Studying the Extreme Behaviour of 1ES 2344+51.4
A. Arbet-Engels, M. Manganaro, M. Cerruti, V. Fallah Ramazani, D. Dorner, V. Sliusar, A. V. Filippenko, T. Hovatta, V. Larionov, J. A. Acosta-Pulido, C. M. Raiteri, W. Zheng
SStudying the Extreme Behaviour of 1ES 2344+51.4
A. Arbet-Engels ∗ , M. Manganaro , M. Cerruti , V. Fallah Ramazani , for theMAGIC † Collaboration, D. Dorner for the FACT ‡ Collaboration, V. Sliusar ,A. V. Filippenko , , T. Hovatta , V. Larionov , J. A. Acosta-Pulido , ,C. M. Raiteri , W. Zheng ETH Zurich, CH-8093 Zurich, Switzerland, E-mail: [email protected] University of Rijeka, Department of Physics, 51000 Rijeka, Croatia Universitat de Barcelona, E-08028 Barcelona, Spain Finnish Centre for Astronomy with ESO, FI-20014 University of Turku, Finland Universität Würzburg, D-97074 Würzburg, Germany University of Geneva, Department of Astronomy, CH-1290 Versoix, Switzerland Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA Miller Senior Fellow, Miller Institute for Basic Research in Science, University of California,Berkeley, CA 94720, USA Astronomical Institute of St. Petersburg State, 198504, St.Petersburg, Russia Instituto de Astrofísica de Canarias, Calle Via Lactea, s/n, E38205 La Laguna, Tenerife, Spain Departamento de Astrofisica, Universidad de La Laguna, E38205 La Laguna, Tenerife, Spain Osservatorio Astrofisico di Torino, I-10025 Pino Torinese, Italy
The BL Lac type object 1ES 2344+51.4 (redshift z = . ∼ Hz.From previous studies of 1ES 2344+51.4 in the very-high-energy (VHE, >
100 GeV) gamma-ray range, its inverse Compton (IC) peak is expected around 200 GeV. 1ES 2344+51.4 was firstdetected in the VHE range by Whipple in 1995 during a very bright outburst showing around60% of the flux of the Crab Nebula above 350 GeV. In 1996, during another flare in the X-rayband, observations with Beppo-SAX revealed a large 0.1-10 keV flux variability on timescalesof a few hours and an impressive frequency shift of the synchrotron peak to above 10 Hz.Later on, this extreme behaviour of the source motivated several multiwavelength campaigns,during most of which the source appeared to be in a low state and showing no clear signs of“extremeness”. In August 2016, FACT detected 1ES 2344+51.4 in a high state and triggeredmultiwavelength observations. The VHE observations show a flux level similar to the historicalmaximum of 1995. The combination of MAGIC, FACT, and Fermi-LAT spectra provides anunprecedented characterisation of the IC peak. It is the first time that simultaneous HE and VHEdata are presented for this object during a flaring episode. We find an atypically hard spectrum inthe VHE γ -rays as well as a hard X-ray spectrum, revealing a renewed extreme behaviour. ∗ Speaker. † https://magic.mpp.mpg.de/ . For collaboration list see PoS(ICRC2019)1177 ‡ For collaboration list see PoS(ICRC2019)1177 c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] A ug he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels
1. Introduction
Blazars constitute a prominent class of sources in the extragalactic very-high-energy (VHE, >
100 GeV) sky . Belonging to the group of active galactic nuclei (AGN), they are characterized bya relativistic plasma jet pointing toward the observer. Blazars are commonly divided into two widecategories, flat spectrum radio quasar (FSRQ) and BL Lac type objects, according to the propertiesof their optical spectrum. FSRQs are identified by strong optical emission lines, while BL Lac typeobjects exhibit spectra with none or a very weak presence of such spectral features.The spectral energy distribution (SED) of BL Lacs typically displays a two-hump structure.While the low-energy hump is generally attributed to synchrotron radiation emitted by highly rel-ativistic electrons, the origin of the higher energy hump is still under debate. Most of the time,the latter component is successfully explained by synchrotron self-Compton (SSC) models, wherethe electron population responsible for the synchrotron radiation up-scatters the same synchrotronphotons to GeV-TeV energies via the inverse-Compton process [1]. However, other scenarios suchas hadronic or lepto-hadronic models have been proposed [1]. BL Lacs can further be dividedinto three subcategories depending on the location of the synchrotron peak ν synch , peak : low-energypeaked BL Lacs (LBL; ν synch , peak < Hz ), intermediate-energy peaked BL Lacs (IBL; 10 Hz < ν synch , peak < ), and high-energy peaked BL Lacs (HBL; 10 Hz < ν synch , peak < ).During the past decade, however, X-ray observations have revealed that a small set of BL Lacsshow a ν synch , peak that is shifted to unusually high frequencies, above 10 Hz. Based on this ex-tremeness, [2] suggested the existence of an additional class of BL Lac objects, dubbed as extremehigh-energy BL Lacs (EHBLs). Having a synchrotron peak located at higher energies, one expectsthe inverse-Compton bump to be also moved to higher energies, peaking at VHE. Consequently,EHBLs typically exhibit a hard spectrum with a photon index Γ (cid:46) ∼ TeV energies. Addi-tionally, a hard TeV spectrum suggests a high minimum electron Lorentz factor for the electronpopulation [3]. Finally, the model parameters describing the SED often require a very low magne-tization in the jet, suggesting that the energy budget in the emission region is far from equipartitionbetween matter and magnetic field.Recent observations suggest very different temporal behaviours among the EHBL family.Some of them, such as the classical EHBL 1ES 0229+200, seem to constantly display an extremebehaviour. On the other hand, several sources behave as EHBL only on a temporary basis and/orduring flaring episodes (e.g., Mrk 501, see [4]). 1ES 2344+51.4 belongs to the latter group. In thiswork, we report multiwavelength observations of 1ES 2344+51.4 during an intense flaring statethat occurred in August 2016. The flare is characterized by hard VHE and X-ray spectra that weinterpret as a renewal of extreme behaviour in these two energy bands.
2. 1ES 2344+51.4 z = .
044 [5], and it is among the first http://tevcat2.uchicago.edu/ he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels extragalactic sources detected in the VHE band. Its discovery in the VHE γ -ray range was reportedby the Whipple collaboration in 1995 [6] during a flare, where the flux above 350 GeV reached ∼ , and the spectrum could be well described by a power law with an indexof 2 . ± .
17 [7]. Later on,
BeppoSAX observations performed during another high X-ray statein 1996 showed strong 0.1-10 keV flux variability on ∼ hour timescale, together with impressivespectral variability [8]. A lower limit to the synchrotron peak frequency was set to 3 × Hz,implying a frequency shift of the synchrotron peak by a factor ∼
30 or more with respect to lowstates, which connotated this source for the first time as EHBL [8]. Interestingly, most recentmultiwavelength campaigns probed the source mostly during quiescent states and not displayingthe similar extremeness in the X-ray band seen during the 1996 flare, which suggests that extremebehaviours occur only during flaring states.
3. Observations
The First G-APD Cherenkov Telescope (FACT) is located at La Palma, Spain (altitude 2200 m)and measures photons at VHE using the imaging air Cherenkov technique [9, 10]. Thanks to theexcellent performance stability of its silicon photomultiplier camera, FACT is an ideal monitoringinstrument for bright TeV blazars. FACT continuously monitors 1ES 2344+51.4 and collectedmore than 1700 hr of observation time for this object after almost 8 years of operations [11].On MJD 57610 (August 10 2016 ), the FACT Quick Look Analysis [12], a low-latency on-siteanalysis, detected the source in an enhanced state. Based on this, the FACT collaboration issued analert to the community. The flux of that night obtained from an off-site analysis was consistent with ∼ (cid:38)
810 GeV. Follow-up observations were performed by several other instrumentssuch as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes [13].MAGIC is a system of two 17-m imaging atmospheric Cherenkov telescopes located at LaPalma, Spain, very close to FACT, and measures γ -rays above E (cid:38)
50 GeV. MAGIC observed1ES 2344+51.4 for two consecutive nights, MJD 57611 & MJD 57612 (August 11 and August12 2016), for a total of ∼ . ∼ σ level, was obtained. In Figure 1, we show the distribution of the θ variable when combiningthe two observation nights. The θ variable denotes the angular distance (in [deg ]) between thereconstructed direction and the expected source position of the gamma candidate events. Thedetection significance is calculated using eq. 17 in [14].We complement VHE data with high-energy γ -ray (HE; E >
100 MeV) observations providedby the Large Area Telescope (LAT) installed on the
Fermi satellite [15]. For this work, we con-sidered all
Fermi -LAT data taken between MJD 57567 & MJD 57644 (June 28 & September 132016). We then built the light curve between 300 MeV and 300 GeV and used a 7-day binningbecause of the faintness of the source in this band, typical of EHBLs. For this source, it is thefirst time that contemporaneous HE data are combined with VHE observations during a flare event,providing the best depiction of the IC peak so far.X-ray observations were performed by the X-ray Telescope (XRT) onboard the
Neil GehrelsSwift Observatory (Swift) [16]. In total, five observations took place close to the MAGIC obser- For a given energy threshold, C.U. is defined as the integral flux of the Crab Nebula above the threshold energy. All dates herein are in UT. he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels vations (MJD 57613, MJD 57620, MJD 57623, MJD 57626, and MJD 57629 – August 13, 20,23, 26, and 29 2016). Additionally to XRT data, we analysed UV data provided by the Ultravio-let/Optical Telescope (UVOT), also onboard the
Swift satellite. All UVOT observations are strictlysimultaneous to the XRT ones, and we considered the following three filters: UVW1, UVM2 &UVW2.1ES 2344+51.4 was also observed in the infrared (IR) and optical bands by several instru-ments: KVA, NOT (Tuorla blazar monitoring program ), Stella, IAC80, AZT-8, and LX-200(Whole Earth Blazar Telescope community ) in the R-band. We also add observations from theKAIT telescope [17] that were performed without filter (i.e., in clear band), which have an effec-tive color close to the R band. In the IR, we use data from the Telescopio Carlos Sánchez (TCS)telescope in the J , H , and K -short filters. For the IR/optical range, we applied a host-galaxy correc-tion following [18], since in these bands the host galaxy significantly contributes to the observedflux.Finally, at the lowest energies, we use radio measurements at a frequency of 15 GHz, whichwere obtained by the OVRO 40-m telescope blazar monitoring program [19]. ] [ deg q e v en t s N Time = 1.10 h 1.6 – = 7.9 off = 86; N on N 9.4 – = 78.1 ex N s Significance (Li&Ma) = 11.87
Preliminary
Figure 1:
Distribution of the θ variable obtained from the overallMAGIC observation time. Darkpoints represent the θ values com-puted in the signal region, while thegray shaded area comes from thebackground region. The significanceis calculated based on eq. 17 in [14],by considering all events on the leftside of the dashed vertical line.
4. Results
We present the multiwavelength light curves from radio to VHE in Figure 2. All fluxes arecomputed in daily bins, except for the two monitoring instruments FACT &
Fermi -LAT, where weuse 7-day binning. Additionally, the last time bin of FACT is integrated over 1 month since thesource faded and the measurements are consistent with no signal.Between MJD 57603 and MJD 57615, the FACT light curve displays a weekly averaged fluxof ∼ − cm − s − above (cid:38)
810 GeV, which is around 0 . F ( >
300 GeV ) = ( . ± . ) × cm − s − , cor-responding to ∼ .
55 C.U. The flux is therefore similar to the historical maximum measured by http://users.utu.fi/kani/1m/ he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels F l u x [ c m s ] FACT 810 GeV F l u x [ c m s ] MAGIC >300 GeV F l u x [ c m s ] FERMI-LAT (0.3-300 GeV) F l u x [ e r g c m s ] SWIFT/XRT 2-10 keV F l u x [ m J y ] SWIFT/UVW1SWIFT/UVM2SWIFT/UVW2 F l u x [ m J y ] KVA R-bandStella R-bandIAC80 R-bandAZT-8 R-band NOT R-bandLX-200 R-bandKAIT Clear filter F l u x [ m J y ] TCS J-bandTCS H-bandTCS Kshort-band
Modified Julian Date [MJD] F l u x [ J y ] P R E L I M I N A R Y OVRO 15GHz
Figure 2:
Multiwavelength light curve of 1ES 2344+51.4 from MJD 57567 (28 June 2016) to MJD 57645(14 September 2016). Light curves are obtained using (from top to bottom) FACT ( (cid:38)
810 GeV), MAGIC(>300 GeV),
Fermi -LAT (0.3-300 GeV),
Swift -XRT (2-10 keV),
Swift -UVOT (W1, M2, and W2 filters),KVA, Stella, IAC80, AZT-8, LX-200, NOT ( R band), KAIT (clear filter), TCS ( J , H , K -short filters), andOVRO (15 GHz). In the Fermi -LAT light curve, we quote an upper limit at 95% C.L. for time bins withTS<3. he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels
Whipple in 1995 [6]. On the second night, MJD 57612, the flux diminished by a factor of ∼ . ∼ daily scale variability at VHE energies. So far,no intranight variability has been observed at VHE in 1ES 2344+51.4.The Fermi -LAT light curve is shown in the third panel. As already mentioned, we present forthe first time contemporaneous HE observations together with VHE data collected during a flaringepisode. The source is relatively faint in this energy band, and was first detected after 5.5-months of
Fermi -LAT operations with a marginal test statistic TS ≈
37 ([20]). Additionally, during previousmultiwavelength campaigns no detection was claimed on timescales of ∼ months. On the contrary,over the considered period shown in the Figure 2, we report a strong detection with TS ≈ F (0.3-300 GeV) = ( . ± . ) × − cm − s − . Thespectrum is best described with a power-law index of Γ = . ± .
1. On the restricted 1-100 GeVband, the flux is F (1-100 GeV) = ( . ± . ) × − cm − s − , which is about two times higher thanin the 3FGL catalog. Moreover, Figure 2 shows an indication of a higher flux that is temporarilycoincident with the flare seen in the VHE. The strong detection as seen by Fermi -LAT provides anunprecedented constraint on the IC peak and will be exploited for the SED modelling.The
Swift -XRT light curve in the 2-10 keV energy band is plotted in the fourth panel. The fluxvaries between ∼ × − erg cm − s − and ∼ × − erg cm − s − , and a decreasing trendis visible along the days. The VHE flare is therefore accompanied by elevated X-ray flux. Sucha state remains, however, moderately high since the maximum flux that has been observed in thisband is ∼ − s − in December 2007 [21].Given the well-known X-ray spectral variability that 1ES 2344+51.4 exhibited in the pastduring high states, we studied the SED measured by Swift -XRT. Furthermore, the location of thesynchrotron peak is a crucial parameter, which defines the EHBL family. All
Swift -XRT spectra arewell described with a power-law model. Additionally, we generally find hard spectra with a photonindex Γ XRT (cid:46)
2. In Figure 3, we plot the simultaneous combined UVOT-XRT SEDs from the fiveobservations. Black data points represent the data taken on MJD 57613, which is one day after theMAGIC observations. For this night, the photon index is the hardest among the five observationsand is equal to Γ XRT = . ± .
06. Thus, Figure 3 and the hard photon index Γ XRT imply thatUVOT and XRT data are describing the rising edge of the synchrotron bump, and we observe ashift of the synchrotron peak to (cid:38) Hz ( (cid:38) ∼ Hz. Consequently, we report a significant frequency shiftof the low-energy hump by ∼
5. Conclusions
We report multiwavelength observations of a strong flaring episode of 1ES 2344+51.4. TheVHE flux was around ∼ .
55 C.U. above 300 GeV, comparable to the historical maximum observedby Whipple in 1995. A significant dimming is evident between the two MAGIC observations,revealing at VHE variability at ∼ daily scale. The VHE flare was further accompanied with an5 he Extreme Behaviour of 1ES 2344+51.4 A. Arbet-Engels Energy [keV] F [ e r g c m s ] P R E L I M I N A R Y MJD 57613.5MJD 57620.8MJD 57623.9MJD 57626.7MJD 57629.0
Figure 3:
Strictly simultaneous com-bined UVOT-XRT SEDs from the fiveobservations around the time of theMAGIC observations. All these obser-vations are in agreement with an ex-treme synchrotron spectrum. enhanced X-ray state. Spectral analysis of the XRT data reveals a power-law spectrum with anindex harder than 2, implying a frequency shift of the synchrotron peak to (cid:38) Hz. Our resultsconfirm the EHBL behaviour already observed during flares. In an upcoming publication, a detailedanalysis of the VHE spectrum will be presented. Moreover, taking advantage of the unprecedentedconstraint of the IC peak obtained when combining
Fermi -LAT data with VHE data, a modeling ofthe broadband emission will be presented and discussed.
6. Acknowledgments
MAGIC Collaboration: https://magic.mpp.mpg.de/acknowledgments_ICRC2019/.
FACT Collaboration: https://fact-project.org/collaboration/icrc2019_acknowledgements.html. The OVRO 40-m monitoring program is supported in partby NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G, and NSF grants AST-0808050 and AST-1109911.This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive ResearchCenter (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy As-trophysics Division of the Smithsonian Astrophysical Observatory. We acknowledge the use of public data from theSwift data archive. This article is based partly on observations made with the 1.5 TCS operated by the IAC in the Span-ish Observatorio del Teide. This article is also based partly on data obtained with the STELLA robotic telescopes inTenerife, an AIP facility jointly operated by AIP and IAC. A.V.F. and W.Z. are grateful for support from NASA grantNNX12AF12G, the Christopher R. Redlich Fund, the TABASGO Foundation, and the Miller Institute for Basic Researchin Science (U.C. Berkeley). KAIT and its ongoing operation were made possible by donations from Sun Microsystems,Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the US National Science Foundation,the University of California, the Sylvia and Jim Katzman Foundation, and the TABASGO Foundation. Research at LickObservatory is partially supported by a generous gift from Google.
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