A luminous and isolated gamma-ray flare from the blazar B2 1215+30

0.26 MB·

by  VERITAS Collaboration

Draftversion January5,2017
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
A LUMINOUS AND ISOLATED GAMMA-RAY FLARE FROM THE BLAZAR B2 1215+30
A. U. Abeysekara1, S. Archambault2, A. Archer3, W. Benbow4, R. Bird5, M. Buchovecky6, J. H. Buckley3,
V. Bugaev3, K. Byrum7, M. Cerruti4, X. Chen8,9, L. Ciupik10, W. Cui11,12, H. J. Dickinson13, J. D. Eisch13,
M. Errando3,14, A. Falcone15, Q. Feng11, J. P. Finley11, H. Fleischhack9, L. Fortson16, A. Furniss17,
G. H. Gillanders18, S. Griffin2, J. Grube10, M. Hu¨tten9, N. H˚akansson8, D. Hanna2, J. Holder19,
T. B. Humensky20, C. A. Johnson21, P. Kaaret22, P. Kar1, M. Kertzman23, D. Kieda1, M. Krause9,
7 F. Krennrich13, S. Kumar19, M. J. Lang18, G. Maier9, S. McArthur11, A. McCann2, K. Meagher24, P. Moriarty18,
1 R. Mukherjee14, T. Nguyen24, D. Nieto20, R. A. Ong6, A. N. Otte24, N. Park25, V. Pelassa4, M. Pohl8,9,
0 A. Popkow6, E. Pueschel5, J. Quinn5, K. Ragan2, P. T. Reynolds26, G. T. Richards24, E. Roache4, C. Rulten16,
M. Santander14, G. H. Sembroski11, K. Shahinyan16, D. Staszak25, I. Telezhinsky8,9, J. V. Tucci11, J. Tyler2,
2
S. P. Wakely25, O. M. Weiner20, A. Weinstein13, A. Wilhelm8,9 & D. A. Williams21 (VERITAS Collaboration),

n S. Fegan28, B. Giebels28 & D. Horan28 (Fermi-LAT Collaboration), A. Berdyugin27, J. Kuan20, E. Lindfors27, K.
a Nilsson29, A. Oksanen30, H. Prokoph31, R. Reinthal27, L. Takalo27 & F. Zefi28
J

4
1DepartmentofPhysicsandAstronomy,UniversityofUtah,SaltLakeCity,UT84112, USA
] 2PhysicsDepartment, McGillUniversity,Montreal,QCH3A2T8,Canada
E
3DepartmentofPhysics,WashingtonUniversity,St. Louis,MO63130, USA,[email protected]
H
4FredLawrenceWhippleObservatory, Harvard-SmithsonianCenter forAstrophysics,Amado,AZ85645, USA
.
h 5SchoolofPhysics,UniversityCollegeDublin,Belfield,Dublin4,Ireland
p 6DepartmentofPhysicsandAstronomy,UniversityofCalifornia,LosAngeles,CA90095, USA
-
o 7ArgonneNationalLaboratory,9700S.CassAvenue,Argonne,IL60439, USA
tr 8InstituteofPhysicsandAstronomy,UniversityofPotsdam,14476Potsdam-Golm,Germany
as 9DESY,Platanenallee6,15738Zeuthen, Germany
[ 10AstronomyDepartment, AdlerPlanetariumandAstronomyMuseum,Chicago,IL60605,USA

1 11DepartmentofPhysicsandAstronomy,PurdueUniversity,WestLafayette, IN47907,USA
v 12DepartmentofPhysicsandCenter forAstrophysics,TsinghuaUniversity,Beijing100084, China.
7 13DepartmentofPhysicsandAstronomy,IowaState University,Ames,IA50011,USA
6
14DepartmentofPhysicsandAstronomy,BarnardCollege,ColumbiaUniversity,NY10027,USA,[email protected]
0
1 15DepartmentofAstronomyandAstrophysics,525DaveyLab,PennsylvaniaState University,UniversityPark,PA16802, USA
0 16SchoolofPhysicsandAstronomy,UniversityofMinnesota,Minneapolis,MN55455, USA
.
1 17DepartmentofPhysics,CaliforniaStateUniversity-EastBay,Hayward,CA94542, USA
0 18SchoolofPhysics,NationalUniversityofIrelandGalway,UniversityRoad,Galway,Ireland
7
1 19DepartmentofPhysicsandAstronomyandtheBartolResearchInstitute, UniversityofDelaware,Newark,DE19716, USA
: 20PhysicsDepartment, ColumbiaUniversity,NewYork,NY10027, USA
v
i 21SantaCruzInstitute forParticlePhysicsandDepartmentofPhysics,UniversityofCalifornia,SantaCruz,CA95064, USA
X 22DepartmentofPhysicsandAstronomy,UniversityofIowa,VanAllenHall,IowaCity,IA52242, USA
ar 23DepartmentofPhysicsandAstronomy,DePauwUniversity,Greencastle, IN46135-0037, USA
24SchoolofPhysicsandCenterforRelativisticAstrophysics,GeorgiaInstituteofTechnology,837StateStreetNW,Atlanta,GA30332-0430
25EnricoFermiInstitute, UniversityofChicago,Chicago,IL60637, USA
26DepartmentofPhysicalSciences,CorkInstituteofTechnology, Bishopstown,Cork,Ireland
27TuorlaObservatory,Department ofPhysicsandAstronomy,UniversityofTurku,Finland
28Laboratoire Leprince-Ringuet, Ecole polytechnique, CNRS/IN2P3, Universit´e Paris-Saclay, 91128, Palaiseau, France, [email protected],
zefi@llr.in2p3.fr
29FinnishCentreforAstronomywithESO,UniversityofTurku,Finland
30Nyrolaobservatory, JyvaskylanSiriusry,Finland
31DepartmentofPhysicsandElectricalEngineering,LinnaeusUniversity,35195V¨axj¨o,Sweden
ABSTRACT
B2 1215+30 is a BL Lac-type blazar that was first detected at TeV energies by the MAGIC atmo-
spheric Cherenkov telescopes, and subsequently confirmed by the VERITAS observatory with data
2
collected between 2009 and 2012. In 2014 February 08, VERITAS detected a large-amplitude flare
from B2 1215+30during routine monitoring observations of the blazar 1ES1218+304,located in the
same field of view. The TeV flux reached 2.4 times the Crab Nebula flux with a variability timescale
of<3.6h. MultiwavelengthobservationswithFermi-LAT,Swift,andtheTuorlaobservatoryrevealed
a correlated high GeV flux state and no significant optical counterpart to the flare, with a spectral
energy distribution where the gamma-ray luminosity exceeds the synchrotron luminosity. When in-
terpreted in the frameworkof a one-zoneleptonic model, the observedemission implies a high degree
of beaming, with Doppler factor δ >10, and an electron population with spectral index p<2.3.
Keywords: galaxies: active — galaxies: nuclei — galaxies: jets — BL Lacertae objects: individual
(B2 1215+30= VER J1217+301)— gamma rays: galaxies
1. INTRODUCTION 2. VERITAS OBSERVATIONS
Extreme flux variability is one of the defining proper- VERITAS (Very Energetic Radiation Imaging Tele-
ties of the blazar class of active galactic nuclei, appear- scope Array System) is an array of four imaging at-
ing at all wavelengths over a wide range of timescales. mospheric Cherenkov telescopes located at the Fred
Flares with amplitudes up to hundred times the quies- Lawrence Whipple Observatory in southern Arizona,
centfluxandvariabilitytimescalesasshortas3minutes USA. VERITAS operates by recording Cherenkov light
have been observed at TeV energies (E & 0.1TeV; see, fromparticleshowersinitiatedbygammaraysintheup-
e.g.,Aharonian et al.2007). Todate,sixflaringBLLac- per atmosphere and is sensitive to gamma-ray energies
typeblazarshavebeendetectedtoexceedthefluxofthe from about 85GeV to more than 30TeV (Park 2015).
Crab Nebula (1Crab= (2.1±0.2)×10−10cm−2s−1 at Table 1 summarizes the VERITAS observations and
E > 0.2TeV, Hillas et al. 1998) at TeV energies. The results on B2 1215+30. Observations were made in
large signal statistics obtained during bright flares en- “wobble” pointing mode (Fomin, et al. 1994) consider-
ableflux-variabilitystudiesonminutetimescales,result- ing the presence of another TeV source in the field of
◦
ing in tighter constraintson the size andlocationof the view (1ES 1218+304,offset 0.76 from B2 1215+30) as
gamma-ray emitting region (see, e.g., Begelman et al. described inAliu et al. (2013). Data wereprocessedus-
2008) and probing the particle acceleration and cooling ingstandardVERITASanalysispipelines(Acciari et al.
mechanisms in blazar jets (see, e.g., Bykov et al. 2012). 2009;Archambault et al.2013). Theenergythresholdof
This paper describes a large-amplitude gamma-ray the analysis is 200GeV, with a systematic uncertainty
flare from the blazar B2 1215+30 detected on UT of 20% on the energy estimation.
date 2014 Feb 08, and compares its broadband prop- A TeV flare from B2 1215+30 was detected in 2013
erties to long-term observations of the source with Feb 07 (MJD 56330, Figure 1) with flux F =
>0.2TeV
VERITAS (TeV energies), Fermi-LAT (GeV energies; (5.1±1.0 ±1.0 ) × 10−11cm−2s−1, or 0.24Crab.
stat sys
0.1 . E . 100GeV), and the Tuorla optical ob- Themeasuredgamma-rayspectrumiscompatiblewitha
servatory. B2 1215+30 (R.A. = 12h17m52s, decl. power-law dN/dE =N ·E−Γ withphotonindexΓ=
0
= +30◦07′00′′1, J2000), also known as ON 325 or 3.7±0.7 ±0.4 ,inlinewithΓ=3.6±0.4reportedin
stat(cid:0) sys (cid:1)
1ES 1215+303, was first detected at TeV energies by Aliu et al. (2013) and Γ = 3.0±0.1 from Aleksi´c et al.
MAGIC (Aleksi´c et al. 2012). At GeV energies it is as- (2012). A fit of the decaying phase of the flare (MJD
sociated with 3FGL J1217.8+3007 (Acero et al. 2015). 56330-56639)toafunctionF(t)=F0 1+2−(t−t0)/tvar
Thereissomeuncertaintyinthedistancetothissource, results in an upper limit on the flux halving time of
(cid:0) (cid:1)
with values of z = 0.130 (Akiyama et al. 2003) and t <52h at a 90% confidence level (c.l.).
var
z = 0.237 (Lanzetta et al. 1993) being quoted for its A brighter subsequent flare from B2 1215+30 was
spectroscopic redshift. Based on the location of its observed on 2014 Feb 08 (MJD 56696, Figure 1) with
synchrotron peak, B2 1215+30 has been either classi- flux F = (5.0±0.1 +4.0sys)×10−10cm−2s−1,
>0.2TeV stat−1.0sys
fied as an intermediate (IBL, Nieppola et al. 2006) or or 2.4Crab. The reconstructed energy spectrum is
high-frequency peaked BL Lac (HBL, Ackermann et al. compatible with a power-law with photon index Γ =
2015). 3.1±0.1 ±0.6 between 0.2 and 2TeV (Figure 2).
stat sys
Throughout this paper we assume a Friedmann uni- The observations targeted 1ES 1218+304 and had a
verse with H = 67.7kms−1Mpc−1, Ω = 0.309 and mean zenith distance of 27◦, accumulating 45min of
0 m
Ω = 0.691. All distance-dependent quantities are cal- live-time exposure. On that night, a high-cloud layer
λ
culated assuming a redshift z = 0.130 (d = 630Mpc) at an altitude of 11.2km a.s.l. was measured by an on-
L
for B2 1215+30. Measurement uncertainties are statis- site Vaisala CL51 ceilometer. On average, 30% of the
tical only unless indicated otherwise. Cherenkov light output in particle showers initiated by
A luminous TeV flare from B2 1215+30 3
Table 1. Summary of the VERITAS and Fermi-LAT results from observations of B2 1215+30 in different epochs. The
VERITASupperlimit is computed at 95% c.l. assuming a power-law spectrum with index Γ=3.0.
Instrument Energy range Dates Livetime Significance Flux [cm−2s−1]
VERITAS >0.2TeV 2013 Jan 06 – 2013 May 12 (MJD 56298–56424) 631min 8.8σ (6.0±1.2)×10−12
2013 Feb 07 (MJD 56330) 25min 10.5σ (5.1±1.0)×10−11
2014 Jan 29 – 2014 May 25 (MJD 56686–56802) 748min 23.6σ (2.4±0.2)×10−11
2014 Feb 08 (MJD 56696) 45min 46.5σ (5.0±0.1)×10−10
2014 Feb 09 (MJD 56697) 25min 1.6σ <1.4×10−11
Fermi-LAT 0.1–500GeV 2013 Jan 06 – 2013 May 12 (MJD 56298–56424) 28.8σ (6.8±0.7)×10−8
2014 Jan 01 – 2014 May 25 (MJD 56658–56802) 34.5σ (1.0±0.1)×10−7
2014 Feb 05 – 2014 Feb 09 (MJD 56693–56696) 17.4σ (4.4±0.7)×10−7
200GeV gamma rays is produced above 11.2km (see, ingfromanaveragedΓ =1.92±0.04duringthe2014
GeV
e.g., Rossi & Greisen 1941). This fraction decreases campaigntoΓ =1.70±0.09inthefourdaysofhigh-
GeV
with increasing gamma-ray energy (see, e.g., Weekes estGeVflux(MJD56693-56696). In2013,theLATlight
2003). If all Cherenkov light emitted above the cloud curve shows no significant flux variability (Figure 1).
layeris lost,VERITAS wouldunderestimate the energy However, the same TeV to GeV flare amplitude ratio
of incoming gamma rays by ∼30%, which added to the seen in 2014 can be accomodated within the error bars
20%systematicuncertaintyontheenergyestimationre- of the 2013 LAT light curve.
sultsintheincreasedsystematicerroronthegamma-ray
4. SWIFT OBSERVATIONS
flux and spectral index measured in 2014 Feb 08. The
largesignalstatisticsduringtheflareallowfluxmeasure- An observation by the Swift Observatory (Ob-
ments in 5-minute time bins (Figure 3). No significant sId 00031906012) was carried out one day after the
flux variability was detected during the 45min expo- VERITAS-detected flare (Figure 3) with an expo-
sure,withthelightcurvedeviatingfromaconstantflux sure of 1.97ks. X-ray Telescope (XRT, 0.2 − 10keV,
hypothesis at a level of 2.8 standard deviations. Obser- Burrows et al. 2005) data were obtained in photon-
vations on the next night (2014 Feb 09) did not show counting mode and processed with the xrtpipeline
anelevatedflux fromB21215+30(Table1), implying a tool (HEASOFT 6.16). The exposure shows a stable
90% c.l. limit on the flux halving time of t <3.6h. source-countrateof∼0.3s−1,suggestingnegligiblepile-
var
up effects.
3. FERMI-LAT OBSERVATIONS
The spectrumwasrebinnedtohaveatleast20counts
The Large Area Telescope (LAT) is a pair-conversion per bin, ignoring channels with energy below 0.3keV,
gamma-raytelescopeonboardtheFermisatellitecover- and fit using PyXspec v1.0.4 (Arnaud 1996). An ab-
ing energies from about 20MeV to more than 500GeV sorbed power law with column density N = 1.68 ×
H
(Atwood et al. 2009). Table 1 summarizes the Fermi- 1020cm−2(Kalberla et al.2005)andphotonindexΓ =
X
LAT observations and results on B2 1215+30. Data 2.54±0.07 givesa gooddescriptionofthe spectraldata
were analyzedusing the unbinned likelihood analysis in (P(χ2) = 0.42). The unabsorbed flux is F0.3−10keV =
LAT ScienceTools (v10r0p5) with P8R2 SOURCE V6 in- (1.28±0.05)×10−11ergcm−2s−1.
strument response functions, selecting photons with en- ToanalyzetheSwift-UVOTdata(E ∼6.0eV),source
ergy 100MeV < E < 500GeV in a circular region of counts were extracted from an aperture of 5.0 arcsec
◦
10 radiuscenteredonthepositionofB21215+30. The radius around the source. Background counts were
energy spectrum of B2 1215+30 was modeled with a taken from four neighboring regions with equal ra-
power law. Further analysis details and standard qual- dius. Magnitudeswerecomputedusingthe uvotsource
ity cuts followed Acero et al. (2015). Light curves were tool (HEASOFT v6.16), corrected for extinction ac-
derived by dividing the data in bins of one and three cording to Roming et al. (2009) using E(B −V) from
days duration. Schlafly & Finkbeiner (2011), and converted to fluxes
A clear flux peak is seen coinciding with the following Poole et al. (2008).
VERITAS-detected flare of 2014 Feb 08 (Figure 1), fol-
5. OPTICAL OBSERVATIONS
lowed by a rapid decay that constrains the flux halving
time to t < 8.9h at 90% c.l. (Figure 3). The GeV Optical R-band data were obtained as part
var
spectrum shows some evidence of hardening (2.2σ), go- of the Tuorla blazar monitoring program
4
-11-1-2] s cm [10F>0.2TeV 0246 VERITAS 2013 -11-1-2]scm [10F>0.2TeV24000 VERITAS 2014 12 CCrraabb
-8-1-2] s cm [10F0.1-500GeV124680000000 56F3e0r0mi-LAT56320 56340 56360 56380 56400 56420 -8-1-2]scm [10F0.1-500GeV124680000000 56F66er0mi-L5A6T680 56700 56720 56740 56760 56780 56800
mJy] 6 56T3u0o0rla 56320 56340 56360 56380 56400 56420 mJy] 6 56T66u0orla56680 56700 56720 56740 56760 56780 56800
ux [ ux [
nd fl 4 nd fl 4
a a
b b
R- R-
2 56300 56320 56340 56360 56380 56400 56420 2 56660 56680 56700 56720 56740 56760 56780 56800
time [MJD] time [MJD]
Figure 1. TeV(top),GeV(middle),andoptical(bottom)lightcurvesofB21215+30in2013(leftpanel)and2014(rightpanel).
Fluxes are calculated in 1-day bins for VERITAS. Fermi-LAT fluxes are calculated with 3-day integration bins (blue crosses)
and1-daybins(orangecrosses)aroundthetimeofthe2014flare. Down-pointingtrianglesindicate95%c.l. upperlimitsderived
fromtheFermi-LATdatafortimebinswithsignalsmallerthan2σ. Theyearly-averagedTeVfluxin2011(8.0×10−12cm−2s−1,
Aliu et al. 2013)is shownbyared-dashedline,andablue-dashedlineindicatestheaverageGeVfluxfrom Acero et al.(2015).
Statistical errors on theTuorla optical fluxesare smaller than thedata points.
(http://users.utu.fi/kani/1m, Takalo et al. 2008). fluxhalvingtimesof∼52and∼3.6hours,respectively.
Observations were taken using a 35cm Celestron tele- Suchlarge-amplitude,short-lived,isolatedflaresarenot
scope attached to the KVA 60cm telescope (La Palma, commoninTeV-emittingblazars. Fastvariabilityistyp-
Canary Islands, Spain) and the 50cm Searchlight Ob- ically measured during longer high-flux states in HBLs
servatory Network telescope (San Pedro de Atacama, (see, e.g., Krawczynskiet al. 2004; Albert et al. 2007),
Chile). Data were analyzed using a semi-automatic while somequasarsandIBLsshowshortperiodsofTeV
pipeline developed at the Tuorla Observatory. The emission in epochs where multiple GeV flares are seen
host galaxy flux of 1.0 mJy (Nilsson et al. 2007) was (Aleksi´c et al. 2011; Arlen et al. 2013).
subtracted from the observed fluxes, and a correction Inthefollowingwesummarizethemainobservational
for Galactic extinction was applied using values from propertiesofthebrightestflareof2014Feb08andinter-
Schlafly & Finkbeiner (2011). The yearly-averaged pretthemintheframeworkofanhomogeneousone-zone
optical flux of (3.27±0.01)mJy in year 2013 is similar leptonic emission scenario:
to historical values dating back to 2003.1 In 2014, (i)Themeasuredfluxabove0.2TeVwas(5.0±0.1)×
B2 1215+30 appeared to be in a long-lasting high 10−10cm−2s−1. This corresponds to an isotropic lumi-
optical state, with average flux of (5.56±0.02)mJy. nosity L =1.7×1046 ergs−1. Todate, only four other
γ
No significant enhancement of the optical emission was blazarshaveepisodicallybeen observedto emit TeVra-
detected in coincidence with the two gamma-ray flares diationwithluminosityexceeding1046ergs−1. Forcom-
reported in Sections 2 and 3. parison, the historical TeV blazar Mrk 421 would have
to exhibit a 35Crab flare to reach the luminosity of the
6. DISCUSSION B2 1215+30 outburst reported here.
With the data presented here and in Aliu et al. (ii) A non-detection by VERITAS 24h after the flare
(2013), VERITAS has published TeV observations of indicatesafluxhalvingtimetvar <3.6hatTeVenergies.
B2 1215+30 spanning over 50 nights between 2008 Causalityimpliesthattheobservedvariabilitytimescale
and 2014, finding no significant deviations from yearly- is related to the size (R) and Doppler factor (δ) of the
averaged fluxes other than the flares on 2013 Feb 07 gamma-ray emitting region by
and2014Feb08 reportedin this paper. These two TeV Rδ−1 ≤ct /(1+z)=3.4×1014cm, (1)
flares had amplitudes of ∼ 6 and ∼ 60 times the av- var
erage quiescent flux from B2 1215+30, with associated (iii) The TeV flare was accompanied by a significant
GeV flare measured by Fermi-LAT that extended over
fourdaysanddisplayedsomeevidenceforspectralhard-
1 http://users.utu.fi/kani/1m/ON 325 jy.html ening, with Γ =1.70±0.09.
GeV
A luminous TeV flare from B2 1215+30 5
(iv)Opticalobservationsdidnotshowenhancedemis-
10-9
sion in coincidence with the GeV and TeV flare, al- SSC
SSC+EC
though the overall optical flux in 2014 was approxi- MJD 56697
LAT MJD 56693-6
mately two times brighter than in previous years. 1]− 10-10 VERITAS MJD 56696
(v) Non-detections by Swift-BAT2 (15-50keV) and 2ms−
c
M56A69X6I)3c(a4n-1b0ekeiVnt)eropnrettheeddaasyaoflimthiet ToenVthflearhea(rMdJXD- [Fergν10-11
ray flux of the order of ν F . 2×10−10ergcm−2s−1 ux, ν
x νx Fl
(Krimm et al. 2013; Hiroi et al. 2013). This effectively 10-12
limits the peak synchrotron luminosity to
Lsyn ≤1046erg s−1. (2) 10 15 20 25
Frequency, log10(ν/Hz)
(vi) No change in the 15GHz radio brightness of
B21215+30wasseeninthe OVROlightcurvesincoin- Figure 2. Broadband SED of B2 1215+30 at different
cidenceoraftertheTeVflare.4 B21215+30isinfactin epochs. Red markers show the state of the source during
the lower third of the OVRO sample in terms of radio the2014 Feb08flare, includingVERITAS(MJD 56696.52),
Fermi-LAT (MJD 56693-56696), Swift-BAT (MJD 56696),
flux variability (Richards et al. 2014).
and Tuorla(MJD56696.72) data. BluemarkersshowSwift-
(vii)Swift-XRT datataken24hafterthe flareshowed XRTandUVOTfluxesandVERITAS95%c.l. upperlimits
an X-rayflux comparable with historicalaveragevalues taken 24h after the flare. Gray markers show archival ob-
servations from Aliu et al. (2013). The numerical SSC and
(Aleksi´c et al.2012;Aliu et al.2013),althoughtheTeV
SSC+EC models described in Section 6 are shown with a
flux was back to a quiescent level at that point. solid and a dashed gray line, respectively. Gamma-ray ab-
A lower limit on δ can be derived by estimating the sorption by the extragalactic background light is applied to
the models following Finkeet al. (2010).
required Doppler boosting for gamma rays with en-
ergy E to escape pair production on a co-spatial syn-
γ
chrotron photon field with density F (E ), where E =
0 0
m c2 2(1+z)−2δ2E−1. ForphotonswithE ∼1TeV
e γ γ (see, e.g., Giebels et al. 2007). Taking the radio spec-
measuredbyVERITASthe meaninteractionenergyfor
(cid:0) (cid:1) trumfromAnto´n et al.(2004)andtheR-bandfluxfrom
pair production is E =76eV. Using the expression for
0 theTuorlaobservatorywederivearadio-to-opticalspec-
opticaldepth from Dondi & Ghisellini (1995), imposing
tral index α = 0.45. If the cooling break5 in the syn-
τ ≤1,andestimatingF (E )fromtheSwift-XRTand ro
γγ 0 chrotronSEDhappensbeyondopticalfrequencies,asas-
UVOT measurements described in Section 4 results in
sumedinAleksi´c et al.(2012)andAliu et al.(2013)and
1/(4+2α)
σ d2 (1+z)2αF (E ) typically observed in BL Lac objects (Tavecchio et al.
δ≥ T L 0 ,
" 5hc2 tvar # 2010), αro determines the power-law spectral index (p)
of the emitting electrons(see,e.g., Rybicki & Lightman
δ≥10.0, (3)
1979):
whereσTistheThomsoncrosssectionandαisthespec- p=1+2αro ≈1.9. (4)
tral index of the synchrotron emission around E . We
0
Beyond the cooling break, the electron distribution has
notethattheSwiftobservationsweremade24hafterthe
to extend to Lorentz factors (γ) of the order
TeV flare (Figure 3). The lower limit on δ is still valid,
however, as long as the density of synchrotron photons γ ≈(1+z)δ−11TeV >2.2×105(δ/10)−1 (5)
was not lower during the flare than that measured on max m c2
e
the subsequent day.
to produce the ∼ 1TeV photons detected by VER-
Thespectralenergydistribution(SED)ofB21215+30
ITAS. In the simplest leptonic emission scenario,
during the flare is shownin Figure 2. TeV emission can
the high-energy component of the SED is produced
beexplainedbyafreshinjectionofrelativisticelectrons,
via the synchrotron self-Compton mechanism (SSC;
wherethe injected perturbationpropagatesdowninen-
Maraschi et al. 1992). In an SSC scenario,the ratio be-
ergy as the plasma cools, explaining the smaller ampli-
tween the synchrotron and inverse-Compton luminosi-
tude of the GeV flare and the lack of optical variability
ties can be used to estimate the magnetic field. Follow-
ing Ghisellini et al. (1996) and using equations (2) and
2 http://swift.gsfc.nasa.gov/results/transients/weak/QSOB1215p303/
3 http://maxi.riken.jp/mxondem/
5 corresponding to emitting electron energies at which the ra-
4 http://www.astro.caltech.edu/ovroblazars/data/data.php?page=ddaiattai_vreectouorlnin&gsoaunrdcee=scJa1p21e7t+im30e0s7calesareequal.
6
(3) to constrain L and δ, we derive
syn
60
B≃(1+z) δ−3 2L2syn 1/2, -1-2]scm 40 VERITA6800S Swift-XRT
Lγc3tvar! -110 40
1
61.8G Lsyn/1046ergs−1 (δ/10)−3. (6) [2TeV 20 20010 20 30 40 50 60
(cid:0) (cid:1) >0. time - MJD 56696.5 [min]
The scarcity of multiwavelength coverage simultane- F 0
ouswiththeTeVflare,speciallyofthesynchrotroncom- 100 5F6e6r9m2i-L5A6T693 56694 56695 56696 56697 56698
ponent, leaves numerical modeling of the SED under- -1]s
constrained. However, even if modeling solutions are -2m 80
c
not unique, they can be used to understand the level -80 60
1
of kinetic and magnetic jet power requiredunder differ- [V 40
Ge
ebnytusscinengatrhioess.taWtieontaesrtytlehpetofenaiscibmiloitdyelooffaBS¨oSttCchsecrenetarailo. 0.1-500 20
(2013),fixingthejetviewingangletoδ−1 forsimplicity. F 0
Models6 within the parameter constraints from equa- y] 5T6u6o9r2la56693 56694 56695 56696 56697 56698
J
tions (1–6) reproduce the measured gamma-ray lumi- m 6
nosity without overproducingthe optical flux measured x [
u
by the Tuorla observatory, and keeping Lsyn . Lγ as nd fl 4
constrainedby the Swift-BAT non-detection(Figure 2). a
b
Thesesolutionswouldindicateanemittingregionwhere R-
the kinetic power of relativistic electrons (Le) exceeds 2 56692 56693 56694 56695 56696 56697 56698
thepowercarriedbythemagneticfield(L )byafactor time [MJD]
B
of ∼ 1200. This is typically the case in SSC modelling
Figure 3. Same as Figure 1 around the night of 2014 Feb
of TeV blazars (see, e.g., Aliu et al. 2013). Higher val- 08(MJD56696). ThetoppanelinsertshowstheTeVfluxon
ues of δ would imply even higher L /L ratios. Given MJD 56696 in 5-minute bins. A fit of the 5-minute binned
e B
TeV light curve to a constant flux (gray-dashed line) yields
the observational uncertainty in the shape of the syn- P(χ2)=4.2×10−3. Averticalblue-dashedlineindicatesthe
chrotronemission,wealsoexploreawiderrangeofelec- time of theSwift-XRTobservation described in Section 4.
tronspectralindicesthanindicatedinequation(4),find-
ing that p<2.3 is required to reproduce the hard GeV
spectrum measured by Fermi-LAT. where the accretion disk luminosity (L ) is assumed to
d
The lack of observable thermal emission from be 4×1043erg s−1 (Ghisellini et al. 2010) and ζ(r )
diss
the accretion disk and associated emission lines in describes the composition of the external radiation
B2 1215+30 supports an SSC emission scenario. How- fields. Equations (7) and (8) constrain the (δ, r ) pa-
diss
ever, the observed Compton dominance (Lγ/Lsyn & rameter space with a marginal solution at δ > 19 and
1) typically points to external Compton models (EC; r >1.2×1017cm thatwouldplace the emitting blob
diss
Dermer & Schlickeiser 1993) to explain the high-energy beyond the broad-line region. A numerical EC model7
emission. Assuming an EC scenario, constraints on δ (B¨ottcher et al. 2013) with an external photon field de-
and the distance of the energy dissipation region from scribedasblackbodyemissionwithT =103Ktypical
ext
theblackhole(rdiss)canbederivedassumingreasonable of hot dust can reproduce the SED with Le/LB ∼ 1
limits on the jet collimation and luminosity of upscat- (Figure 2).
tered synchrotron photons. Following Nalewajko et al. Particle acceleration in relativistic shocks or
(2014) results in: through magnetic reconnection can explain the
short flux-variability timescale observed in B2 1215+30
1/2
(1+z) r (Sironi & Spitkovsky 2009; Giannios 2013). The
diss
δ(r )< , (7)
diss ct hard electron spectrum (p . 2.3) derived from the
(cid:20) var (cid:21)
1/8 1/4 multiwavelength SED is usually obtained in semi-
9 L (1+z)r
γ diss
δ(r )> , (8) analytical calculations of relativistic shock acceleration
diss
2ζ(r )L 2ct
(cid:20) diss d(cid:21) (cid:20) var (cid:21)
6 E.g., Le = 1.05×1045ergs−1, qe = 1.9, δ = γmin = 40, 7 Le =5×1043ergs−1, qe =1.9, δ=γmin =40, γmax =105,
γmax = 105, B = 0.03G, R = 1.3×1016cm, ηesc = 1, see B=0.3G,R=1016cm,uext=2×10−6ergcm−3,Text=103K,
B¨ottcher etal. (2013) for parameter definitions not included in ηesc =1, see B¨ottcher etal. (2013) for parameter definitions not
thetext. includedinthetext.
A luminous TeV flare from B2 1215+30 7
(Achterberg et al. 2001), but more recent fully ki- atE´colePolytechniqueandColumbiaUniversity. VER-
netic particle-in-cell simulations derive significantly ITAS research is supported by grants from the U.S.
softer spectra (Sironi & Spitkovsky 2009). Magnetic Department of Energy Office of Science, the U.S. Na-
reconnection events can produce harder electron spec- tional Science Foundation and the Smithsonian Institu-
tra than those originating from shock acceleration tion, and by NSERC in Canada. We acknowledge the
(Sironi & Spitkovsky 2014), easily reproducing p ∼ 1.9 excellent workof the technical supportstaff at the Fred
derived from the synchrotronspectrum of B2 1215+30. Lawrence Whipple Observatory and at the collaborat-
Recently, Sironi et al. (2015) have suggested that ing institutions inthe constructionandoperationofthe
magnetic reconnection is a more viable scenario for instrument. The VERITAS Collaborationis grateful to
particle acceleration in relativistic jets, disfavoring TrevorWeekes for his seminalcontributions andleader-
shock models for their inability to simultaneously shipin the fieldofVHE gamma-rayastrophysics,which
dissipate energy and accelerate particles beyond ther- madethisstudypossible. TheFermi-LATCollaboration
mal energies. Efficient magnetic reconnection requires acknowledges generous ongoing support from a num-
an emitting region in rough equipartition between ber of agencies and institutes that have supported both
particles and magnetic field (L /L .1). The EC the development and the operation of the LAT as well
e B
scenario presented above does fulfill this condition, as scientific data analysis. These include the National
while our attempts to describe the observed SED with Aeronautics andSpace Administration and the Depart-
SSC models persistently resulted in particle-dominated ment of Energy in the United States, the Commis-
emitting regionswhere the magnetizationof the plasma sariat`a l’EnergieAtomique and the Centre National de
would be too low for efficient magnetic reconnection to la RechercheScientifique/Institut Nationalde Physique
take place. Nucl´eaire et de Physique des Particules in France, the
VERITAS will continue to monitor B2 1215+30. Agenzia Spaziale Italiana and the Istituto Nazionale di
Events like the extreme flare of 2014 Feb 08 should FisicaNucleareinItaly,theMinistryofEducation,Cul-
be within the sensitivity reach of HAWC (Lauer et al. ture,Sports,ScienceandTechnology(MEXT),theHigh
2015). Futureobservationswillshowhowfrequentthese Energy Accelerator Research Organization (KEK) and
extreme gamma-ray flares are and whether or not they JapanAerospaceExplorationAgency(JAXA)inJapan,
are present in the majority of TeV blazars. and the K.A. Wallenberg Foundation, the Swedish Re-
search Council, and the Swedish National Space Board
in Sweden. Additional support for science analysis dur-
The authors thank Markus B¨ottcher for valuable dis-
ingtheoperationsphaseisgratefullyacknowledgedfrom
cussions about leptonic emission models, and David
the Istituto Nazionale di Astrofisica in Italy and the
Sanchez for providing useful comments on the draft.
Centre National d’E`tudes Spatiales in France.
RM acknowledges support from the Alliance Program
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