VERITAS Observations of day-scale flaring of M87 in 2010 April
E. Aliu, T. Arlen, T. Aune, M. Beilicke, W. Benbow, A. Bouvier, S. M. Bradbury, J. H. Buckley, V. Bugaev, K. Byrum, A. Cannon, A. Cesarini, L. Ciupik, E. Collins-Hughes, M. P. Connolly, W. Cui, R. Dickherber, C. Duke, M. Errando, A. Falcone, J. P. Finley, G. Finnegan, L. Fortson, A. Furniss, N. Galante, D. Gall, S. Godambe, S. Griffin, J. Grube, R. Guenette, G. Gyuk, D. Hanna, J. Holder, H. Huan, G. Hughes, C. M. Hui, T. B. Humensky, A. Imran, P. Kaaret, N. Karlsson, M. Kertzman, D. Kieda, H. Krawczynski, F. Krennrich, M. J. Lang, S. LeBohec, A. S Madhavan, G. Maier, P. Majumdar, S. McArthur, A. McCann, P. Moriarty, R. Mukherjee, P. D Nunez, R. A. Ong, M. Orr, A. N. Otte, N. Park, J. S. Perkins, A. Pichel, M. Pohl, H. Prokoph, J. Quinn, K. Ragan, L. C. Reyes, P. T. Reynolds, E. Roache, H. J. Rose, J. Ruppel, D. B. Saxon, M. Schroedter, G. H. Sembroski, G. D. Şentürk, C. Skole, D. Staszak, G. Tešić, M. Theiling, S. Thibadeau, K. Tsurusaki, J. Tyler, A. Varlotta, V. V. Vassiliev, S. Vincent, M. Vivier, S. P. Wakely, J. E. Ward, T. C. Weekes, A. Weinstein, T. Weisgarber, D. A. Williams, B. Zitzer
aa r X i v : . [ a s t r o - ph . C O ] D ec VERITAS OBSERVATIONS OF DAY-SCALE FLARING OF M 87 IN 2010APRIL
E. Aliu , T. Arlen , T. Aune , M. Beilicke , W. Benbow , A. Bouvier , S. M. Bradbury ,J. H. Buckley , V. Bugaev , K. Byrum , A. Cannon , A. Cesarini , L. Ciupik ,E. Collins-Hughes , M. P. Connolly , W. Cui , R. Dickherber , C. Duke , M. Errando ,A. Falcone , J. P. Finley , G. Finnegan , L. Fortson , A. Furniss , N. Galante , D. Gall ,S. Godambe , S. Griffin , J. Grube , R. Guenette , G. Gyuk , D. Hanna , J. Holder ,H. Huan , G. Hughes , C. M. Hui , T. B. Humensky , A. Imran , P. Kaaret , N. Karlsson ,M. Kertzman , D. Kieda , H. Krawczynski , F. Krennrich , M. J. Lang , S. LeBohec ,A. S Madhavan , G. Maier , P. Majumdar , S. McArthur , A. McCann , P. Moriarty ,R. Mukherjee , P. D Nu˜nez , R. A. Ong , M. Orr , A. N. Otte , N. Park , J. S. Perkins , ,A. Pichel , M. Pohl , , H. Prokoph , J. Quinn , K. Ragan , L. C. Reyes , P. T. Reynolds ,E. Roache , H. J. Rose , J. Ruppel , , D. B. Saxon , M. Schroedter , G. H. Sembroski ,G. D. S¸ent¨urk , C. Skole , D. Staszak , G. Teˇsi´c , M. Theiling , S. Thibadeau ,K. Tsurusaki , J. Tyler , A. Varlotta , V. V. Vassiliev , S. Vincent , M. Vivier ,S. P. Wakely , J. E. Ward , T. C. Weekes , A. Weinstein , T. Weisgarber , D. A. Williams ,B. Zitzer Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA95064, USA Department of Physics, Washington University, St. Louis, MO 63130, USA Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA School of Physics, University College Dublin, Belfield, Dublin 4, Ireland School of Physics, National University of Ireland Galway, University Road, Galway, Ireland Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA Department of Physics, Purdue University, West Lafayette, IN 47907, USA Department of Physics, Grinnell College, Grinnell, IA 50112-1690, USA Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA16802, USA Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA;[email protected] School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA Physics Department, McGill University, Montreal, QC H3A 2T8, Canada Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE19716, USA Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA DESY, Platanenallee 6, 15738 Zeuthen, Germany Physics Department, Columbia University, New York, NY 10027, USA Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland School of Physics & Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW,Atlanta, GA 30332-0430 CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA. University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA. Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67-Sucursal 28, (C1428ZAA) Ciudad Autnomade Buenos Aires, Argentina Institut f¨ur Physik und Astronomie, Universit¨at Potsdam, 14476 Potsdam-Golm,Germany
ABSTRACT
VERITAS has been monitoring the very-high-energy (VHE; >
100 GeV) gamma-ray activity of the radio galaxy M 87 since 2007. During 2008, flaring activity on atimescale of a few days was observed with a peak flux of (0 . ± . × − cm − s − at energies above 350 GeV. In 2010 April, VERITAS detected a flare from M 87 withpeak flux of (2 . ± . × − cm − s − for E >
350 GeV. The source was observedfor six consecutive nights during the flare, resulting in a total of 21 hr of good qualitydata. The most rapid flux variation occurred on the trailing edge of the flare with anexponential flux decay time of 0 . +0 . − . days. The shortest detected exponential risetime is three times as long, at 2 . +1 . − . days. The quality of the data sample is suchthat spectral analysis can be performed for three periods: rising flux, peak flux, andfalling flux. The spectra obtained are consistent with power-law forms. The spectralindex at the peak of the flare is equal to 2 . ± .
07. There is some indication thatthe spectrum is softer in the falling phase of the flare than the peak phase, with aconfidence level corresponding to 3.6 standard deviations. We discuss the implicationsof these results for the acceleration and cooling rates of VHE electrons in M 87 and theconstraints they provide on the physical size of the emitting region.
Subject headings: galaxies: individual (M87, VER J1230+123) - gamma rays: galaxies
1. Introduction
M 87 is a giant radio galaxy located in the Virgo cluster at a distance of 16.7 Mpc (Mei et al.2007). It is believed to harbor a supermassive black hole of mass (3 . ± . × M ⊙ (Macchetto et al.1997), derived from gas kinematics, or (6 . ± . × M ⊙ (Gebhardt et al. 2011), derived fromstellar kinematics. Its jet is misaligned with the line of sight; this, along with the proximity ofM 87, allows for detailed observations of its structure in the radio (e.g., Cheung et al. 2007), optical(e.g., Biretta et al. 1999), and X-ray (e.g., Marshall et al. 2002; Wilson & Yang 2002) wavebands.Apparent superluminal motion is observed in the radio and optical wavebands (Biretta et al. 1995,1999). Month-scale flaring activity has been observed in various energy ranges at the nucleus andat HST-1, the jet feature closest to the nucleus (Perlman et al. 2003; Harris et al. 2009). The jetknot HST-1 is located 0.85 arcsec ( ≈
69 pc projected) from the nucleus and is resolved from thenucleus in the radio, optical, and X-ray energy bands.Very-high-energy (VHE) gamma-ray emission from M 87 was first detected by HEGRA in1998/1999 at energies above 730 GeV (Aharonian et al. 2003) and has since been confirmed by Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bishopstown, Cork, Ireland
Chandra reported historically maximalflaring from HST-1 (Harris et al. 2006). Through the causality argument, the timescale of theenhanced TeV emission implies an emission region size of about R ≤ × δ cm, where δ isthe Doppler factor of the radiating region. Aharonian et al. (2006) preferred the nucleus overHST-1 as the VHE gamma-ray production region due to an unrealistically small opening angle( ∼ . × − δ deg) required to channel energy from the central object to the HST-1 knot. However,the Very Long Baseline Array (VLBA) imaged compact knots in HST-1 that are not resolvedwith semi-minor axes ≤ × cm (Cheung et al. 2007), and Stawarz et al. (2006) proposed jetreconfinement at the HST-1 location which can in turn produce TeV emission. Therefore, HST-1remains a candidate for TeV emission. However, a VHE gamma-ray flare in 2008 coincided withthe historical maximal X-ray flux from the nucleus detected by Chandra , while HST-1 remained ina low state at that time and its X-ray flux was below that of the nucleus. In addition, increasingradio flux from the nucleus, but not from the jet, was observed by the VLBA, lasting up to twomonths past the VHE gamma-ray flare (Acciari et al. 2009). The 2008 observations therefore favorthe nucleus as the origin of the VHE gamma-ray emission.After the launch of the
Fermi Gamma-ray Space Telescope in the summer of 2008, M 87 wasalso detected in the MeV–GeV energy range by the
Fermi
Large Area Telescope (LAT; Abdo et al.2009). However, no significant flaring activity was detected in 2009 at any wavelength.M 87 has been monitored every year in VHE gamma rays since 2003 by at least one of the threemajor atmospheric-Cherenkov telescope arrays — H.E.S.S., MAGIC, and VERITAS. In 2010, VHEflaring activity up to 20% of the Crab Nebula flux was detected from M 87 in the span of several days(Ong & Mariotti 2010), and gamma-ray, X-ray, optical, and radio observations were subsequentlytriggered. Detailed results from the VERITAS observations are presented in this paper, and themultiwavelength light curve will be presented in a separate publication (Abramowski et al. 2011). 5 –
2. Observations and Analysis
VERITAS is an array of four 12 m diameter imaging atmospheric Cherenkov telescopes locatedat the Fred Lawrence Whipple Observatory in southern Arizona, 1.3 km above sea level. Thetelescopes are situated approximately 100 m apart, forming a convex quadrilateral. Each telescopeis equipped with a camera of 499 photomultiplier tubes (PMTs) arranged in a hexagonal latticecovering a field of view with a diameter of 3 . ◦ . The array is sensitive to photons with energyfrom ∼
150 GeV to more than 30 TeV, with an angular resolution of ∼ . ◦ and an effective area of ∼ m at 1 TeV. Further description of the VERITAS observatory and its performance are givenin Perkins et al. (2009) and Holder et al. (2006).M 87 was observed between 2009 December and 2010 May for 53.1 hr. Observations wereconducted at a range of zenith angles between 19 ◦ and 40 ◦ , with low elevation excursions (up to60 ◦ from zenith) during the nights of April 9 through 11 when episodes of flaring were detected.More than 95% of the data were taken with the full four-telescope array and the remainder witha three-telescope sub-array. To enable simultaneous estimation of source and background signals,the data were accumulated in “wobble mode” for which the source is offset from the camera centerby 0 . ◦ in alternating directions every 20 minutes. The analysis presented in this paper is basedon 44.6 hr of live time which satisfied data quality and integrity selection criteria.The data are analyzed with the algorithm described in Acciari et al. (2010). Atmosphericgamma-ray shower images are first corrected for dispersion in PMT gain and timing using infor-mation obtained from nightly laser calibrations (Hanna 2008). Then, an image-cleaning processis applied to select pixels with a signal significantly above the night-sky background level. Aftercleaning, the images are parameterized (Hillas 1985) and the shower direction is reconstructed usingthe stereoscopic technique (Hofmann et al. 1999). Events are then selected as gamma-ray-like if atleast three camera images pass selection criteria optimized for a source with 1% of the Crab Nebulaflux. The results reported in this paper have all been confirmed by an independent secondaryanalysis package described in Daniel (2008).
3. Results
During the six-month observation period, M 87 was detected at a level of 25.6 standard de-viation ( σ ) above the background, with an average flux of (5 . ± . × − photon cm − s − at energies above 350 GeV, equivalent to 5% of the Crab Nebula flux above 350 GeV. The follow-ing sub-sections first present the daily light curve obtained over six months of observation, thenthe April flaring episode light curve binned in 20 minute intervals, followed by the timescale andspectral analyses of the April flare. 6 – Figure 1 shows the daily flux recorded by VERITAS between 2009 December and 2010 May.Applying a constant-flux fit to the daily light curve gives a χ /dof value of 269.4/29, a strongindication the flux was not constant during the observation period.In 2010 February, the MAGIC Collaboration reported an increased activity of M 87 with morethan 10% of the Crab Nebula flux on February 9 (Mariotti 2010). At that time, VERITAS ob-servations were hampered by poor weather conditions, but M 87 was detected by VERITAS ina typical state two nights after the MAGIC alert. In 2010 April, VERITAS detected M 87 withan elevated flux during a week of observations between April 5 and 11 and triggered subsequentmultiwavelength observations (Ong & Mariotti 2010). Figure 2 shows the light curve binned in 20 minute intervals during the flare. Observationsaround the peak of the flare were carried out up to high zenith angles; this reduces the sensitivityat low energies, and as a result, the data points taken at the end of April 10 and the beginning ofApril 11 have larger uncertainties. The flaring episode began with increasing flux during the nightsof April 5 and 6, reaching 10% of the Crab Nebula flux on April 8. On the following three nights,M 87 was observed for more than five hours each night. The average flux on April 9 and 10 was15% of the Crab Nebula flux, reaching as much as 20% of the Crab Nebula flux in individual 20minute bins. The average flux on April 11th was 5% of the Crab Nebula flux. VERITAS continuedto monitor M 87 for two hours each night from April 12 to 15, when the flux level returned to afew percent of the Crab Nebula flux, comparable to the low-state flux measured in the past. Allflux comparison with the Crab Nebula is at energies above 350 GeV.
Using the data from April 9 and 10 (MJD 55295 and 55296) when maximal activity occurred,we searched for variability within each night. On April 9, fifteen 20 minute exposures were takenin total and a constant-flux fit yields a χ /dof value of 9.3/14 and a corresponding χ probabilityof 0.81. On April 10th, twenty-one 20-minute exposures were taken and the constant-flux fit givesa χ /dof value of 19.8/20 and a corresponding χ probability of 0.47. In order to investigatevariability within a single day of observation in more detail, the wavelet analysis described byPrice et al. (2011) is applied to the April 9 and 10 data sets. The highest confidence level for theApril 9 data set is obtained for a variability timescale of 80 minutes. However, the confidence levelis only 86.2%, implying that on an 80 minute timescale, the evidence for variability is only at thelevel of 1 . σ . The highest confidence level for the April 10 data set is obtained for a variability 7 – Date ( MJD )55180 55200 55220 55240 55260 55280 55300 55320 55340 ) - s - G e V ( ph c m I n t e g r a l F l u x > -4-20246810121416 -12 ×
10% Crab5% Crab MAGIC triggerAtel
Fig. 1.— Daily light curve of M 87 observed by VERITAS in 2010. Clear evidence of flaring activityis seen in 2010 April (MJD 55291–55298). Trigger alerts sent by MAGIC on February 10 (MJD55237) and by VERITAS and MAGIC on April 9 (MJD 55295) are indicated by vertical lines.The average nightly flux during the peak of the flare exceeds 10% of the Crab Nebula flux at thesame energy threshold of 350 GeV. A constant spectral index of 2.5 was assumed for the daily fluxcalculation. 8 –
Date ( MJD )55291.2 55291.4 55291.6 55291.8 55292 55292.2 55292.4 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 ×
20% Crab10% Crab5% Crab 2010 Apr 5, 6
Date ( MJD )55294.1 55294.2 55294.3 55294.4 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 × Date ( MJD )55295.1 55295.2 55295.3 55295.4 55295.5 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 × Date ( MJD )55296.1 55296.2 55296.3 55296.4 55296.5 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 × Date ( MJD )55297.1 55297.2 55297.3 55297.4 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 × Date ( MJD )55298.1 55298.2 55298.3 55298.4 ) - s - G e V ( ph c m I n t e g r a l F l u x > -505101520253035 -12 ×
20% Crab10% Crab5% Crab2010 Apr 12
Fig. 2.— VERITAS light curve with 20 minute binning during the flare period between 2010 April5 and April 12 (MJD 55291–55298). The flux scale is the same for all six panels, and dashed linesindicating 5%, 10%, and 20% of the Crab Nebula flux are included. A constant spectral index of2.5 was assumed for the flux calculation. 9 –timescale of 160 minutes at 97.5%, or 2 . σ . Therefore, no evidence for intra-night variability isfound.Figure 3 shows the daily light curve of the April flaring episode. To characterize the timescalesof the flare, an exponential function of the form Φ = p e ( t − /p is fitted to different periodsof the April daily light curve by χ minimization. The parameter p represents the characteristictime of the flux variation. For the days leading up to the flare (MJD 55291–55295, April 5–9), theminimal χ /dof value of 0.3/2 is obtained for p = 2 .
87 days. The error bars of the fit parameters p and p are determined by finding the parameter ranges with χ between χ and χ + 2 . χ is the smallest χ value. The same χ calculation is repeated for data from the peak fluxonward. The details are presented in Table 1. For the period between MJD 55296 and 55304 (April10–18), the exponential decay time is 1 . +0 . − . days. An even shorter decay time of 0 . +0 . − . daysis obtained by restricting the fit to the period between MJD 55296 and 55298 (April 10–12). Toinvestigate the possibility of a second flare between MJD 55299 and 55301, a constant-flux fit isapplied to data points between MJD 55298 and 55304 (April 12–18). The χ /dof value of theconstant-flux fit is 9.6/4 with a corresponding χ probability of 0.05. In spite of this low confidencelevel for the constant-flux hypothesis, there is nevertheless insufficient evidence to confirm thepresence of a second, separated flare component around MJD 55299–55301 (April 13–15). Figure 4 shows the spectra measured during the rising period between April 5 and 8 (MJD55291–55294), during the peak on April 9 and 10 (MJD 55295–55296), and during the falling periodbetween April 11 and 15 (MJD 55297–55301). Power-law fits of the form Φ = Φ ( E/ TeV) − Γ areapplied to all three periods, and the corresponding power-law fit parameters are listed in Table 2.The spectral index of the peak period differs from that of the falling period by 2 . σ , and from thatof the rising period by 1 . σ . The peak period has the hardest spectrum of all three periods.A hardness ratio (HR) test is also applied to investigate further the possibility of spectralvariability between these three different periods. The HR may provide more sensitivity as it isTable 1. χ minimized parameters of the April flare light curve in Figure 3 (Fit functionΦ = p e ( t − /p ). The error bars of p and p are statistical only.Period (MJD) χ /dof χ Probability p (cm − s − ) p (days)55291–55295 0.3/2 0.88 2 . +1 . − . × − . +1 . − . . × − . +17 . − . × − − (1 . +0 . − . )55296–55298 2.1/1 0.15 1 . +4 . − . × − − (0 . +0 . − . ) 10 – Date ( MJD )55290 55292 55294 55296 55298 55300 55302 55304 ) - s - G e V ( ph c m I n t e g r a l F l u x > -12 × Fig. 3.— Fits to the 2010 April VHE gamma-ray light curve of M 87 leading up to the flare andtrailing the flare, with fit errors included and shown as shaded regions. The exponential timescaleis 2 . +1 . − . days for the rising flux portion, and 0 . +0 . − . days for MJD 55296–55298 segment of thefalling flux.Table 2. Spectral power-law fit parameters and hardness ratios for the three periods of the M 87flare in 2010 April: Rising, Peak, and Falling. Errors given are statistical only.Periods MJD Date Flux Normalization Spectral Index χ /dof Hardness RatioConstant Φ Γ10 − (cm − s − TeV − )Rising 55291–55294 1 . ± .
42 2 . ± .
31 4.1/4 0 . ± . . ± .
29 2 . ± .
07 4.3/5 0 . ± . . ± .
16 2 . ± .
18 5.2/4 0 . ± .
04 11 –
Energy ( TeV )1 10 T e V ) - s - F l u x ( ph c m E -13 -12 -11 rising (55291-55294)peak (55295-55296)falling (55297-55301) Fig. 4.— Spectral measurements during three periods: leading up to the flare (MJD 55291–55294,April 5–8), peak of the flare (MJD 55295, 55296, April 9 and 10), and trailing the flare (MJD55297–55301, April 11–15). The lines are power-law fits to the data, with the values for the fluxnormalization constant and the spectral index given in Table 2. 12 –obtained in a straightforward way from the energy distribution of the excess events, whereas thespectral index calculation requires multiple binning and fitting of the data. The HR used here isdefined as the ratio of the integral flux in the energy range 1–10 TeV to that in the range 0.35–1 TeV. For the rising period, HR = 0 . ± .
12; for the peak period, HR = 0 . ± .
03; and for thefalling period, HR = 0 . ± .
04 (see Table 2). The HR for the peak period is found to be largerthan that for the falling period with a statistical significance of 3 . σ , compared to the 2 . σ forthe spectral index difference of the same time periods. The increased significance may result froma higher sensitivity of the HR to spectral variability. However, we also note that the HR for therising period is 2 . σ larger than that for the falling period while the spectral indices from theseperiods are identical. This may be a result of statistical fluctuation due to the poor statistics ofthe rising period spectral measurements.Figure 5 shows the spectral index (Γ) plotted against flux normalization constant (Φ ) for the2010 April flare spectra (open circles), together with archival VHE gamma-ray spectra from 2004onward (Aharonian et al. 2006; Acciari et al. 2008; Albert et al. 2008; Acciari et al. 2009, 2010). Aconstant-flux fit to the 2010 April flare flux-index data yields a χ probability of 0.05. A linear fit ofthe form Γ = p + p log Φ yields a χ probability of 0.67, with the parameter p = − . ± . . σ away from zero. Although the fit may suggest a possible correlation between thespectral index and the flux normalization constant, the data do not provide definitive evidence forspectral variability during this flaring episode. Using all the flux-index data available since 2004,a constant-flux fit yields a χ probability of 0.26, while a linear fit yields a χ probability of 0.52with p = − . ± .
4. Discussion
VERITAS first detected M 87 in 2007 in a low state, with emission at ∼
2% of the Crab Nebulaflux above 250 GeV (Acciari et al. 2008). In 2008, VERITAS detected flaring activity up to 10%of the Crab Nebula flux above 250 GeV during a joint monitoring campaign in which correlationsbetween VHE gamma rays, X-ray, and radio (Acciari et al. 2009) were found. In 2009, M 87 wasobserved to be in a low state again at ∼
1% of the Crab Nebula flux above 250 GeV (Acciari et al.2010). In 2010 April, VERITAS observed the brightest emission ever seen from M 87, with a fluxup to 20% of the Crab Nebula flux above 350 GeV. In comparison to previous constraints frompast flares (Aharonian et al. 2006; Albert et al. 2008; Acciari et al. 2010), the 2010 VERITAS dataset yields the fastest exponential flux-changing time (0 . +0 . − . days) ever observed for M 87. Thistime constraint gives a new upper limit on the emission region size that is lower than those derivedfrom previous observations. Using the exponential decay time, the emission region size has radius R ≤ R var = δ c ∆ t = 2 . × δ cm ≈ . δ R s , where R s is the Schwarzschild radius of the M 87black hole (= 2 GM BH /c ≈ . × cm, with M BH = 6 . × M ⊙ scaled from Gebhardt et al.(2011) to the distance used in this paper) and δ is the relativistic Doppler factor. As in earlierfindings, this may point to the black hole vicinity as the actual origin of the VHE radiation. While 13 – ) -1 TeV -1 s -2 ( ph cm Φ Flux normalization constant -13 -12 -11 Γ S p ec t r a l i nd ex VERITAS 2010 dataVERITAS archival dataMAGICHESS
Fig. 5.— Spectral index vs. flux normalization constant using spectra from the three periods(rising, peak, and falling) and archival spectra from 2004 onward. The dashed line represents aconstant fit with a χ probability of 0.26, and the dash-dotted line represents a linear fit of theform Γ = p + p log Φ with a χ probability of 0.52. The values of p and p are given in thetext. 14 –an increased X-ray flux from the nucleus seems to support this hypothesis, no increase of the radioflux from the nucleus could be found (Abramowski et al. 2011), in contrast to the contemporaneousradio and VHE gamma-ray flares observed in 2008 (Acciari et al. 2009).Another notable characteristic of the 2010 flare is the large difference between the rise timeand the decay time of the flux, a feature which has not been seen in previous flares. Since previousVHE flares in 2005 and 2008 were not sampled at a comparable accuracy and their onsets were notas well defined as the 2010 flare, this is the first M87 VHE flare that allows the determination ofthe rise and fall times. The shape of the 2010 flare also seems less erratic as compared to the earlierflares, which could point to a different production mechanism. However, given the lower statisticsof the earlier flares, this is difficult to quantify and requires future observations to disentangle.From a compilation of multiwavelength data sets spanning decades, Wagner et al. (2009) pre-sented a spectral energy distribution (SED) of M 87, along with hadronic and leptonic models.The hadronic synchrotron-proton blazar (SPB) model (Reimer et al. 2004) suggests gamma-rayemission from synchrotron radiation by protons or by muons and pions. However, the SPB modelSED produced using archival data before 2004 shows a steep drop-off at TeV energies that is notcompatible with the spectra obtained from the 2010 data set or with any previous VHE spectralmeasurements. Barkov et al. (2010) proposed a scenario where a red giant star, with an envelopeof loosely bound material, interacts with the base of the jet. VHE gamma rays are produced nearthe supermassive black hole via proton–proton interactions between the jet and the red giant cloud.The gamma-ray light curve produced from this model shows an exponential increase/decay timeof ∼ ≤ .
01 G. The spine-sheath model, however, seems to facedifficulties in achieving a harder spectrum due to absorption of TeV photons from interactions withthe optical–IR photons from the spine. As pointed out by Tavecchio & Ghisellini (2008), severegamma-ray photon absorption can be alleviated by increasing the emission region size, which woulddecrease the absorption optical depth. However, this would be limited by the observed short-termvariability. Abdo et al. (2009) fitted a homogeneous one-zone SSC model using 2009 VLBA radio,
Chandra
X-ray, and
Fermi -LAT measurements when M 87 was observed to be in a low state fromradio to VHE gamma rays. A contemporaneous spectral measurement in the VHE range was not 15 –possible due to low statistical significance (Acciari et al. 2010), but compared to archival low-stateVHE measurements, the one-zone SSC model seems to underestimate the VHE gamma-ray flux byalmost an order of magnitude. Georganopoulos et al. (2005) and Lenain et al. (2008) demonstratedthat one-zone homogeneous models are unlikely to reproduce the observed VHE spectrum.Giannios et al. (2010) presented a scenario where minijets are formed within the jet due to flowinstabilities. These minijets move relativistically with respect to the main jet flow. VHE gammarays are produced from the interactions between the minijets and the jet, and are beamed withlarge Doppler factor when the minijets are aligned with our line of sight. The minijets model SEDis compatible with the 2010 data. A satisfactory solution for the high state observed by VERI-TAS in 2010 is also possible within the magnetosphere model (e.g., Neronov & Aharonian 2007;Rieger & Aharonian 2008; Vincent & Lebohec 2010; Levinson & Rieger 2011). The magnetospheremodel is dependent on the injected plasma which suggests that a vacuum gap with a large electricfield that is capable of accelerating electrons to very high energies may be formed during a periodof low accretion rate.We cannot discriminate between different leptonic models based on this VHE data alone. Thespectral change with flux level would serve as an important input for the modeling once it isconfirmed by a second flare. Leptonic models tend to predict a more direct correlation betweenX-ray and VHE gamma rays. For the 2010 flare of M 87, extensive follow-up observations ofthe VHE gamma-ray flare (Ong & Mariotti 2010) were carried out in X-ray, optical, and radiowavebands. The result is a much more complete sampling across different energy bands than inthe case of previous M 87 flares, providing a data set that will help to constrain the emission regionand the radiative processes involved. A separate, upcoming publication (Abramowski et al. 2011)will present the multiwavelength result, which spans 16 decades of energy.This research is supported by grants from the U.S. Department of Energy Office of Science,the U.S. National Science Foundation and the Smithsonian Institution, by NSERC in Canada, byScience Foundation Ireland (SFI 10/RFP/AST2748) and by STFC in the U.K. We acknowledgethe excellent work of the technical support staff at the Fred Lawrence Whipple Observatory andat the collaborating institutions in the construction and operation of the instrument.
Facility:
VERITAS
REFERENCES
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