Optical and X-ray Variability in The Least Luminous AGN, NGC4395

19 Pages·0.58 MB·English

by  P. Lira

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLATEXstylefilev1.4)
Optical and X-ray Variability in The Least Luminous
AGN, NGC 4395
⋆
P. Lira,1 A. Lawrence,1 P. O’Brien,2 R. A. Johnson,3 R. Terlevich4 & N. Bannister2
1 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland
2 Department of Physics & Astronomy, Universityof Leicester, UniversityRoad, Leicester,LE1 7RH, UK
3 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
4 Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ, UK
9
9
9 1February2008
1

n
a ABSTRACT
J

5 We report the detection of optical and X-ray variability in the least luminous
2 known Seyfert galaxy, NGC4395. Between July 1996 and January 1997 the feature-
less continuum changed by a factor of 2, which is typical of more luminous AGN.

1 The largest variation was seen at shorter wavelengths, so that the spectrum becomes
v ‘harder’duringhigheractivity states.During the sameperiodthe broademission-line
8 flux changed by ∼ 20−30 per cent. In a one week optical broad band monitoring
4
program, a 20 per cent change was seen between successive nights. The difference in
3
fluxobservedbetweenthe spectroscopyofJuly1996andthe broad-bandobservations
1
impliesvariationbyafactorof3at4400˚Ainjustonemonth.Inthesameperiod,the
0
spectral shape changed from a power law with spectral index α∼0 (characteristic of
9
9 quasars)toaspectralindexα∼2(asobservedinotherdwarfAGN).ROSAT HRIand
/ PSPC archivedata show a variable X-raysourcecoincidentwith the galactic nucleus.
h AchangeinX-rayflux byafactor∼2in15dayshasbeenobserved.Whencompared
p
with more luminous AGN, NGC4395 appears to be very X-ray quiet. The hardness
-
o ratio obtained from the PSPC data suggests that the spectrum could be absorbed.
r We also report the discovery of weak CaIIK absorption, suggesting the presence of a
t
s youngstellarclusterprovidingoftheorderof10%ofthebluelight.Thestellarcluster
a may be directly observedas a diffuse component in HST opticalimaging. Using HST
:
v UV archive data, together with the optical and X-ray observations, we examine the
i spectralenergydistributionforNGC4395anddiscussthe physicalconditionsimplied
X
bythenuclearactivityunderthestandardAGNmodel.Wheninthelowstate,theex-
r trapolated UV continuum is insufficient to explain the observed broademission-lines.
a
This could be explained by intrinsic variability or absorption or may imply an extra
heating sourcefor the BLR.The observationscanbe explainedby either an accreting
massiveblackhole emitting atabout10−3 LEdd orby a singleoldcompactSNR with
an age of 50 to 500 years generated by a small nuclear starburst.
Key words: galaxies: individual: NGC4395 – galaxies: nuclei – Seyfert
1 INTRODUCTION Seyfertgalaxies. Itslowluminositynucleushasablueabso-
lute magnitude M 11, a luminosity 104 times fainter
The dwarf Seyfert nucleus in NGC4395 was first reported B ∼ −
than a classical Seyfert galaxy like NGC4151. The detec-
by Filippenko & Sargent (1989) almost a decade ago. Op-
tionofafeaturelessUVcontinuumgavesupporttotheidea
tical spectroscopy showed high ionization narrow lines as
thatNGC4395wasarealexampleofdwarfnuclearactivity
well as broad permitted emission-lines. The detection of a
(Filippenko,Ho&Sargent1993). However,Shields&Filip-
compact radio source (Sramek 1992) added support to the
penko (1992), after several years of spectroscopic monitor-
ideathatNGC4395isafeebleversionofthemoreluminous
ing, reported that no evidence was found for continuum or
linevariability.Sincevariability isone ofthemost common
characteristics of AGN,this result was quitesurprising.
⋆
VisitingProfessorattheInstitutoNacionaldeAstrof´ısica,Op-
ticayElectro´nica,A.P.51y216.71200Puebla;M´exico. NGC4395 is a nearly face-on dwarf galaxy (B 10.7,
∼
(cid:13)c 0000RAS
2 Lira et al.
Table 1.JournalofObservationsofNGC4395
Date Telescope Mode ArchiveData
7April1988 Hale Spectroscopy No†
2July1992 ROSAT PSPC Imaging Yes
15&19July1992 HST FOS Spectroscopy Yes
17July1992 ROSAT PSPC Imaging Yes
5December1995 HST WFPC2 Imaging Yes
5–16June1996 JKT Imaging No
23June1996 ROSAT HRI Imaging No
13July1996 WHT Spectroscopy No
15January1997 WHT Spectroscopy No
† DataprovidedbyFilippenkoetal.SeeFilippenko&Sargent1989.
M 17.9, assuming a distance of 5.21 Mpc - see below) of σ = 4 arcsec. In this way thenoise in the image is artifi-
B
∼−
with morphological classification Sd III-IV in the extended ciallysuppressedandcanbeusedtodrawthecontourlevels.
Hubble system, as defined by Sandage & Tammann in the Contours in figure 1 were drawn at 3, 6, 12, 24, 48 and 96
RSA catalogue (Sandage & Tammann 1981). It exhibits a times the standard deviation in the smoothed background
star-like nucleus and an extremely low surface-brightness (count rates, fluxes and related errors elsewhere in this pa-
disk.Thelooseanddisconnectedspiralarmsshowsomeblue peraredeterminedfromtherawdata).Themostprominent
knotsofstarformation activity(foracolourplateseeWray sourceinthefigure(S2)hasnoobviousopticalidentification
1988). although theDSSimage showssome diffuseemission in the
WewilluseadistancetoNGC4395of5.21Mpc,based area. We have identified the possible source S1 as the ten-
on the observed recession velocity and a Virgo flow model tativeX-raynuclear emission of NGC4395. The slight shift
withaninfallvelocityof220kms−1 fortheGalaxy(Kraan- between the optical nucleus and the X-ray emission ( 3.5
Korteweg 1986). This value agrees well with the distance arcsec)isconsistentwiththeaccuracyoftheROSAT p∼oint-
modulus of 28.5 reported by Sandage & Bedke (1994) and ing.Indeed,anotherX-raysourceapproximately7arcminto
gives a scale of 25 parsecs per arcsec on the sky. the west of S1 and with a clear optical identification shows
Aspartofamultiwavelengthimagingandspectroscopy thesame shift.
project to study a volume-limited sample of very nearby The count rate for the nucleus was calculated by sum-
galaxies we have acquired ROSAT HRI data and optical ming over all the counts within a circle centered on the
spectroscopy for NGC4395. B and I images of the galaxy source. To estimate the background two circles of radius
were obtained as part of an AGN monitoring program. Fi- 160 arcsec free of evident X-ray sources and away from the
nally,HST and ROSAT PSPC datafor NGC4395 were re- galaxy wereused. Tofind theoptimum radiusfor theaper-
trieved from public archives. In this paper we report the tureradialprofilesofseveralpointsourceswereexamined.A
detection of optical and X-ray variability in the nucleus of finalapertureof10arcsecwasadoptedwhichshouldencircle
NGC4395. In Section 2 we present the observations and 99 per cent of the photons at 0.2 keV and 86 per cent
∼ ∼
reduction of the X-ray imaging, optical spectroscopy, and ofthephotonsat1.7keV(Davidetal.1997).Thenetcount
broad-band optical data. Results are given in section 3. A for S1 was 7.6 4.6 photons, i.e., it is not a significant de-
±
discussion ispresentedinSection4,andtheconclusionsare tection.ForS2wefindanetcountof169.2 14.3photons.
summarized in Section 5. The count rates are 6.6 4.0 10−4 and 15±.0 1.3 10−3
photonss−1 for S1 and S±2 res×pectively. ± ×
2 OBSERVATIONS AND DATA REDUCTION
2.2 PSPC data
A journal with the data used in this paper can be found
in table 1. It includes ground based and HST imaging and Two sets of PSPC data were retrieved from the ROSAT
spectroscopy,, and ROSAT data. This section will describe archive. One of these data sets is presumably that referred
the observations and reduction of data which do not come to as a private communication from Snowden & Belloni in
from public archives. Filippenko,Ho&Sargent(1993).Thefirstsetwasobtained
onthe2ndofJuly1992with7,755secondsofexposuretime,
while the second set was obtained 15 days later with 8,764
2.1 HRI data
seconds exposure. Comparing both data sets it is easy to
An X-ray image of NGC4395 was obtained with the High identifyavariableX-raysourcewhichisconsistentwiththe
Resolution Imager (HRI) on board ROSAT on the 23rd of position of the nuclear source for NGC4395 marginally de-
June1996aspartoftheAO7cycleofpointedobservations. tected from ourHRI image.
The total exposure time was 11,253 secs. For total count extraction of a PSPC point source, an
Figure 1 shows a contour map of the central part of apertureof2arcminshouldbeadequate.Reducingtheaper-
the X-ray frame overlaid on the Digital Sky Survey data tureto1arcminloses15-20percentofthecountsatthesoft
for NGC4395. The pixels of the HRI frame were binned to endofthespectrum(E<0.1keV)becauseofthewiderPSF
2 2arcsec2 and theimage was convolvedwith aGaussian atlowerenergies(Hasin∼geretal.1992). However,apertures
×
(cid:13)c 0000RAS,MNRAS000,000–000
NGC4395 3

35’

34’

33’

32’

31’
o
+33 30’
h m s s s s s s
12 26 00 55 50 45 40 35
Figure 1.ROSAT HRIcontourplotoverlaidontoaDSSplateofNGC4395.Contoursweredrawnat3,6,12,24,48and96timesthe
standarddeviationinthesmoothedbackgroud.S1correspondstoNGC4395nucleus.S2isastrongX-raysourcewithoutobviousoptical
identification.
larger than 30 arcsec around the NGC4395 nuclear source 2.3 Optical spectroscopy
wouldincludeotherknotsofX-rayemission, ascanbeseen
Longslit spectraof thenucleusofNGC4395 wereobtained
in our HRI image. To estimate the flux due to these extra-
onthe13thofJuly1996andthe15thofJanuary1997atthe
nuclear sources, we measured the net counts in the HRI
Cassegrain focusofthe4.2mWilliamHerschelTelescopeat
image using an annuli centered on the nuclear source with
an inner radius of 10 arcsec and an outer radius of 1′. We theRoquedelosMuchachosObservatory.TheR316Rgrat-
ing was installed in the red arm of the ISIS double spec-
find 23.5 14.7 counts in the annuli, so we expect some
± trographandtheR300Bwasusedinthebluearm,together
contamination within the 1 arcmin aperture, but it should
withdichroic5400(halfpowerpointofcrossoverat5470˚A).
not be significant.
Thewavelengthcoveragewas 3650-6750˚A.Asmallgap
∼
between5220 and5270 ˚Awas notobserved duringJanuary
Fora 1arcmin aperturecenteredat theposition of the 1997.ATEKCCDandaLoralCCDwereusedduringJuly
NGC4395 nucleus,thenetcountswere 67.2 10.7 photons 1996 for the red and the blue camera, respectively. Both
±
and 139.7 14.1 photons for the first and second PSPC cameraswereequippedwithTEKCCDsfortheruninJan-
±
data sets, respectively. For the background estimation we uary1997. Theslit width was 1arcsec on thesky,resulting
used a large circle of radius 600 arcsec far away from any in a spectral resolution of 3.0 ˚A FWHM for the red arm
∼
contaminationbyotherX-raysources.Theassociatedcount and 3.5 ˚A FWHM for the blue arm ( 3.8 ˚A FWHM
rates are 8.7 1.4 10−3 photons s−1 on the 2nd of July with∼the Loral chip). The seeing varied be∼tween 0.8 and
1992,and15.9± 1.6×10−3 photonss−115dayslater,giving 1.2 arcsecs during both runs. The dispersion ac∼hieved in
± × ∼
a variability of about a factor of two. the red arm was 1.47 ˚A per pixel (0.061 ˚A per micron). In
(cid:13)c 0000RAS,MNRAS000,000–000
4 Lira et al.
Figure2.OpticalspectraobtainedwiththeWHTshowinghighandlowstateofNGC4395.ThelowerspectrumwasobtainedonJuly
1996andtheupperwasobtainedalmostexactly6monthslater.Forplottingpurposes,thebluecontinuumfromtheJuly1996spectrum
hasbeenslightlysmoothedinordertosuppressnoise.Fluxesareinunitsofergscm−2 s−1 ˚A−1.
the blue arm the dispersion was 0.96 ˚A per pixel with the empirical aperture correction was applied to the NGC4395
Loraland1.54˚AperpixelwiththeTekdetector(0.065and spectra. The correction was calculated by comparing the
0.064 ˚A permicron, respectively).In both runstheslit was standardstarspectrausedinthefluxcalibrationwithspec-
positioned at the parallactic angle to minimize light losses. tra of the same stars obtained by software which simulated
The CCDs were windowed to cover 4 arcmin in the spa- a narrow slit. A slight variation with wavelength was found
tial direction with a scale of 0.36 arcsec per pixel for the and the NGC4395 spectra were scaled by a factor of 1.4
∼
red arm and 0.20 arcsec per pixel for the blue arm. In July attheblueend(λ 3700˚A)andbyafactorof 1.3atthe
∼ ∼
1996 conditions were photometric during the whole night, red end (λ 6800 ˚A). The resulting spectra are shown in
∼
although the presence of Saharan dust in the atmosphere figure 2. The blue continuum from the July 1996 spectrum
hampered some of the observations. The conditions during hasbeensmoothedslightlysothatitsoverallshapeiseasier
January 1997 were not photometric throughout the night, to follow.
but the data for NGC4395 and associated standard stars TheinterstellarGalacticabsorptiontowardsNGC4395
were acquired duringclear periods of thenight. is just A = 0.008 mag (Burstein & Heiles 1984), and no
V
The data were reduced using iraf software. Bias cor- correction toaccount for thisextinction was used in there-
rection and flat-fielding of the 2D frames were performed ductionofthespectra.Thesignaltonoiseperpixelachieved
in the usual way. Spectra of the nucleus were extracted duringJuly 1996 was 16 for thered arm and 8 perfor
∼ ∼
using an effective aperture of 1 2 arcsec2. For wave- thebluearm.ForJanuary1997thesignaltonoiseperpixel
lengthcalibrationafifthorderL≈egen×drepolynomialwasfit- was 42 and 35 for thered and bluearm respectively.
∼ ∼
ted to the strongest non-blended emission-lines of copper- HST Planetary Camera observations of the NGC4395
neon and copper-argon lamps. The frames were flux cali- taken in the narrow band F502N filter show that the
bratedusingthespectrophotometricstandardsBD+284211 [OIII]λ5007 emission region has a diameter of < 0.4 arcsec
andPG1708+602 inJuly1996,andFeige34andG191-B2B (Filippenko, Ho & Sargent 1993). In contrast,∼the FWHM
in January 1997, and using the mean extinction curve for for the spatial profile of an unresolved star in our data is
theObservatory(E ).Theextinctioncorrection isair-mass > 1 arcsec. Inspection of the profile of the emission-lines in
λ
dependentandcanbeexpressedasA E (inmagnitudes), ∼ourspectrashowsthattheNarrowLineRegion(NLR)is,as
λ
×
where A is the air-mass during the observations. For the expected, spatially unresolved and no aperture effects have
13thofJuly1996agreyshiftcorrectionof0.13magnitudes¡ tobetakenintoaccountwhencomparingnarrowlinefluxes
(asmeasured on sitebytheCarlsberg Meridian Circle) was from our two spectra.
appliedtothemeanextinctioncurvetoaccountforthedust MeasurementsofthenarrowlinefluxesinJuly1996and
extinction.Thisispossiblebecausethewind-blownSaharan January1997agreetowithin25percentatworst(seetable
dust in theatmosphere abovetheCanary Islandsis grey to 2). Since we have used a 1 arcsec slit, the accuracy of the
an accuracy < 5 per cent between 0.32 and 1.0 microns absolutecalibration is expectedtobeof theorderof 30per
(Stickland et∼al. 1987; Whittet, Bode & Murdin 1987). For cent. Comparison of the narrow line fluxes measured from
January 1997 the flux calibration was done using the stan- our data with the values measured from data obtained in
dardextinctioncurvewithnoothercorrections.Asthestan- 1988, and kindlyprovided by Filippenko and collaborators,
dard stars were observed with a wider slit (8 arcsecs), an have also been included in table 2 and 3. The spectra were
(cid:13)c 0000RAS,MNRAS000,000–000
NGC4395 5
Table 2.NarrowLineFluxes
BlueArm RedArm
Date [OII] [NeIII] [OIII] [OIII] [OI] [OI] [NII] [NII] [SII] [SII]
λ3727 λ3869 λ4959 λ5007 λ6300 λ6363 λ6548 λ6583 λ6716 λ6731
April1988† – – 64.41 – 23.02 7.59 – – 19.77 22.97
July1996 36.83 19.19 66.84 207.60 19.60 6.52 4.29 12.86 13.00 16.57
January1997 36.34 21.55 69.72 227.82 24.69 7.77 4.80 14.40 16.34 19.81
Fluxesinunitsof10−15 ergss−1 cm−2
†:LinefluxesfromthespectrumacquiredbyFilippenkoetal.
Table 3.BroadandNarrowDeblendedLineFluxes
Date HβN FWHM HβB FWHM HαN FWHM HαB FWHM1 FWHM2
April1988† 28.58 4.0 17.43 21.3 – – – – –
July1996 21.80 4.0 14.29 22.4 55.20 3.5 82.90 10.0 36.4
January1997 24.20 4.0 18.49 20.4 59.85 3.6 120.86 9.7 33.9
Fluxesinunitsof10−15 ergss−1 cm−2
†:Linefluxesfromthespectrum acquiredbyFilippenkoandco.
examined using the same software packages used in the re- al. 1990). Within each CCD frame several nearby stars of
duction of our data. The line fluxes agree to within 30 per similar magnitude to the AGN nucleus were identified, and
cent, except for [SII]λ6716, where the difference is slightly their counts calculated using a circular photometric aper-
bigger. ture6arcsec inradius.Alarge aperturewas usedtoensure
The blue spectrum obtained during the July 1996 ob- all the point-source light was enclosed allowing for possible
servation shows a depression just after the [OIII]λ5007 smallvariationsinseeing.Foreachstartheskybackground
emission-line. This is due to a cosmetic defect in the CCD was removed by subtracting a scaled average of the counts
which produces a dark tail after bright pixels, as can be inanannulusofinnerandouterradius9and11arcseconds
seen in figure 3. The problem is only apparent whenever respectively centered on the photometric aperture. A simi-
pixels have > 2000 counts in the raw frame. The depth of larprocedurewasusedfortheAGN,excepttheannuluslay
the depressio∼n seems to be correlated with the peak count. ontopofthegalaxytherebypermittingafirst-ordercorrec-
The data shown in figure 3 were produced with the largest tion for the galaxy-light within the photometric aperture.
cosmic ray available in the image (49447 peak counts) and No other attempt was made to correct for galaxy contam-
the depression extends over up to 300 pixels ( 270 ˚A). ination. We note, however, that the central region of the
∼
PeakcountsfortheNGC4395emission-linesarelargerthan galaxy of NGC4395 is quite faint even relative to the low-
2000countsonlyforthe[OIII]lines,makinganydepressions luminosity nucleus: the surface brightness of the galaxy in
negligible for wavelengths shorter than 5200 ˚A. Unfortu- the nucleus vicinity is 20.5 mag arcsec−2. The light dis-
∼ ∼
nately,fluxesfor Hβ could be slightly affected. tribution is also flat in the spatial direction (i.e., no strong
central bulge appears present). Hence the adopted galaxy-
subtractionprocedureappearsquiteadequatefor obtaining
2.4 Ground-based broad-band observations
agoodmeasurementoftheintrinsic,nuclear-variabilityam-
AspartofanAGNmonitoringprogram,NGC4395wasob- plitude.
servedon5th–11th ofJune1996 withtheJacobusKapteyn
Telescope (JKT). Observations of NGC4395 were obtained
2.5 HST WFPC2 observations
overaperiod ofafewhoursatthestart ofeachnightusing
theJKT CCD camera, usingB and I bandfilters. Aninte- NGC4395wasobservedwiththeWFPC2onboardHST in
gration time of 5 minutes was used for each exposure. The theF450W ( B)andF814W ( I)bandfiltersonthe5th
∼ ∼
atmosphericconditionswerejudgedtobegoodandverysta- ofDecember1995aspartoftheGTOproposal6232. These
ble over the entire week. This was confirmed by inspection datawereretrievedfromtheHST dataarchiveandanalysed
oftheextinctiondatarecorded independentlybytheCarls- usingiraf.Resultsfrompreviousanalysisoftheseobserva-
berg automatic Meridian Circle, which showed the average tions can be found in Matthews et al. (1996;1998). Thenu-
V-bandextinctionat thezenithwasconstantto 0.01 over cleus of NGC4395 was imaged on the PC chip which has a
±
theobserving run. pixelsizeof0.046arcsec.ThreeexposuresofNGC4395were
The CCD data were reduced in a standard way using takenineachfilter.TheF450W exposuretimeswere1 60s
the iraf packages. Although photometric standard stars and2 400sandtheF814Wexposuretimeswere1 60s×and
× ×
were observed during the monitoring campaign, we re- 2 300s. Wehaveonlyusedtheshort exposureobservations
×
stricted our variability analysis to photometry of the nu- hereas thenucleuswas saturated in the long exposures.
cleus of NGC4395 relative to several stars within the same
CCD frames. This procedure is better suited to searching
forrapid,small-amplitudevariabilityforwhichatmospheric
3 RESULTS
changes can significantly affect the results (e.g., Done et
(cid:13)c 0000RAS,MNRAS000,000–000
6 Lira et al.
Figure 3.DarktailproducedafterbrightpixelsinarawframeobtainedwiththeLoralCCD.
3.1 X-ray imaging and spectral analysis
Table 4.ROSAT (0.1–2.4keV)Fluxes
The conversion to fluxes of the HRI and PSPC count rates
HRI PSPC
for the NGC4395 nuclear source were done assuming a
LowState HighState
power law spectrum (F ν−α) with energy index α = 1
ν ∝ α=1.5 0.37 1.11 2.06
and 1.5, and using the energy range 0.1 – 2.4 keV. Adopt-
α=1.0 0.32 1.10 2.05
ing a Galactic hydrogen column density of 1.31 1020
cm−2 (Stark et al. 1992) the HRI count rate gives×a flux Fluxesinunitsof10−13 ergss−1 cm−2
3.5 10−14 ergs s−1 cm−2, as can be seen in table 4.
∼ ×
For a distance of 5.21 Mpc this implies an X-ray nuclear al. (1997). This value of HR, with a Galactic hydrogen col-
luminosity for NGC4395 of 1.1 1038 ergs s−1. umndensity,impliesaspectralindexofα 1.IfNGC4395
× ∼
Since at least 100 counts are required to perform spec- isindeedatypicalSeyfert1wewouldexpectαx >1.5(Laor
tralanalysisofPSPCobservations,itwasnotpossibletofit et al. 1997 ; Walter & Fink 1993), which would∼require an
the data using the 34 energy channels of the detector. In- additional hydrogen column density of 3 1020 cm−2 to
∼ ×
stead, the total counts (0.1–2.4 keV)for each data set were explaintheobservedHR.ThehardnessratioforNGC4395,
binnedintoasinglechanneltoestimatethefluxes.Apower therefore,isconsistentwithamodestintrinsicabsorptionof
law spectrum with α = 1 and 1.5, and a hydrogen column thesoft part of theX-ray spectrum.
density of 1.31 1020 cm−2 were assumed. The results are
×
shown in table 4.
3.2 Analysis of optical spectra
Insection3.2wediscussthepossibilityofayoungstellar
clusterinthenucleusofNGC4395.Ifthisispresentitcould Figure 2 shows the spectra obtained in July 1996 and Jan-
bethedominantsourceofverysoftX-rays.Duringthehigh uary1997. Within 6monthsthecontinuumhaschangedby
state the count rate in the soft 0.1-0.4 keV band was 4.7 a factor of 1.3 at red end of the spectra and by a fac-
1.0 10−3 photons s−1, contributing about 30 percent o±f tor of 2.2∼at the blue end. The nuclear source becomes
the×totalflux(8.7 1.4 10−3 photonss−1).Duringthelow bluer w∼hen brighter, with a change in the spectral index
± ×
state the soft count rate drops proportionately, continuing from α 2 to α 1 (see Section 4). From the narrow
to provide about 30 percent of the total flux. The source line fluxe∼s quoted i∼n table 2 it seems that the flux at the
istherefore variable, implyingthat thesoft fluxis probably redendofthespectrumobtainedinJanuary1997mightbe
dominated bynuclear emission. slightly overestimated when compared to theJuly 1996 ob-
Inordertoobtain spectral information from thePSPC servation. It is then possible that there is negligible change
data,wehaveusedthehardnessratio(HR)technique,which at the red end of the spectra, and an even more dramatic
gives an ‘X-ray colour’ for objects with few net counts changein colour between July 1996 and January 1997. The
(Hasinger1992;Ciliegi etal.1997).BydefinitionHR=(H- continuum becomes harder when it is brighter, which is a
S)/(H+S), where S is the number of net counts within the general characteristic of classical AGNs (Kassebaum et al.
channels11-42 ( 0.11 0.43 keV),andHisthenumberof 1997; Kaspi et al. 1996a; Reichert et al. 1994; Peterson et
∼ −
countsinthechannels51-201( 0.51 2.02 keV).Valuesof al. 1991; Edelson, Krolik & Pike 1990).
∼ −
HRcloseto-1indicatethatthesourcehasanextremelysoft Two and three Gaussians were fitted to Hβ and Hα
spectrum,whilevaluescloseto+1showthatthesourcehas respectively: a narrow component with a fixed (instrumen-
a hard or heavily absorbed spectrum. For NGC4395 HR = tal)FWHMasmeasuredfromthenarrowlines,andone(or
0.37 0.11, where the error was calculated as in Ciliegi et two) free parameter broad components to fit the extended
±
(cid:13)c 0000RAS,MNRAS000,000–000
NGC4395 7
Figure 4.GaussianprofilefittingtoHα(top)andHβ (bottom). Twoandonebroad components werefittedtoeachline,respectively.
The narrow components had a fixed width measured from other narrow lines. The left panel shows the data and individual model
components (the continuum level is not included). The right panel shows the residuals between the data and model. All fluxes are in
unitsof10−15 ergscm−2 s−1 ˚A−1.
wings.Thecontinuumlevelandslopewerealsofreeparam- componentsto fittheextendedwings. The continuumlevel
eters duringthefitting. A Lorentzian profile was also fitted and slope were also a free parameters during the fitting.
tothebroad components,buttheresultsweremuchpoorer Figure 4 shows the fit to Hα and Hβ as individual Gaus-
than when using Gaussian profiles. Figure 4 shows the fit siancomponentsandresiduals.ALorentzianprofilewasalso
to Hα and Hβ as individual Gaussian components and as fitted to the broad components but the results were much
residuals between thedataand model. Errors for thebroad poorer than when using Gaussian profiles. Errors for the
componentswerefoundtobelessthan3percent.Theywere broad line fluxeswere found to be less than 3 percent. The
computedasthesquarerootofthediagonalelementsofthe errors were computed as thesquare root of the diagonal el-
covariancematrixofthenon-linearmodel(ie,theyrepresent ements of the covariance matrix for each parameter of the
68percentconfidenceintervalsforeachparametertakensep- non-linear model (ie, they represent 68 percent confidence
arately). Table 3 gives the fluxes and FWHM obtained for intervalsforeachparametertakenseparately).Table3gives
each line. For Hα the values for both fitted broad compo- the fluxes and FWHM obtained for each line. For Hα the
nentsare shown. valuesfor both fitted broad components are shown.
Thenarrowlinewidthsarenotresolvedinourdata(res- The error on theabsolute line fluxesis probably of the
olution 3.5˚A).Tofindthefluxesgivenintable2thelines order of 30 per cent, but the relative changes in Hα and
∼
were fitted with a single Gaussian (except for the nitrogen Hβ can beobtained to within afew percent bynormalizing
doublet). Gaussian profile fitting to the Balmer lines deter- to nearby narrow line fluxes. At a distance of 5.21 Mpc the
mined the broad component luminosities. Two and three NLRhasalinearsizeof 10pc(fromHST PCnarrowband
∼
Gaussians were fitted to Hβ and Hα respectively: a narrow observations centred on the [OIII]λ5007 line; Filippenko,
componentwithafixed(instrumental)FWHMasmeasured Ho & Sargent 1993). Although fairly modest, this diameter
fromthenarrowlines,andone(ortwo)freeparameterbroad impliesatravelingtimeofmorethan30yearswhichshould
(cid:13)c 0000RAS,MNRAS000,000–000
8 Lira et al.
Figure 5. Narrow line flux ratios between July 1996 and January 1997 as a function of wavelength. A linear fit to the data is shown
as a solid line. Dashed lines correspond to 1 standard deviation from the fit. The empty circles correspond to the ratio of the narrow
linecomponent of Hαand Hβ which were not used inthe fit. The stars correspond to the ratio of the Hα and Hβ broad components.
TheratiobetweenthenuclearcontinuumobservedinJuly1996andinJanuary1997isalsoshownasacurvedlineatthebottomofthe
figure.
ensure that any variations in the central continuum source rowlines, havealso beenincludedin figure5(open circles).
will be smeared out within the NLR and that the narrow Applying the linear correction found using the narrow line
line emission is fairly constant. flux ratios to those for the broad components, we find that
Given the observed size of the nuclear region (see sec- Hα96/Hα97 = 0.81 0.05 and Hβ96/Hβ97 = 0.84 0.05,
B B ± B B ±
tions 2.3 and 3.2) there should be no significant aperture where the error has been assumed to be equal to the typi-
effects to take into account, so we can use the ratios of the cal scatter of the narrow line ratios about the fit. However,
narrowlinefluxesobservedinJuly1996andJanuary1997to we expect the errors to be somewhat larger than this since
assessanyrelativecalibrationdifferences.Theobservednar- it is more difficult to measure the flux in broad lines than
row line ratios indicate that there is a slight variation with in narrow lines. Clearly, we have detected real broad line
wavelength in the relative calibration. By fitting to these variability, but with a considerably smaller factor than the
ratios, and assessing their scatter about the result, we can variations seen in thebluecontinuum.
estimate thesignificance of thebroad line variations. Figure 5 also shows the ratio of the fitted continuum
Figure 5 shows the narrow line ratios taken from table observed in July 1996 and January 1997 (see section 4.1)
2andplottedagainstwavelength.Thestraightlineisanun- versus wavelength. This shows that there was a large vari-
weightedbestfit.Ifthetwonarrowlinespectraareidentical, ation in the blue, but that the variation in the red is only
apart from a linear flux correction, then the difference be- marginally significant.
tweentheindividuallineratiosandthislinereflectthenoise Inspectingthespectrabyeye,itappearsthatthebroad
inthedata.Thestandarddeviationofthescatteraboutthe components of higher order Balmer lines, such as Hγ and
fit is 0.05, shown as dashed lines. Hδ, have varied by a larger factor. However, we have not
ThelineratiosforthebroadcomponentsofHαandHβ attempted to quantify this given the noisiness of the data
are also shown and clearly differ from the observed trend and theproblem of blended lines.
in the narrow lines (filled stars). If the error in the ra- An absorption line, identified as CaIIKλ3933, can be
tio of the broad lines is similar to that inferred from the seen near the blue end of the January 1997 spectrum.
narrow lines then we estimate the significance of the dif- (CaIIHλ3970coincidescloselywithHεsoisnoteasilyseen).
ference to be greater than 3 times the standard deviation. Weseeanadditionaltentativeabsorption lineat 4055˚A,
∼
The ratio of the Hα and Hβ narrow components, which for which we have no identification. These lines, as well as
are consistent with the ratios found from the other nar- someotherweakfeatures,arenotseenintheJuly1996spec-
(cid:13)c 0000RAS,MNRAS000,000–000
NGC4395 9
Figure 6. JKT I and B relative photometry for the NGC4395 nucleus. The count ratios between two field stars (open circles) are
comparedwiththeratiobetweenthenuclearphotometryandoneofthestars(filledcircles).Thestar–starcomparisonsuggestsatypical
errorof∼2%sothevariationinthenuclearphotometry issignificant.
trum due to the poor S/N of the data (note that the spec- cluster plus direct measurement of its velocity dispersion
truminfigure2hasbeenslightlysmoothed).Theequivalent thus its dynamical mass plus an estimate of dilution factor
widthoftheobservedCaIIKlineisjustabove1˚A.Theline in the nearinfrared.
is quite broad, with a FWHM of 10 ˚A ruling out the
∼
possibility of it being caused by interstellar absorption. Its
profileisslightlyasymmetric,whichmayindicatemorethan 3.3 Broad-band variability
onecomponent.However,thequalityofthedatapreventsus
AftercarefulexaminationoftheJKTBandIimages,noev-
from reaching any firm conclusions. There is noevidence of
idencewasfoundfornuclearvariabilitywithinanyofthetwo
otherimportantmetallinessuchasCNλ4200,theGbandof
hourobserving windows each night.Therefore, thedatafor
CHλ4301,MgI+MgHλ5175orNaIDλ5892inthedata.Nei-
eachnightwereaveraged.Tominimiseanycolour-dependent
ther is the 4000 ˚A break observed, suggesting a very young
effectsduetodifferencesintheintrinsicspectralenergydis-
stellar population. Bica (1988) shows that in a sequence of
tribution of the nucleus, galaxy and stars, only data from
stellar population types from spiral galaxies the equivalent
framesfor whichtheairmass wasless than1.5wereusedto
width(EW)oftheCaIIKfeaturedecreasestowardsyounger
construct thenightly averages. The average airmass is very
populations as well as towards lower metallicities. The ab-
similarforeachnightastheobservingperiodwassimilarin
sence of the 4000 ˚A break means that of Bica’s templates
UT.
only groups S6 and S7 can apply. For S7 most of the light
The average B and I band CCD count ratios for two
is thought to come from populations of age 108 years,
∼ stars and for the nucleus and one of the stars are shown in
and still has a EW of CaIIK 3.5 ˚A. Although we cannot
∼ figure6.Thevalueshavebeennormalisedtounityusingthe
separate the effects of age and dilution, it seems likely that
datafrom thelast four nightsof therun.Based on thestar
thepopulation isyoung(conservatively<1Gyr)andhasa
ratios,weconservativelyestimatetheone-sigmauncertainty
dilution of at least 70%.
for flux variability to be 2 per cent. The nucleus is clearly
Our detection of the CaIIK absorption line suggests variable over the first few nights, with the largest change
that other stellar features characteristic of young clusters being a brightening by about 20 percent from night two to
may also be detectable. Among these, the near infrared three. The changes are similar in form and amplitude in
CaIIλ8498,8542,8662 triplet should be the strongest (Ter- both bands. Other small-amplitude variations are possibly
levich et al. 1990). A detection of theIR CaII triplet would present during the second half of the week, although these
provide confirmation of the presence of the nuclear young are not of high significance.
(cid:13)c 0000RAS,MNRAS000,000–000
10 Lira et al.
Absolute photometric magnitudes were derived using and its physical scale is a diameter of 18 pixels = 0.8
∼
several flux standards and field stars all observed at very arcsec = 20 pc, similar to the size of the extended [OIII]
low airmass (< 1.1) so colour differences are insignificant. emission detected byFilippenko, Ho&Sargent (1993) - see
Thisimpliesnuclear(galaxysubtracted)magnitudesofB= also Matthews et al. (1996). Indeedit is quitepossible that
16.8 and I = 15.8 for NGC4395 during the JKT run. We atthe5-10percentfractionofthecorethatweareconsider-
estimate 1-sigma errors of 0.1 magnitudes. This result is inghere,thediffuselightintheF814W filterisentirelydue
±
fairly consistent with the trend shown by our spectroscopy: tonebularemission. Ontheotherhand,thedetectionofan
thecontinuumbecomesharderwhenbrighter.Filippenko& underlying young stellar component (see section 3.2) could
Sargent (1989) quote a B magnitude of 17.3 (0.42 mJy) explain the diffuse emission as contamination by a nuclear
∼
basedonthefluxdensityat4400˚Afromtheirspectroscopic stellar cluster.
data. From our spectroscopy shown in figure 2, the fluxes In the blue (F450W) observation, the data has two
at 4400 ˚A imply B magnitudes of 18.0 and 17.5 for July brightpixelsandanypointsourceisprobablycentredsome-
1996andJanuary1997respectively.Thisimpliesahistorical where between these. The type of crude analysis we used
flux variation by a factor 2, and strongly supports our above is, therefore, even less reliable, and so we defer de-
∼
detection of variability in NGC4395. tailed discussion of this image. Very roughly however, the
data are at least consistent with the same story - an unre-
solved core and diffuse emission at the10 per cent level.
3.4 Analysis of HST images
Photometry in standard bands was derived from the HST
observations using a circular aperture of 1 arcsec diameter.
The background emission was negligible. This gave values 4 DISCUSSION
of B =16.91 and I =16.23, comparable to thevalues from
4.1 Spectral energy distribution
the JKT broad-band imaging, further supporting evidence
of variability. For the determination of the Spectral Energy Distribution
HST imaging has been previously discussed by Filip- (SED)ofNGC4395,ultraviolet HST spectraobtainedwith
penko, Ho & Sargent (1993). However those observations theFaint Object Spectrograph (FOS)in July 1992 werere-
were taken by WFPC1, whereas the data we discuss here trievedfromthepublicarchive.Theoriginaldatawerepub-
waretakenwith WFPC2. Wethereforeexaminetheimages lished byFilippenko, Ho& Sargent (1993).
foranysign ofresolved structure.Model pointspread func- TodeterminethefeaturelessUVandopticalcontinuum
tions (PSFs) for both the F450W and F814W filters at the a low degree polynomial was fitted to the spectra, allowing
relevantchippositionswerecalculatedusingTinyTimsoft- for the rejection of all the emission-line features during the
ware.Standardirafroutineswerethenusedtofindthecen- fitting procedure. The resulting UV continuum will include
troids and radial profiles of both the real and model data. broad quasi-continuumsuch asBalmer continuumand FeII
Comparison ofHST datawith modelPSFsintheverycen- emission-lines. Some starlight contamination is expected as
tralregionsisverysensitivetocentroidlocationwithrespect stellarsignatureshavebeenobservedinthenuclearemission
topixelcentres,aseventhePCissomewhatundersampled. butitisonlyoftheorderof10%ofthetotalfluxat4000˚A
We do not attempt a proper model test here, but rather (see sections 3.2 and 3.4).
havesimply scaled themodel PSFbyeyetomatch thereal For the X-ray data an effective energy for the ROSAT
data at radii of 1-2 pixels, in order to look for evidence of HRIbandpassof0.8keVwasdeducedassumingapowerlaw
extendedstructure. spectrumofindexα=1.5.ForthePSPCthevalueadopted
The F814W data have a clear peak pixel, so a direct was 1.0 keV.
comparisonwithaPSFisrelativelysecure.Thecomparison Figure8showstheobservedSEDforNGC4395.Dashed
with the PSF is shown in figure 7. It can be seen that the lines have been drawn as examples of power law functions
core of the PSF matches the data very well, so we confirm with spectral indices α = 0, 1 and 1.7 (f ν−α). Several
ν
∝
the finding of Filippenko, Ho & Sargent (1993) that most striking features are evident in the SED plot. The change
of the flux from the nucleus of NGC4395 comes from an in the continuum shape between our two spectroscopic ob-
unresolved point source. We have not performed a fit, but servations is dramatic. The featureless optical continuum
an intrinsic FWHM of more than about half a pixel would obtained in July 1996 is verysteep, with abest fittedspec-
have been easily detected. At our adopted distance of 5.21 tral index α 1.7. The extrapolation of this continuum
≈
Mpc this corresponds to a physical size of 0.6 pc. to shorter wavelengths seems to agree with the observed
In the wings of the profile, however, the source has a X-ray luminosities. Compared with more luminous AGN,
clear excess above the PSF. Although this is a very small NGC4395 seems to be a very quiet X-ray source, unless
number of counts, it is a large factor above the predicted the PSPC and the HRI observations were made during ex-
wings, so it is unlikelytobeaccounted for byaslightly dif- tremely low activity states.
ferent normalisation of the PSF without disagreeing badly TheopticalcontinuumlooksflatterandbrighterbyJan-
with the core. In this region the Tiny Tim PSF should be uary 1997 with a best fitted α 1.1, but starts to raise for
very reliable and thespacecraft jitter during these observa- wavelengths shorter than 4000≈˚A. The HST UV data from
tionswasonly0.14pixels.Thereis,then,evidenceofdiffuse 1992 shows a sharp break at 2200˚A. After the break the
∼
emissionsurroundingthecoreofNGC4395.Withoutproper UV spectrum seems to agree with an α = 1 spectral dis-
modelling, which we defer to a later paper, it would be un- tribution. The bump observed at 3000 ˚A nicely matches
∼
wisetoquantifytheexcess,butveryroughlytheintegrated theopticalobservationsof1997.However,duetotheuncer-
diffusefluxcouldbeasmuchas10percentofthecoreflux, tainties in the spectral absolute flux calibration and the 4
(cid:13)c 0000RAS,MNRAS000,000–000