Mrk 421 showed an unprecedented X-ray flaring behaviour in 2013 April 10–17 when the con- secutive outbursts by a factor of 8–16 were observed in the 0.3–10 keV and 3–79 keV bands, and it became one of the brightest objects in the X-ray sky. During these events, the source was also very active on intraday timescales with fractional variability amplitude of 2–47 per cent, and showed the flux doubling and halving time scales of 1.16–7.20 hr and 1.04–3.54 hr, respectively. We have revealed up to 20 cases when the flux varied with 3σ significance within 1 ks. In this period, Mrk421 also underwent the most extreme X-ray spectral variability ever reported for BL Lacertae objects. The location of the synchrotron SED peak moved from about 0.1 keV to almost 20 keV. The photon index at 1 keV and curvature parameter showed the ranges a=1.68–2.83 and b=0.09–0.57, respectively, and varied along with the flux on diverse time scales down to the in- tervals shorter than 1 ks. While the X-ray outburst were accompanied by very strong TeV-flares, the source was relatively less active in the radio-UV and GeV energy ranges

Kapanadze, B.

Kapanadze, S.

ROMANO, Patrizia

Tabagari, L.

VERCELLONE, STEFANO

OA@INAF - Istituto Nazionale di Astrofisica

2016Publication Year2020-06-01T16:20:48ZAcceptance in OA@INAFUnprecedented X-ray Flaring Activity of Mrk 421 in 2013 AprilTitleKapanadze, B.; Kapanadze, S.; ROMANO, Patrizia; VERCELLONE, STEFANO; Tabagari, L.Authors10.22323/1.269.0033DOIhttp://hdl.handle.net/20.500.12386/25875HandlePOS PROCEEDINGS OF SCIENCEJournal269NumberPoS(FRAPWS2016)033Unprecedented X-ray Flaring Activity of Mrk 421 in2013 AprilBidzina Kapanadze ab, Sergo Kapanadze ∗a, Patrizia Romano b, Stefano Vercellone b,Lela Tabagari aa E. Kharadze Abastumani Astrophysical Observatory, Ilia State University, Colokashvili Av. 3/5,Tbilisi, Georgia, 0162b INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, Palermo, Italy,I-90146E-mail: bidzina_kapanadze@ilauni.edu.ge , romano@ifc.inaf.it ,stefano@ifc.inaf.it , sergo_kapanadze.1@ilauni.edu.geMrk 421 showed an unprecedented X-ray flaring behaviour in 2013 April 10–17 when the con-secutive outbursts by a factor of 8–16 were observed in the 0.3–10 keV and 3–79 keV bands, andit became one of the brightest objects in the X-ray sky. During these events, the source wasalso very active on intraday timescales with fractional variability amplitude of 2–47 per cent, andshowed the flux doubling and halving time scales of 1.16–7.20 hr and 1.04–3.54 hr, respectively.We have revealed up to 20 cases when the flux varied with 3σ significance within 1 ks. In thisperiod, Mrk421 also underwent the most extreme X-ray spectral variability ever reported for BLLacertae objects. The location of the synchrotron SED peak moved from about 0.1 keV to almost20 keV. The photon index at 1 keV and curvature parameter showed the rangesa=1.68–2.83 andb=0.09–0.57, respectively, and varied along with the flux on diverse time scales down to the in-tervals shorter than 1 ks. While the X-ray outburst were accompanied by very strong TeV-flares,the source was relatively less active in the radio-UV and GeV energy ranges.Frontier Research in Astrophysics− II,23-28 May 2016Palermo, Italy∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/opyright o ned by the author(s) under the ter s of the reative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo Kapanadze1. IntroductionBL Lacertae objects (BLLs) are active galactic nuclei (AGN) of elliptical galaxies, which withquasi-featureless spectra, high and variable optical/radio polarization, compact radio-morphology,strong flux variability in all spectral bands and broad continuum extending from radio to veryhigh energyγ-rays. These properties are explained as a result from a relativistically boosted non-thermal emission produced in the jet closely aligned to the line-of-sight (see [1] and referencestherein). Their spectral energy distribution (SED) shows the presence of two broad components.The lower-energy one is explained via the synchrotron radiation emitted by relativistic electronsin the jet, while the inverse Compton (IC) scattering of synchrotron photons (produced locally, orhaving an external origin) by the same electron population and hadronic processes are consideredas possible sources for the high-frequency bump [2].In the following, some results from the timing and spectral study of the unprecedented X-rayflaring activity of Mrk 421 in 2013 April are presented.2. High-energy Flux Variability from Past StudiesMrk 421 is a nearby (z=0.031, [3]) high-energy peaked BL Lac object (HBL) which was thefirst TeV-detected extragalactic object [4]. This detection was followed by numerous multiwave-length (MWL) campaigns which revealed an extreme X-ray and TeV variability in this object.Their highlights are briefly reviewed in the next two subsections.2.1 TeV-band Variability• Whipple observations of 1994 May 9-10 revealed an increase in the TeV flux by a factor of∼10 compared to its quiescent level [5].• The 1995 April-May MWL campaign showed a rapid variations in the TeV light curve withdoubling and decay times of about 1 d [6].• The source underwent an extremely rapid VHE flares on 1996 May 7. In the first outburst,which had a doubling time of about one hour, the flux increased above the relatively quiescentvalue by more than a factor of 50, making Mrk421 the brightest TeV source in the sky. Inthe second outburst, which lasted about 30 min, the flux increased by a factor of 20–25 [7].• Strong TeV outbursts during December 1999 – May 2001 were reported by [8] from theHEGRA observations, accompanied by a rapid variability with the flux doubling time of20 min.• Whipple observations in 2000–2001 revealed exceptionally strong and long-lasting flaringactivity with the TeV flux of 0.4–13 Crab [9].• Several strong TeV flares in Mrk 421 was reported by [10] from the observations with TibetIII air shower array, Whipple, HEGRA and CAT in 2000–2001.2PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo KapanadzeFigure 1: X-ray images of Mrk 421 from theSwift-XRT observations perormed in the photon counting (PC;left) and windowed timing (WT; right) regimes.• Mrk 421 was detected in an extraordinarily high state at TeV energies with CANGAROO-IIin 2001 January–March [11]. The same was reported by [12] from the 2000 February–Maycampaign with HEGRA. The TeV observations on February 7/8 showed a substantial TeVflux variability on the 30 min timescale.• During the 2006 April–June monitoring, the TeV flux doubling time of 36 min was observedwith MAGIC [13].• On 2008 May 2/3, bright flares were detected with the TeV fluxes reaching the level of10 Crab [14].2.2 X-ray Variability• BeppoSAXobservations in 1998 April revealed multiple flares, occurring on timescales ofabout 1 day [15].• Target of Opportunity (ToO) observation of Mrk 421 withBeppoSAXin 1998 June revealedthe source in a high and hard X-ray state with a significant temporal and spectral variabilityon timescales down to 500–1000 s [16].• BeppoSAXobservations in 1997–2000 showed a variability in the 15–90 keV count rate bya factor of 33.4 [17].• The threeXMM-Newtonpointings performed in 2000–2004 showed significant spectral vari-ations on time scales as short as∼500–1000 s [18].• The 3–20 keV flux from theRXTE-PCA observations in 2000 February–May increased anddecayed with e-folding times as short as about 5 hr [12].3PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo Kapanadze• Swift-XRT observations in 2005 March–July showed large variations in the 2–10 keV flux,by a factor of about 20 [19].• From theNuSTARobservations in 2013 April, Paliya et al. [20] reported about the detectionof the shortest flux doubling time of 14.01±5.03 min, which is the shortest hard X-ray (3–79 keV) variability ever recorded for Mrk 421.3. ResultsFor our study, we used theSwift-XRT andNuSTARdata, retrieved from the publicly availablearchive, maintained by HEASARC1, and processed according to the standardHEASOFTproce-dures described in detail in [21]. For the observations showing the count rates above 100 cts s−1, apileup correction was performed according to the recipe presented in [22].3.1 Flux VariabilityThe source exhibited an unprecedented X-ray flaring behaviour in April 10–17, when we ob-serve the two consecutive outbursts by a factor of 12 and 8.1 in the 0.3–10 keV band (followed bya minor flare). The count rate exceeded the level of 200 ctss−1 for the first time since the start oftheSwiftobservations of Mrk 421, and it became one of the brightest sources in the X-ray sky (seeFig. 2a).In the 3–79 keV band, we observe the three strong flares (Fig. 2b), and Mrk 421 was detectedby MAXI with 5σ significance every day in this period.The source showed several X-ray flux doubling/halving events during 2013 April 10-17. Forexample, The 3–79 keV count rate showed an increase by a factor of 8.84 in 23.6 hr, superimposedby a minor flare during the prolongedNuSTARobservation on April 11/12. Mrk 421 exhibited twosuccessive flares by a factor of 1.74 and 2.75 in 4.7 hr and 6.3 hr, respectively, reaching the highesthistorical 0.3–10 keV count rate on April 12/13.The fastest events X-ray flux variability is found from the separate orbits of the XRT observa-tion ObsID 00080050019 (April 12/13), each lasting less than 1 ks and exhibiting a flux variabilitywith 99.9% confidence with fractional variability amplitude of 2.2(0.4)%–5.9(0.3)% (Fig. 2c)2A strong TeV flaring behaviour was also evident from the MAGIC and FACT observations(see [21]). The optical–UV light curves sometimes showed an uncorrelated behaviour with theX-ray ones. TheFermi-LAT (0.3–300 GeV) flux from this period was not the highest in the firsthalf of 2013 [21].3.2 X-ray SpectroscopyWe performed the X-ray spectral analysis by fixing the Hydrogen column density to the Galac-tic valueNH = 1.90×1020 cm−2, taken from Leiden/Argentine/Bonn (LAB) Survey of Galactic HI[23], and using the log-parabolic model (LP; [17])F(E) = K(E/E1)−(a+blog(E/E1)) ph cm−2 s−1 keV−1, (3.1)1http://heasarc.gsfc.nasa.gov/docs/archive.html2A comprehensive and detailed description of the X-ray flux and spectral variability of Mrk 421 in the2013 April 10–17 period is presented in [21].4PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo Kapanadze1000 1500 2000 2500 3000 3500050100150MJD−53000 [d]0.3−10 keV Count Rate [cts s−1] (a)501502500 2 4 6204060MJD−56392 [d]Count Rate [cts s−1]NusSTAR(b)      XRT0 200 400 60022022523023524080050019 Orbit 70 200 400 600220240260 80050019 Orbit 80 200 400 600120130140150160Time [s, since the orbits start] 80050019 Orbit 20 100 200 300 400150160170Count Rate [cts s−1]80050019 Orbit 1(c)Figure 2: (a) Long-term 0.3–10 keV light curve of Mrk 421 from theSwift-XRT observations in 2005-2016with the dashed area representing the giant outbursts of 2013 April 10–17; (b) the XRT andNuSTARlightcurves of Mrk 421 from the 2013 April 10–17 period; (c) fastest X-ray flux variability detected from differentorbits of ObsID 00080050019.5PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo KapanadzeFigure 3: The powerlaw (left) and logparabolic (right) fits to the spectrum of Mrk 421 extracted from thesecond orbit of theSwift-XRT pointing ObsID 00080050016 (April 11). In the first case, the reduced Chi-square is 1.283 with 244 d.o.f., while it is equal to 0.932 with 242 d.o.f. for the logparabolic fit. In thesecond case, the curvature parameter is significant (b=0.36±0.04), and the distribution of the fit residualsshow that the logparabola model is acceptable for this spectrum, while the powerlaw fit yields a prominenttrend in the residuals.with E1, the pivot energy fixed to 1 keV and 10 keV for the XRT andNuSTARspectra, respectively;a, the photon index at the energyE1; b, the curvature parameter;K, the normalization factor. Dur-ing 2013 April 10–17, the source also underwent the most extreme X-ray spectral variability everreported for BLLs. The spectra mainly were curved and fitted well with the LP model (see Fig. 3for the corresponding example). In the case of the XRT spectra, the location of the synchrotronSED peak moved from about 0.1 keV to almost 20 keV. The photon index at 1 keV and curvatureparameter showed the rangesa=1.68–2.83 andb=0.09–0.47, respectively, and varied along withthe 0.3–10 keV flux on different time scales from a few weeks down to intervals shorter than 1 ks.The curvature parameter showed an even larger range of 0.13–0.57 in the 3–79 keV band.However, some XRT and NuSTAR spectra showed the values of the curvature parameter belowb=0.08, and the fit with the LP model did not give better statistics than that with the simple power-law (PL) F(E) = KE−Γ, whereΓ is the photon index throughout the observation band. Therefore,the latter model was chosen for those spectra (see Fig. 4). The parameterΓ also showed broadrangesΓ=1.70–2.76, andΓ=2.38–3.16 for the XRT andNuSTARspectra, respectively.4. Conclusions• Mrk 421 showed an unprecedented X-ray flaring behaviour in 2013 April 11–16 when theconsecutive outbursts by a factor of 8–16 were observed in the 0.3–10 keV (Swift-XRT) and3–79 keV (NuSTAR) bands.• The 0.3–10 keV count rate exceeded the level of 200 cts s−1 for the first time since the startof the Swift observations of Mrk 421, and it became one of the brightest objects in the X-raysky.6PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo KapanadzeFigure 4: Same as Fig. 3 from the first 300 s segment of the second orbit of theSwift-XRT pointing Ob-sID 00035014063 (April 14/15). In the first case, the reduced Chi-square is 0.921 with 329 d.o.f., whileit is equal to 0.908 with 328 d.o.f. for the logparabola fit. In the second case, the curvature parameter isvery small (b=0.06±0.03), hinting on the insignificant spectral curvature. Therefore, we chose the powerlawmodel for this spectrum.• During these events, the source was also very active on intraday timescales with fractionalvariability amplitudes of 2–47 per cent, and showed the flux doubling and halving time scalesof 1.16–7.20 hr and 1.04–3.54 hr, respectively.• We have revealed up to 20 cases when the flux varied with 3 sigma significance within 1 ks,and numerous IDVs occurring within 1 hr.• In this period, Mrk421 also underwent the most extreme X-ray spectral variability ever re-ported for blazars. The location of the synchrotron SED peak moved from about 0.1 keVto almost 20 keV. The photon index at 1 keV and curvature parameter showed the rangesa=1.68–2.83 andb=0.09–0.57, respectively, and varied along with the flux on diverse timescales down to the intervals shorter than 1 ks.• While the X-ray outburst were accompanied by strong TeV-flares, the source was less activein the radio–UV and GeV energy ranges that that can be explained via the global long-termchange in the efficiency of the acceleration mechanism, shifting the entire synchrotron bumpto higher energies.5. AcknowledgementsBK, SK and LT acknowledge Shota Rustaveli National Science Foundation and Ilia StateUniversity for the grant FR/377/6-290/14. SK thanks Franco Giovannelli and LOC for his invitationto the workshop and hospitality. PR acknowledges the contract ASI-INAFI/004/11/0. This researchhas made use of XRTDAS. We acknowledge the helpful service of HEASARC.7PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo KapanadzeReferences[1] Blandford, R. D.& Rees, M. J.,Extended and compact extragalactic radio sources - Interpretationand theory, Physica Scripta, 17, 265 (1978)[2] H. Krawczynski et al.,MULTIWAVELENGTH OBSERVATIONS OF STRONG FLARES FROM THETeV BLAZAR 1ES 1959+650, ApJ, 601, 151 (2004)[3] G. Colla et al., Radio and optical data on a complete sample of radio faint galaxies,A&AS 20, 1(1975)[4] M. Punch et al.,Detection of TeV photons from the active galaxy Markarian 421, Nature, 358, 477(1992)[5] A. D. Kerrick et al.,Outburst of TeV Photons from Markarian 421, ApJ, 438, L59 (1995)[6] J. H. Buckley et al.,GAMMA-RAY VARIABILITY OF THE BL LACERTAE OBJECTMARKARIAN 421, ApJ, 472, L9 (1996)[7] J. A. Gaidos et al.,Extremely rapid bursts of TeV photons from the active galaxy Markarian 421,Nature, 383, 319 (1996)[8] F. Aharonian et al.,Variations of the TeV energy spectrum at different flux levels of Mkn 421 observedwith the HEGRA system of Cherenkov telescopes, A&A, 493, 89 (2002)[9] F. Krennrich et al.,DISCOVERY OF SPECTRAL VARIABILITY OF MARKARIAN 421 AT TeVENERGIES, ApJ, 775, L9 (2002)[10] M. Amenomoti et al.MULTI-TeV GAMMA-RAY FLARES FROM MARKARIAN 421 IN 2000 AND2001 OBSERVED WITH THE TIBET AIR SHOWER ARRAY, ApJ, 598, 242 (2003)[11] K. Okomura et al.,OBSERVATION OF GAMMA RAYS GREATER THAN 10 TeV FROMMARKARIAN 421, ApJ, 579, L9 (2002)[12] H. Krawczynski et al.,SIMULTANEOUS X-RAY AND TeV GAMMA-RAY OBSERVATION OF THETeV BLAZAR MARKARIAN 421 DURING 2000 FEBRUARY AND MAY, ApJ, 559, 187 (2001)[13] J. Aleksic et al.,MAGIC TeV gamma-ray observations of Markarian 421 during multiwavelengthcampaigns in 2006, A&A, 519, 321 (2010)[14] V. A. Acciari et al.,TeV AND MULTI-WAVELENGTH OBSERVATIONS OF Mrk 421 IN 2006–2008,ApJ, 738, 25 (2011)[15] T. Takahashi et al.,COMPLEX SPECTRAL VARIABILITY FROM INTENSIVEMULTIWAVELENGTH MONITORING OF MARKARIAN 421 IN 1998, ApJ, 542, L105 (2000)[16] A. Malizia et al.,The 0.1–100 keV spectrum and variability of Mrk 421 in a high state, MNRAS, 312,123 (2000)[17] E. Massaro et al.,Log-parabolic spectra and particle acceleration in the BL Lac object Mkn 421:Spectral analysis of the complete BeppoSAX wide band X-ray data set, A&A, 413,489 (2004)[18] W. Brinkmann et al.,XMM-Newton timing mode observations of Mrk 421, A&A, 443, 397 (2005)[19] A. Tramacere et al.,Signatures of synchrotron emission and of electron acceleration in the X-rayspectra of Mrk 421, A&A, 466, 521 (2007)[20] V. S. Paliya et al.,Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April, ApJ,811, 143 (2015)8PoS(FRAPWS2016)033X-ray Flaring Activity of Mrk 421 in 2013 April Sergo Kapanadze[21] B. Kapanadze et al.,X-RAY FLARING ACTIVITY OF MRK 421 IN THE FIRST HALF OF 2013, ApJ,831, 102 (2016)[22] P. Romano et al.,Panchromatic study of GRB 060124: from precursor to afterglow, A&A, 456, 917(2006)[23] P. M. W. Kalberla et al.,The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI. Final data releaseof the combined LDS and IAR surveys with improved stray-radiation corrections, submitted toApjA&A, 440, 775 (2005)DISCUSSIONJIM HOWARTH BEALL: What was the source of your Inverse Compton “seed”photons?Sergo KAPANADZE: We adopted so-called synchrotron self-Compton scenario when the low-energy photons are up-scattered by the same electron population which produce them via the syn-chrotron mechanism.9

Unprecedented X-ray Flaring Activity of Mrk 421 in 2013 April

Sergo Kapanadze

Bidzina Kapanadze

Patrizia Romano

Stefano Vercellone

Lela Tabagari

Crossref

https://openaccess.inaf.it/bitstream/20.500.12386/25875/1/FRAPWS2016_033.pdf

Unprecedented X-ray Flaring Activity of Mrk 421 in 2013 April

Abstract

Similar works

Full text

Available Versions

OA@INAF - Istituto Nazionale di Astrofisica

Crossref