polarization and Spectral Variability in MS7 3
TABLE 1
PHOTOMETRY OF M87 COMPONENTS
Date (No.) Core Fluxes (pJy) HST-1 Fluxes (~Jy)
F606W F330W F250W F220W F606W F330W F250W F220W
Dec 07 2002 (1) 671 ±7 305±2 146 ± 11 232 ± 2 2318, ± 2 141b ± 6
m
Dec 10 2002 630±6 305±2 146 ± 11 237±2 231 ± 2 141 ±6
Nov 29 2003 478±5 137 ± 11 427±4 244±7
Nov 28 2004 itl 1066 ± 11 475±2 306 ± 14 226 ± 14 970 ± 10 882±4 674 ± 12 631 ± 12
Dec 26 2004 1306 ± 13 363 ± 10 1113 ± 11 719 ± 10
Feb 09 2005 (6) 891 ±9 280 ±9 1224 ± 12 738 ± 10
Mar 27 2005 (7) 1037 ± 10 328 ± 10 1404 ± 14 903 ± 11
May 09 2005 (8) 932±9 446±3 274±9 217± 9 1333 ± 13 1209 ± 5 878 ± 14 853± 14
Jun 22 2005 (9) 839±8 273±9 1150 ± 12 877 ± 11
Aug 01 2005 i(l1l0)) 639±6 192±7 1117±11 664± 10
Nov 29 2005 735±7 349±2 217±8 170±8 1019 ± 10 872±6 604± 12 616 ± 12
Dec 26 2005 12) 756±8 234±8 987 ± 10 590 ±9
Feb 08 2006 (13) 631 ±6 201 ±8 160 ± 8 771±8 482 ±8 469±8
Mar 30 2006 \14l 780±8 232 ±8 656±7 404±7
May 23 2006 15 862±9 372±2 193 ±7 156.4 ± 7 592 ± 6 488±4 361 ± 7 357±7
Nov 28 2006 ?6~ 1370 ± 14 636±3 323 ±9 862±9 720±4 486 ± 10
Dec 30 2006 17 1006 ± 10 276±9 682±7 414±7
Nov 25 2007 (18) 1292±17 372±4
a Observation~ taken Dec. 10, 2002.
b Observations taken Nov. 30, 2002.
TABLE 2
POLARIMETRY AND SPECTRAL INFORUATION
No. Core HST-1
F606W F606W F330W F330W F606W F606W F330W F330W
P(%) EVPA(O) P(%) EVPA(O) Cto-uv p(%) EYPA(O) P(%) EVPA(O) Q'o-uv
1 3.1 ± 0.3 -79.9±3.12 4.5±0.7 -82.5 ±4.7 1.50 ± 0.06 40.2 ± 4.0 -65.8± 3.0 34.011. ± 3.5 -67.5 ±3.1 0.07± 0.04
2 3.7 ±0.4 -79.6±3.17 4.5±0.7 -82.5 ±4.7 1.41 ± 0.04 39.9 ± 4.0 -65.3± 3.0 34.0±3.5 -67.5 ± 3.1 0.12 ± 0.04
3 5.5 ±0.6 -70.3 ±3.07 1.23 ± 0.08 39.9 ± 4.0 -51.3 ± 3.0 0.58 ± 0.03
4 1.4 ± 0.2 -10.4 ±3.37 13.4 ± 1.5 73.7 ± 3.4 1.54 ± 0.06 23.4 ± 2.3 -58.2± 3.0 24.8±2.6 -64.7± 3.1 0.32±0.03
5 2.0 ±0.3 -55.4 ±3.69 1.58 ± 0.04 27.7± 2.8 -56.9± 3.0 0.57±0.02
6 2.2 ±0.3 -98.9 ±3.76 1.42 ± 0.04 36.7±3.7 -58.4± 3.0 0.66±0.02
7 6.1 ± 0.6 -53.2± 3.1 1.41 ± 0.04 42.5 ± 4.3 -61.5 ± 3.0 0.57±0.02
8 4.1 ±0.4 -78.9 ±3.05 10.0 ± 1.3 -86.8± 3.8 1.42 ± 0.04 34.5 ± 3.5 -64.7± 3.0 38.1 ± 3.9 -62.7 ± 3.0 0.37±0.02
9 10.7 ± 1.1 -36.0 ±3.04 1.37 ±·0.04 32.4 ± 3.2 -63.8± 3.0 0.35±0.02
10 8.6 ±0.9 -62.0 ±3.09 1.48 ± 0.05 36.4 ± 3.7 -60.6± 3.0 0.68 ± 0.02
11 6.6±0.7 -69.1 ±3.03 10.1 ± 1.4 -77.2±4.0 1.45 ± 0.05 32.6 ± 3.3 -62.2± 3.0 29.3±3.0 -62.0 ± 3.0 0.45 ±0.03
12 4.4 ±0.5 -90.0 ±3.26 1.44 ± 0.05 28.1 ± 2.8 -62.4 ± 3.0 0.67 ± 0.02
13 2.5 ±0.3 -69.4±3.24 1.39 ± 0.07 26.3 ± 2.6 -59.5± 3.0 0.55 ± 0.03.
14 2.6 ±0.3 -71.4±3.7 1.50 ± 0.05 24.1 ± 2.4 -59.0± 3.0 0.63±0.03
15 1.0 ± 0.1 -1.2 ± 3.91 3.0 ± 1.0 -25.7± 9.8 1.63 ± 0.05 20.7 ± 2.1 -62.7± 3.0 24.4±2.7 -50.2 ± 3.3 0.47±0.03
16 6.6±0.7 -39.1 ± 3.01 2.0±0.7 -57± 10 1.50 ± 0.04 34.9 ± 3.5 -66.1 ± 3.0 34.8±3.6 -64.2± 3.1 0.48±0.03
17 10.2 ± 1.0 -46.4 ±3.04 1.60 ± 0.04 20.3 ± 2.1 -58.7± 3.0 0.73 ± 0.03
18 6.7± 3.0 -130.1 ± 1.30 22.2 ± 3.0 -76.5±0.7
a Observations taken Dec. 10, 2002.
two concentric apertures for the core (one with a radius flatness of the remaining background. A similar strat
of 11 pixels, the other with a radius of five pixels) and egy was used for the F330W images, where the galaxy
one ce:ltered on HST-l with a radius of 11 pixels. The contributon was minimal. Aperture correction was done
smalle:- of the two core apertures excludes a knot that to account for the wide wings of the HST point spread
emerged from the core in later images, while the larger function (PSF). As the nucleus and HST-l are either un
core aperture includes it. While the two lightcurves show resolved or nearly so, we modeled them as PSFs, using
the same behavior, we use only the smaller aperture in TINYTIM (Maybhate et ai. 2010; Bohlin 2007a). The
this pape,. For the lone WFPC2 dataset (epoch IS) we aperture corrections were generally 10-20% in flux, simi
used apertures of 4 pixels radius for both the core and lar to those found by Sirianni et al. (2005); however, our
HST-l, and also excluded in each aperture an annular method allowed us to account for the fact that the posi
region between 4-6 pixels from the other. This alternate tion of these components varied widely on the chip from
procedure was made necessary by the much larger pixels observation to observation. Count rates were converted
of the WFPC2/WFC chips. For the F606W images, af to flux using SYNPHOT, recomputing PHOTFLAM val-·
ter galaxy subtraction rectangular regions located quasi ues by assuming a spectral index a = 0.7 (Fv ex: v-a), ap
randomly throughout the galaxy were used to verify the propriate for most of the MS7 jet (Perlman et al. 2001).
4 Perlman et al.
Typical errors for this procedure are < 5%. Finally, we ,
1800,1
also accounted for Galactic extinction using data from j
NED, which gives AB = 0.096, as well as standard ex 1600,,~
tinction curves. The resulting fluxes in Stokes I are listed
t-
in Table I, while the fractional polarizations and EVPA >; 1400 o
•
are given in Table 2. 2~-
High-quality UV photometry of these two components 1200t 'I
l •
was published recently by 1ladrid (2009). We have uti "" 1000 0 , •
ltiizoends . tRhoasteh emr ethasaunr ecmorernectst,i ngal baleli tf luwxeitsh tos oam sei nmgloed wifiacvRe- -e5"n) BOO r~ • •. • I
length as in the Madrid paper, we have utilized the un [~ ~,
corrected fluxes in both F220W and F250W. This mini 600 ,
mizes the number of assumptions, and also allows us to c •
make use of all the data in the 6 epochs where observa 4002t0 0:1 ~OIJ-1 'iliDa "-2oon·-~iln':;--··WDB
tions were taken in both bands, increasing the accuracy •• Il"
of the measured spectral indices in those epochs and also ,
allowing us to check for significant emission in the Mg 14 ~
II A 2798 line, which falls near the center of the F250W 1 1
ibdaenndcpea ossf, thbiust liisn eo uetmsiidssei otnh ew Fas2 2f0oWun db).a ndTphaes sr e(andoe re vi s 12,,~ i,1t , t,
10~ r
referred to that paper for reduction steps. We list the ' 1
Y
resulting fluxes in Table 1. In Table 2 we list the spec l
Bt-
traEl rirnod"i,c ews eOre:U Vp r-o0p.a gated in both datasets using stan 0: 6~ ~, T ••
!
dard techniques. The propagated errors include Poisson H
errors, an additive noise term (the RMS background cal 4r~1' ~ ~
~J
culated post galaxy subtraction) and the read-out and 2r
discretization noises, as well as an additional! % term to •1 I~ •
oL.
account for uncertainties in SYNPHOT models (Bohlin 2003 WOo ·~ob.ff 20Q8 20~~ 2C nR
et al. 2007b). For this analysis, we have ignored errors in nl~
the F606W galaxy model. The resulting uncertainties in
the Stokes Q and U images are approximately Gaussian f
in nature, with their values being approximately equal I.B ~
to the sum in quadrature of the individual polarizer im
"
age errors. Hence, Gaussian error propagation for P is '0
appropriate for our purposes. .5 t 1
4. RESULTS ;;o;; 1.6f.~L- ~1 ~ w.1 ,If,r 1 f1 ~
opFtiigctaLi"-eUSV 1 sapnedc tr2a ls hionwde xo,u rf rraecstiuolntsa lf opr otlharei ztaottiaoln falunxd, ~ 14~ J of
EVPA variations of both the nucleus and HST- 1. The -3 tl
total fluxes shown are in the F606W ba"d, while all f
other panels use all available data. The total flux varia L2t l,
tions mirror those previously shown for the near-UV by
Madrid (2009), showing the very large flare in HST-1 300:) 20001 ~O~.'" 20~09- -.-taO""-, -l-!O.\ )B
as well as two smaller flares in the nucleus during the
same time. Because the large majority of these obser of i
vations occurred during 2004-2006, when the knot was
-~o~
already very bright, we do not see the full dynamic range
of the variability exhibited by knot HST- 1 during its ~ t '. ' ", f ~
flare - nearly a factor 150 at 2500 A, where its flux in -40 t
creaseci from 6 p.Jy in 1999 to a peak of 854 p.Jy in early t~ ~ 1 j
-60 'I' 'I'
2005. We used our data to measure the distance between -80~ _____ ~--------;---~I-':~.----------~
the two features, which in all epochs is consistent with
0.885 == 0.010 arcsec, with no evidence of motion. This &j f f if: i~
sets an upper limit of 1.56 pc (2<7) on positional change -100~ 4
o! the flux maximum of HST-l during the roughly 5-year -120~ j
timespan covered by these data.
Comparing Figures 1 and 2 we see that the nucleus - 14of , 20'07 ;
and HST-I display very different characteristics in p0- 2003 2004 300O5a te ~006 200B
larization and spectral index variability. Both display
large changes in polarization characteristics during the FIG. 1.-Variations in Total flux (i.e., Stokes I), fractional p0-
larization, spectral index and EVPA, are plotted for the nucleus of
six-year timespan of our observations. The nucleus is
M87. A dashed. line in the EVPA panel reflects the jet PA. The
seen to range between 1-13% polarization during this F606W observations are plotted as squares, while in the second
and fourth panel the F330W polarimetry is plotted as diamonds.
See §3.2 and 4 for discussion.
.Polarization lUld Spectral Variability in M87 5
time, with the EVPA changing by as much as 90 de
, ,
1400 ~ , grees. HST-l is much more highly polarized, with its po
larization ranging from 20-45%, but much less variability
1200 ,~ , 0 • (marginally significant at most) in the EVPA, which in
>: f 0 I our data ranges from roughly -500 to -750 with a typ
,
2~- 1000 ~ " '10 ~ ical value of ~ -620, about 7-8 degrees away from tbe
I nominal jet direction of -69.50 but rather closer (4 de
•w ~ ~, i grees) to the average PA of the radius vector for HST-l
".3 800 0 " jI during VLBA observations (Paper IV) of _660, The dif
m 600 • ference between the two angles is only 1 for anyone
(j
l point, but in 17 of 18 epochs the EVPA is significantly
400 ,f " displaced towards the north from the radius vector, mak
200t~ ~orlQa ing the difference significant at roughly the 30" level (see
2003 2004 200 200'5 also §5.1).
8 at. For the most part the polarization characteristics in
F330W track the ones seen in F606W. This is true for
50 i-
t 1 all epochs for knot HST-1. Thus that knot displays no
t , evidence for frequency-dependent polarization behavior.
!J r!
~!T. For the nucleus, however, one epoch shows significant (>
40 1 i ~
rj 20-) differences between polarization characteristics in the
• 1
f! 1 TT ! two bands, namely 28 Nov 2004. As can be seen in Figure
" 30P l~l~ ,i, i 1, this difference is highly significant both in P (8 0") as
n: !, f,,l) "'"n1l TLj l~T ., swoeulir caess E oVf PinAst (r1u0m 0e"n), taaln der wroer hianv teh eel iFm3i3n0aWted a anldl pFo6s0si6bWle
it;
Uj ~ data for this epoch. We note that this epoch is very near
T the peak of a flare in the nucleus; however, this flare does
20
not exhibit a spectrum that is significantly different than
surrounding epochs. Unfortunately the next epoch does
1020L03- . 2004 2005 .OU6 20(]7 ~aIo s not have UV polarimetry so we are unable to comment
Dali! further on whether any frequency-dependent polarization
pattern developed during this flare.
0.7 r I ! i ! ""J tioAnnshoitph ebre ptweresepne cftluivxe acnadn fbrea cgtaioinneadl pboyl aloroizkaitnigo na.t Trehlais
-B•"0 0.6 ,tE t II ? ..,,,,~ iksn odto nHe SiTn- lF i(gbuorteto 3m, upsainnegl ),t hteh isF 6r0e6veWal sd aa tast roonnlyg. coIrn
~
-0.5 ! ~ relation between total flux and fractional polarization,
"0 ~ ! ij particularly between 2004 November and 2006 December
~•u 0.4 ~ h I, (epochs 4-17), when the flux variability was dominated
'" !
"' 0.3 f ! 1 by the main part of the flare and the monitoring was
most frequent. The other four points represent times
"'E"'- 0.2 Eqf ~l hwahde rae stihgen iffilcuaxn tv acorinatbriilbituyt iowna sf roemith semr adlolemr isncaatleed e vbeyn tosr.
t!
o 12 003 2004 2005 2006 2007 ...2..0"J0 8 Up s=i n0g. 8o4n2l ya nepdo Pch =s 41-.167 X, a1 0Sp-4e•a rInmsapne'cst irohno -otfe stht ein Fd3ic3a0tWes
-J Dale data shows they also follow this correlation. During this
time the EVPA is nearly constant (Figure 1), although
there is possible evidence for a quasi-sinusoidal modu
lation. This gives the clear impression of the variability
1 being dominated by a single component with a highly or
" ! ~ dered magnetic field. We discuss the implications further
-••w ,[ ! , in §5.2.
~ .
•~ ~ itfj! ~ 1'~1, IT A pattern is much more difficult to pick out for the
"0 -60 j nucleus. We have therefore added red arrows to Fig
.>0C{1- 'Oft ~' ~IlI' l l,1 1 ~ ~,, i(t, " ------I,:j! wurhee r3e ttoh egruei dies ethveid eeynec ed oufr ian gp athttee rpna. rtD oufr tihneg ecpamocphasi g6n-
--------- 15, there was a coherent increase and then decrease in
1 the fractional polarization of the nucleus, at a time when
1
the flux variations where small (Figure 1), This trans
~
-80 [ j lates into a near-vertical "loop" in the polarization-flux
2003 2004 2008 2006 2007 2008
ate plane (Figure 3, top pane0, with the initial increase in
polarization being correlated with the decrease in flux
FIG. 2.-Variations in Total flux, fractional polarization, spectral
from epoch 5'8 maximum, and then a nearly monotonic
index and EVPA, are plotted for knot HST~1. A dashed line in
the EVPA panel reflects the jet PA. The F606'\\' observations are decrease being observed over epochs 9-15 while the flux
plotted as squares, while in the second and fourth panel the F330W
polarimetry is plotted as diamonds. See §3.2 and 4 for discussion.
6 Perlman et a1.
varied by only 25%. This is discussed further in §5.3. oscillations in the plane in between (denoted on Figure
4 by blue arrows). Both "looping" patterns are known
to arise for particular relations between the acceleration
and cooling timescales controlling the spectral evolution
of the radiating particles (in the framework of simplified
models for such an evolution) and are discussed further
in §5.2.
0.7
~ 0.6 f3
""
E
_ 0.5
e
~o 0.4
"~
20~0~~6~0~0~~8~0~0~~1~0~0~O~~J~2~0~0~~1~4~00 Ul
0.3
Nuclear Flux tuJy)
I
b;
50 l . "' 0.2
[
45 ~ 0.1 ~
t
~ ! I 200 400 600 800 1000) 1200 1400
40 ~ , ~ • 3 HST-1 Flux (uJy
~ i 1 T' i
r ! I ~ lob
" 35 ,~ IjII 16 $T 1 t: lT 9I j EaFcIhG .e p4o.c-h Thhase beeveonlu ltaiobne lolefd o s:euqvu-eont ivaellrys,u sa sf liunx Ffiogr ukrne o3t. HNSoTti-cle.
0. 30.~• ' I .,i the strong 'looping' behavior during the maximum of the flare in
t>- II!I l.2f1f!i ~ 5 1., HST-1, as well as after (green and red arrows). See §§4, 5.1 for
25 [I . iT, 14, j'1 13 l" ~ l discussion.
.t 18 11',5 117 T1 1j. 5. DISCUSSION
'20 ~ ' ~'
It is interesting to explore further the reasons behind
15kr~'~-7~~~~~~~~~~~~~~1 the highly disparate· behaviors of the nucleus and HST-1,
200 400 600 800 1jJOO) 1200 1400 as seen in our data as well as other bands. X-ray vari
HST-l Flux l uJy
ability of the nucleus and HST-1 was studied in Papers
I, III and V. Variability has also been seen at gamma
FIG. 3.~ Graphs of fractional polarization versus flux for the ray energies, where M87 has been detected in both TeV
core (to:;» and HST-l (bottom). The observation epochs have been
(Aharonian et al. 2003) and GeV (Abdo et al. 2009)
labelled sequentially. Notice that for HST-l, epochs 4-18 display a
very strong correlation between the flux and polarization, whereas energies (although, n.b., in the gamma-rays the 1.187 nu
the behavior for the nucleus is very different, with a 'loop' seen cleus and jet cannot be resolved from one another). Dur
between epochs 9-15, but otherwise no organized pattern. ing the peak of the flare of HST-1 in 1\larch-May 2005,
TeV variability on timescales of a few days (Aharonian
Table 2 and Figures 1-2 also describe the evolution of et a1. 2006) was seen, leading Stawarz et al. (2006) to
the UV- optical spectral index Q'UV -0. As can be seen, associate the enhanced TeV emission seen in 2005 with
Quv -0 behaves differently for the nucleus than it does the flare of HST-l. However, the origin of other features
for HST-l. For the core, we do not see significant vari in the TeV lightcurve is unclear. A major work compil
ability in the optical spectral index. By contrast, the ing all multi-wavelength and TeV gamma-ray variability
spectral index is strongly variable for HST-l. For most of data for 1\187 was recently completed by Abramowski et
the time, the variability of the spectral index appears un a1. (2011). That work concluded that while it remains
correlated with flux, but during the brightest part of the plausible that both the unresolved nucleus and HST-1
flare (epochs 4-9, denoted on Figure 4 by red arrows), we contribute to the TeV emission observed from M87 sys
see that the spectral index is larger (Le., steeper) when tem during the quiescence epochs and also the 2005 flare,
the flux is higher. As can be seen in Figure 4, during during the 2008 and 2010 epochs of the enhaced "I-ray
this time HST-1 describes a deflnite 'loop' in the flux activity of the source the nucleus is more likely to have
spectral index plane. This pattern has been called "hard contributed the majority of the TeV flux.
lags" in the blazar and optical variability literature (see In the radio, both Paper IV as well as Chen et a1.
e.g., Zhang 2002, Zhang et a1. 2002; Fiorucci, Ciprini & (2011) studied the variability of knot HST-1, with Paper
Tosti 2004; the term "counterclockwise looping" is also in IV using VLBA data, while Chen et a1. (2011) used data
use, but note that those papers use the opposite sign con from the VLA, both from roughly 2003-2007. The lat
vention for a than we do). Looping in the other direction ter work has angular resolution comparable to HST and
is seen for epochs 13-17 (denoted on Figure 4 by green includes an analysis of polarization data, and finds a vari
arrows), e.g., "soft lagging" (Zhang et al. 2002), with able P, EVPA and rotation measure, as well as a radio
Polarization and Spectral Variability in M87 7
spectrum that softened when the knot was brighter. It is by a few factors. Primary among these is its compres
difficult to compare their data to ours in detail because sion ratio, k = r dfld/ (r" flu) (Laing 1980; Wardle et a1.
Chen et a1. (2011) used a subset of the avallable data, 1994), where r u,d is the bulk Lorentz factor and flu,d is
including only 4 datasets between 2004-2006 (and none the bulk speed with the subscripts referring to upstream
in 2005), when HST-1 was most active. However their and downstream quantities respectively. One can also
findings, while consistent with the idea of non-cospatial think of the compression ratio in terms of the compres
radio &nd optically emitting particle populations (Perl sion of a unit length. Ignoring the plasma magnetiza
man et al. 1999), are difficult to reconcile with the much tion, the shock jump conditions relate the upstream and
longer radiative lifetimes of radio synchrotron emitting downstream speeds via fl"fld = 1/3 (Landau & Lifshitz
particles, as well as the similar radio and UV lightclirves. 1987) for a plasma with a fully relativistic equation of
To further explore the physical implications of our re state. The other factors that describe the shock are the
sults, it is necessary to discuss the physics of shocks and spectral index 0, Doppler factor 0 and viewing angle Bob.
other disturbances in jets, which may explain the be More specifically, in this model, Kollgaard et al. (1990)
havior we see. As will be seen, we do this because we found that the degree of polarization is given by (their
believe that both behaviors may be explained by s"ch equation (2); note that our convention for the spectral
disturbances. Following this, we will then present phys index a is the opposite of the one in Kollgaard et al.)
ical interpretations of the behaviors of knot HST- 1 and
the nucleus. P = 3 + 3a 82(1- k2) sin2 eob . (1)
5 + 3a 2 - 82(1 - k2) sin2 eob
5.1. Shocks, Helical Distortions and Polarization
Variability This model is obviously dependent on the geometry cho
sen for the jet magnetic field and for the shock - in par
The behaviors we see in both the nucleus and HST-1
ticular ignoring any helical component to the field and
are direct reflections of the physics in the emitting region.
a strong, perpendicular shock. Such a model can eas
Since the data 'we have analyzed in this paper includes
ily produce a correlation between flux and polarization,
fluxes, optical-UV spectra and also polarimetry, we have
along with a roughly constant EVPA parallel to the jet
information both on the interplay between particle accel
direction, as seen in knot HST- 1. We will discuss in §5.2
eration and cooling, as well as the magnetic field struc
the application of this model to HST-1.
ture that was either associated with this behavior or pro
If, however, a different type of disturbance is envi
duced it. In order to motivate the discussion herein, it is
sioned, quite different polarization behavior can be pro
useful to summarize the commonalities in the behavior of
duced. The first possibility we will consider is a time
both regions and then discuss why those commonalities
varying upstream speed, flu. This is supported by the
argue for an origin in shocks and waves. Both HST-l and
variable superluminal speeds observed in AGN jets(e .g.
the nucleus exhibit coherent patterns in the (1, P) plane.
Lister et a1. 2009). In this second model, the varying fl"
In the case of HST-1 the pattern is simple: polarization
is related to the downstream fiow speed via fl"fld = 1/3
is correlated strongly with intensity, while at the same
which induces variability in the shock compression fac
time the EVPA remains essentially constant very close
tor, k. To induce a variation in the shock compression
to the PA of the jet. By comparison, in the nucleus we
factor, the stochastic variation in f3u is modeled as being
see a 'loop' in the (1, P) plane, with somewhat more com
sinusoidal:
plicated EVPA behavior, featuring wild swings of up to
1000, albeit around a dominant orientation that is once flu(t) = flu,o + A~ sin (w~t - <p~) (2)
again :!lear the PA of the jet's radial motion vector. The
fact that both HST-1 and the nucleus display coherent The parameters (fl",o,A~,w,J,<P~) are free parameters of
variability patterns in (I,P) while maintaining a single, the model, and t is time. If the jet contains a disordered
dominant orientation of EVPA indicates that in both re field component Br, no ordered toroidal field, and an or
gions, the details of the local magnetic field structure are dered poloidal field Bp, then the polarization depends
tightly related to the efficiency of the particle accelera on the magnetic field through the ratio E = IBp/Brl.
tion. That component must have a reasonably.well or Since the process of transforming an initially ordered
dered magnetic field structure, particularly in the case of large-scale field to a tangled one is probably governed
HST-1 because of its very high polarization (see §5.2 for by the kink instability (Spruit, Foglizzo & Stehle 1997;
further elaboration on this issue). These behaviors are Begelman 1998; Marscher 2009), it is natural to assume
all consistent with having been produced through shocks that the value of Ea t the shock fluctuates in time. Sim
and/o: waves, although the details of the physics may be ilar to f3u, we model the ~ variation as
different in the two regions.
(3)
What types of shocks may be consistent with these be
haviors? Perhaps the simplest type of .shock to discuss The parameters (Eo, A" we, <P,) are free parameters of the
is localized, planar, and oriented along or near the jet model, and t is time. Equations (2) and (3) allow the cal
perpendicular. This type of model, often known as a culation of the post shock intensity, 1, and polarization
"Laing sheet" due to the fact that shocks of this type P (e.g. Kollgaard et a1. 1990):
fciheladr ainctteor ais tthicianl lsyh ceoemt wpritehss m aang inneittiica lfliye lrda anldoonmg tmhea gsnheeetitc, I ()( 8~+ak-2 B; {2 + [3k2"€ - (1 - k2)] sin2 e~b} (4)
haa been investigated extensively in the literature, par 3 + 3a 82 [(1- k2) - 3k2e] sin2 eob
atincdu lKaralyll gina aLrda ientg a(11. 98(109),9 0H).u gIhne sth, iAs lmleor d&e l,A tlhleer p(r1o9p8e5r) , P "" 5 + 3a 2 - 82 (1 - k2) sin 2 eob + 382~2k2 sin 2 eob •
ties of a relativistic shock can be completely determined (5)
8 Perlman et al.
\>
Note taat equation (4) is an apprOximation as it results
from integrating over the line of sight with a set to unity
for convenience (Wardle et al. 1994), and that equation
(5) as expected reduces to equation (1) for ~ = O. The
downstream Doppler factor, 8 = (rd - r dlid cos 000)-1,
raised to the power 2 + a consistent with a steady jet
(Lind & Blandford 1985), is included in the expression
for intensity since the flow speed varies in the emission
region which is downstream of the shock. In OUf conven~
tion, the EVPA is parallel to the jet axis for P > 0, and
is perpendicular to it for P < O. The factor B~ in equa
tion (4) is heidas constant in our model. Otherwise, if
used as another time varying parameter, it would merely
modify the amplitude of the intensity fluctuations.
Another plausible configuration is one where the jet
contains a significant helical component to its mag~
netic field. Such a scenario might be particularly op
YI~
erative in the nucleus, since AGN jets are generally
thought to be launched and collimated within the cen
tral ~ parsec by magnetic fields that dominate otber
sources of pressure. Its applicability to regions further
from the nucleus is less certain because it is not known
how fer downstream from the launching site the jet re
tains a magnetically dominant ordered helical field since
jets are thought to be unstable to the m = 1 kink 'C3=::::::>lv
mode (Narayan et al. 2009; Marscher 2009; Spruit Mllgnetlc
2010). Despite this theoretical uncertainty, some obser "Sy'r,rv netry
vational evidence suggests that jets contain helical mag
FIG. 5.- Schematic of the helical distortion model illustrated
netic fields on parsec scales: parsec-scale buik acceler
with toroidal loops of magnetic field. As shown by the velocity
ation (Vlahakis & Konigl 2004), Faraday rotation gra vector.:; (labeled by v), the velocity field is uniformly directed de
dients (Asada et al. 2002; Gabuzda, Murray & Cronin spite the kinked jet. As the kinked region of the jet 'propagates
2004; Zavala & Taylor 2005; Kharb et al. 2009), through the standing shock's emission region (gray region), the
orientation of the magnetic field with respect to the line of sight
and observed asymmetries in the transverse pro
changes with time. This change in orientation is parametrized as
fileS of polarization, brightness and spectral index 8'(t) in equation (7). The effects of shock compression, the poloidal
(Clausen-Brown, Lyutikov & Kharb 2011). magnetic field and the change in the magnetic pitch angle, VI(t),
Let us assume that a component in the jet contains a are not shown.
large-scale helical magnetic field that is variable due to
the growth of the m = 1 kink mode. The jet's magnetic where t is time and (A~,A~,W8)WtP,<P8)"'\b,6~,t/Jo) are
symmetry axis (parallel to the Bz direction) will then paran:eters of the model. The jet frame viewing an
deform into a large scale helix that is carried with the gle, 0;"', is actually set by the relation to the observer
jet's velocity field (Mizuno et aI. 2011), which we as frame viewing angle by sin9:x, = 6sin6ob. However, as
sume to be uniform in this work. If the helical deforma the Doppler factor for the inner jet is unconstrained, (J~
tion passes through a standing shock, as shown in Figure is treated as a free parameter as long as the required
5, then the magnetic structure of the post-shock region Doppler factor is within reasonable bounds. It should be
will change in time, thereby producing fluctuations in the noted that when the magnetic field passes through the
post-shock synchrotron emission. Let us further assume standing shock, the field components lying in the shock
that toe ratio of B'o/ B~ varies in time in the post-shock plane will be amplified by shock compression. However,
flow. Kink mode simulations have found that, to avoid this only modifies the form of ""(t) and O'(t); it does not
total jet disruption, B'o/ B~ dynamically relaxes to ~ 1 prevent quasi-periodic variations from occurring.
To calculate the total intensity and fractional polariza
(Nakamura et aI. 2007), while, in competition with this
tion from the standing shock, the emission region elec
process, jet conical expansion always increases the ratio.
tron distribution function is assumed to be a power law,
Therefore, this competition will produce fluctuations in
B:'/ B: at the standing shock. dn = K,E-PdE, where the spectral index is related to
the electron distribution function by p = 2a + 1. We
Within this scenario muitiple types of disturbances can
assume the jet is unresolved and the emission is mostly
be envisioned. Herein we consider two. Firstly, we may
concentrated in a cylindrical shell centered on the local
introduce sinusoidal variations both in tbe jet frame mag
symmetry axis so that the total intensity and fractional
netic pitch angle "" = tan-l(B~/ B~), and in the angle polarization are (Lyutikov, Pariev & Gabuzda 2005):
betwen€ the magnetic symmetry axis and the jet frame
line of sight, 0', as shown in Figure 5 : I RJ K(cos2 "" + cos2 0' - 3(cosO' cos ,p')2 + 1) (8)
PRJ 3 + 3" -2(1 + 3 cos 2"") sin2 0' ,
'i/(t) = A~ sin (w",t - 4>'1) + "'~ (6) 5 + 3" 5. - cos 20' - cos 2",' - 3 cos 20' cos 2",'
O'(t) = A~ sin (w.t - 4>.) + O~b (7) (9)
Polarization and Spectral Variability in U87 9
where (J' and VI are the variables defined in equa strain the kinematics in the local flow under certain as
tions (7) and (6) respectively, and K is an arbi sumptions. Th illustrate, we choose k = 0.25, appropri
trary constant that depends on emission region details ate for a strong relativistic shock (Meisenheimer et aJ.
such as the beaming factor I emission region size, rel 1989), spectral index 0.45 S QI S 0.55, corresponding to
ativistic particle density, and magnetic field strength the mean observed during both flare "loops" (Fignres 2,
(Lyutikov, Pariev & Gabuzda 2005). The sign of Pin 4), and 0.35 S P S 0.45, consistent with the peak value
dicate; whether the EVPA is parallel (P > 0) or perpen attained at both flux maxima. We then allow the view
diculru' (P < 0) to the jet frame local magnetic symmetry ing angle 80b and Doppler factor 6 to vary. We constrain
axis in the standing shock. At any time t', the values of B' the solutions so that the apparent speed fl.p" falls within
and ,p' represent the particular orientation of the kinked the range 4.0 < flapp < 4.5, the values reported for VLBI
magnetic field and the value of the magnetic pitch angle components in Paper IV. The result is shown in Figure
in the standing shock respectively. (See the appendix for 6. As can be seen, the permitted values of Oob range be
a derb'ation of equations 8 and 9.) tween 11 - 18°, and 8 can range between 2.5 and 5.5.
The Lorentz factor, r is more stahle, however, with per
5.2. Polari.ation and Spectml Behavior of HST-j:
mitted values ranging between 4.1 and 4.8. These values
Interpretation
are in agreement with the upper limit of 8 = 8 calculated
We believe that the most consistent explanation for the by Waters & Zepf (2005) based on other considerations,
variability observed in HST-l is that the Hare occurred and are also consistent with the requirement of 8 < 19°
0b
in a shock within the jet, with the maximum polariza from the somewhat faster superluminal speeds seen in
tion coming at the time of maximum compression and HST monitoring during the 1990s (Biretta et aJ. 1999).
also maJCimum optical flux. The high polarization (P at It should be mentioned that with these values of Ct, P
maximum in excess of 40%) and alignment of the EVPA and k there are no allowed solutions with r < 2, and
with the jet axis in HST-l, rules out the conical recon moreover, nearly all the allowed solutions with r < 3
finement hydrodynamic shock model of Nawalejko (2009; require values of (Job> 20°, which is not allowed.
see also Bromberg & Levinson 2009). Assuming that InterestingIY,however, we see no evidence for motion of
the je, magnetic field is weak and tangled, such mod the flaring region of HST-I in our data, as our astromet
els cannot produce a polarization higher than - 30%. ric results (all epochs having identical distances between
Furthermore, such a model would predict a substantially the nucleus and HST-I to a tolerance of ±O.OI") translate
differe,t orientation of EVPA for small and intermediate to an upper limit of 1.02 c (20') on motion of the flaring
jet inclinations. The EVPA we observe in HST-l is very region itself. While this seems to conflict with the VLBA
nearly perpendicular to the jet, and nearly constant, so a measurements of Paper IV, as well as our calculation of
more consistent explanation for the data is that the non 8 from the shock model, this should not be too concern
thermal activity is restricted to a localized, perpendicular ing. It is entirely possible that the flaring region itself
(possioly stationary) strong shock v!ithin the interior of represents a standing shock, not flowing along with the
the flvw, as envisioned in §5.1 (see eq. (I)), which is plasma. Under such a scenario we could not expect the
also more in line with the observed radio morphology. observed speed to accord with the local jet Lorentz fac
The feature producing the' flare may then be associated tor. Indeed, the VLBA maps (Paper IV) show a station
with the ~lach disk produced around the nozzle (con ary upstream end to the HST-l region. For most of the
verging point) of the reconfinement shock (Stawarz et time period covered by Paper IV, the observed speeds are
a1. 2006, Paper IV, Gracia et aI. 2009). In the latter consistent with our limit, as the flaring region (compo
model (Gracia et aI. 2009) the sYnchrotron emissivity nent HST-1c in their nomenclature), while dov:nstream
is proportional to the comoving frame electron density from the stationary upstream end (HST-ld), has a speed
and magnetic field strength and configuration, modulo of 1.14 ± 0.14 c , measured relative to HST-ld. Begin
6(2+<». rather than heing dependent on hydrodynamic ning around epoch 2006.0, however, HST-lc splits into
energy diSSipation as assumed in'Nawalejko (2009). two components, with the faster, downstream one (ac
Using the doubling/halving timescales calculated for counting for the majority of the radio flux) accelerating
the optical/UV emission in Madrid (2009) as well as to 4.3±0.7 c relative to HST-1d. While this latter speed
those in Paper II for the X-ray emission, we can estimate is on the surface highly inconsistent with OUf astrometric.
a size of S 0.56 light-years (where 8 is the Doppler fac results, it is important to realize that Our data are not
tor) for the size of the flaring region, with the irregularity very sensitive to this time period, as it contains only 5 of
of the available measurements perhaps arising because of our 17 ACS epochs (the lower-resolution WFPC2 obser
the complexity of the compression mechanism (we do not vations of epoch 18 are much less useful for astrometry).
believe it arises because we did not sample adequately, Using only our ACS data during 2006, the limit on mo
given the smoothness of the ohserved lightcurve). This tion of the flaring region would be much less restrictive,
is consistent with the fact that HST -I is not resolved i.e., ~ 5.5 cat 20'. Use of later epoch data would improve
by HST, which seta a hard upper limit of 2 pc on the this result; however, there was a gap in UV monitoring
radius of the optically emitting componer.ts (Paper II, of the M87 jet between late 2006 and mid-2009, a time
Madrid 2009). Interestingly, howeve" as already noted interval that featured a further factor - 5 decrease in
in Paper II and Madrid (2009), the X-ray and opticaljUV the X-ray flux of HST-I (e.g., Paper V and later data),
doubli!lg/halving timescales do not lead to significantly which could make it very difficult to identify the flaring
different constraints for the region size. components. The necessary data do, however, exist in
If we then apply equation (1) to knot HST-I, we can the VLBA archive, and it would be highly interesting to
reproduce the type of behavior seen in the (1, P) plane track the motion of the flaring region in both bands.
(Figure 3). But equally interestingly, we can also con- If indeed the giant HST-1 flare was a result of en-
10 Perlman et al:
6 Interestingly, as shown in Paper V, in both the UV and
X-rays the derivative dI / dt changed sign between 2005.4-
2005.5, Le., epochs 9 and 10. If indeed this was related
to the relationship between the acceleration and cool
5
ing timescales, then the oscillations seen in epochs 10-13
- which occurred during a time when the flux was de
creasing monotonically - become important. The mul
4
tiwavelength spectral characteristics of HST-1 discussed
>-<
above could be possibly explained in the framework of
r the scenario in which the primary loss mechanism in the
r optical-UV is Comptonization of external radiation and
3~ the optical-UV emitting electrons are near the transition
, 1 between the Thomson and Klein-Nishina regimes (see the
discussion in §6, below).
L J As discussed in §4, the mean EVPA in HST-1 is some
21
what different from the PA of the jet. Thus while a shock
8 10 12 14 16 18
morphology is likely for HST-1, the polarization data
/Job [deg] hints at a more complex morphology than indicated by
6, the simple, unresolved appearance shown by the images.
Two interpretations are possible. The first possibility is
r
that HST-1's optical polarized flux comes from two or
f
more regions. While this might seem the simplest inter
5~ pretation, it is difficult to reconcile this with the strong
1 correlation between flux and polarization, combined with
the constant EVPA. In addition, the unresolved nature of
4r the flaring region in our data and that of l\ladrid (2009),
'<:> constrains the maximum separation of these components
L to "-' 2 parsecs, which translates to a constraint on the
cooling timescale that, as already discussed, is consis
tent with the spectral evolution we see. Alternatively,
3
it could indicate either a twist in the field within the
shocked region and/or an oblique shock, as suggested for
the knot A region by Bicknell & Begelman (1996). In
21 the latter case, one might expect to see evidence of a
8 10 12 14 16 18 slight local deviation in the flow direction, and indeed,
the VLBA components seen in Paper IV do have a sig
/Job [deg]
nificant range of radius vector PAs as measured from
FIG. 6.- Allowed values of beaming parameters for our shock the standing feature at the upstream end of the HST-1
model of knot HST-l. At top, we show the allowed range for the
Lorentz factor r and viewing angle 9ob. while at bottom, we show complex. Furthermore, there is weak evidence of small
the cor:'esponding range for the Doppler factor 8 plotted against changes in EVPA over time, with the EVPA near the
viewing angle Bob. In both panels, light gray colors refer to spectral two flux maxima (epochs 8, 9 and 16) being within 2-3
indices c¥ = 0.45, gray colors refer to a = 0.5, and black colors refer degrees of the PA of the radial vector from the nucleus
to a = 0.55 See §§5.1, 5.2 for discussion.
to HST-1 from the VLBA data, while in the other 15
epochs the EVPA is more closely aligned with the mo
hanced particle acceleration occurring within a compress tion vector of the fa:ntest feature of the HST-1 complex
ing shock, then it would be logical to ascribe the cool as seen on the VLBA maps, namely component "a", at
ing during the main fiare to relaxation of the compres the downstream end. Note however that if the shock \.ies
sion that occurred within the shock. In that case, the well within the interior of the jet these deviations might
energy dependence - or lack thereof - of the variabil not be indicative of an overall deflection of the flow (as in
ity timescales (in optical and other bands) may be set knot A). On a somewhat related note, Nakamura et al.
by the dynamical timescale of the compression, as origi (2010) recently proposed a model for the overall struc
nally noted in Paper II and explored further in Paper V ture of the 1\187 jet, in which the main shocks in the M87
and Madrid (2009). For example, if the compression and jet have a helical magnetic field structure, which could be
subsequent expansion were adiabatic, one vlOuld expect produced via two methods. Bicknell & Begelman (1996)
to see frequency-independent variability behavior unless suggested that the knots in the M87 jet are generated by
there was an intrinsic, pre-existing break in the spec the helical modes, which can produce lateral oscillation of
trum. This would be modified where the particle cooling the entire jet but has trouble producing filaments within
and acceleration timescales (taee and teool respectively) it. Hardee & Eilek (2011), by contrast, postulate that the
are eqclal to or less than the dynamical timescales. As al elliptical Kelvin-Helmholtz modes dominate, and model
ready noted, we observe hard lags in the optical-UV dur the inner ~!87 jet as having twisted, high-pressure fil
ing the brightest part of the flare (epochs 5-9, Figure 4). aments generated by elliptical Kelvin-Helmholtz insta
To explain such hard lags, tace must be similar to teaol, bilities which eventually disrupt the jet beyond knot A.
whereas when soft lagging was observed (epochs 13-17), Assuming that the magnetic field within the components
the opposite relationship v:ould hold, i.e., taee < teool. is perpendicular to the local bulk velocity vector (rather