Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380
Y. T. Tanaka, C. C. Cheung, Y. Inoue, L. Stawarz, M. Ajello, C. D. Dermer, D. L. Wood, A. Chekhtman, Y. Fukazawa, T. Mizuno, M. Ohno, D. Paneque, D. J. Thompson
aa r X i v : . [ a s t r o - ph . H E ] S e p D RAFT VERSION J ULY
9, 2018
Preprint typeset using L A TEX style emulateapj v. 04/17/13
FERMI
LARGE AREA TELESCOPE DETECTION OF TWO VERY-HIGH-ENERGY ( E >
100 GEV) γ -RAY PHOTONSFROM THE Z = 1 . - Y. T. T
ANAKA , C. C. C HEUNG , Y. I NOUE , Ł. S TAWARZ , M. A
JELLO , C. D. D ERMER , D. L. W OOD , A. C HEKHTMAN ,Y. F UKAZAWA , T. M IZUNO , M. O HNO , D. P ANEQUE , D. J. T
HOMPSON Draft version July 9, 2018
ABSTRACTWe report the
Fermi
Large Area Telescope (LAT) detection of two very-high-energy (VHE, E >
100 GeV) γ -ray photons from the directional vicinity of the distant (redshift, z = 1 .
1) blazar PKS 0426 - . σ confidence level. We therefore claim that at least one of the two VHE photons is securely associated withPKS 0426 - Fermi -LAT constraints on the Extragalactic Background Light (EBL) intensity, which imply a z ≃ ≃
100 GeV photons. The LAT detection of the two VHE γ -rays coincided roughly with flaringstates of the source, although we did not find an exact correspondence between the VHE photon arrival timesand the flux maxima at lower γ -ray energies. Modeling the γ -ray continuum of PKS 0426 -
380 with dailybins revealed a significant spectral hardening around the time of the first VHE event detection (LAT photonindex Γ ≃ .
4) but on the other hand no pronounced spectral changes near the detection time of the second one.This combination implies a rather complex variability pattern of the source in γ rays during the flaring epochs.An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi -LATspectrum accumulated over the ∼ Subject headings: acceleration of particles — radiation mechanisms: non-thermal — galaxies: active — galax-ies: jets — quasars: individual (PKS 0426 - INTRODUCTION
Blazars are radio-loud active galactic nuclei (AGN) withrelativistic jets viewed at small angles to the Earth’s lineof sight. Their broad-band spectral energy distributions(SEDs) are typically dominated by two non-thermal emis-sion components widely believed to be due to synchrotron andinverse-Compton emission from a single population of ultra-relativistic electrons accelerated within the innermost parts ofthe jets (e.g., Urry & Padovani 1995). The exact particle ac-celeration processes at work, as well as the location and struc-ture of the dominant energy dissipation zone in blazar sources(hereafter ‘the blazar zone’), are still under debate. Blazarsare typically sub-divided into Flat Spectrum Radio Quasars [email protected] Hiroshima Astrophysical Science Center, Hiroshima University,Higashi-Hiroshima, Hiroshima 739-8526, Japan Space Science Division, Naval Research Laboratory, Washington, DC20375-5352, USA W. W. Hansen Experimental Physics Laboratory, Kavli Institutefor Particle Astrophysics and Cosmology, Department of Physics andSLAC National Accelerator Laboratory, Stanford University, Stanford, CA94305, USA Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai,Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan Astronomical Observatory, Jagiellonian University, 30-244 Kraków,Poland Space Sciences Laboratory, 7 Gauss Way, University of California,Berkeley, CA 94720-7450, USA Praxis Inc., Alexandria, VA 22303, resident at Naval Research Labo-ratory, Washington, DC 20375, USA Center for Earth Observing and Space Research, College of Science,George Mason University, Fairfax, VA 22030, resident at Naval ResearchLaboratory, Washington, DC 20375, USA Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan Max-Planck-Institut für Physik, D-80805 München, Germany NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (FSRQs) and BL Lacertae objects (BL Lacs) based on theequivalent widths of the emission lines in their optical spectra(e.g., Urry & Padovani 1995). BL Lacs are characterized bymuch weaker emission lines than FSRQs, or even featurelessoptical continua, that can be understood in terms of distinctaccretion rates in an otherwise homogeneous population ofsources (e.g., Ghisellini et al. 2011).Since the launch of the
Fermi satellite in 2008, high-redshift blazars have been established as very-high-energy(VHE; >
100 GeV) emitters (Neronov et al. 2011, 2012), withthe highest-redshift case reported being a 126 GeV photonfrom B2 0912+29, a z = 1.521 BL Lac, though the redshiftis still uncertain (see Neronov et al. 2012). Although col-lectively detected, individually these associations are basedon single photons and the chance coincidence probabili-ties for the VHE events to be detected around the sourcesare not low enough to claim discovery of a single source.Detection of sub-TeV VHE photons from z ∼ γ -ray emissionfrom the blazar PKS 0426 - L BLR / L Edd ≃ × - . In thecase of PKS 0426 - . × erg s - (Sbarufatti et al. 2005). This leads to L BLR / L Edd ≃ - for a Tanaka et al.black hole mass M BH ≃ M ⊙ (see Section 4), and thereforethe FSRQ classification according to the proposed scheme(Ghisellini et al. 2011).The VLT detection of the broad [Mg II] λ λ z = 1 . d L ≃ .
52 Gpc, assuming stan-dard cosmology with H = 71 km s - Mpc - , Ω m = 0 .
27, and Ω Λ = 0 . z = 1 .
111 based on the single [Mg II] λ Hubble
Space Telescopeimage (Urry et al. 2000) is consistent with the high redshift.Given all these findings, we conclude that the redshift deter-mination for PKS 0426 -
380 is robust.The
Fermi -LAT detection of a VHE event from near thedirection of PKS 0426 -
380 in January 2010 was previouslyreported (Ackermann et al. 2011; Neronov et al. 2012). How-ever, the probability that the VHE event originated from othersources (including background/foreground diffuse emissions)was relatively large, and the significance did not reach 5 σ . Inthis Letter, we report the Fermi -LAT detection of a secondVHE event from the directional vicinity of PKS 0426 -
380 inJanuary 2013, and claim convincingly that it is now the mostdistant VHE emitter currently detected. DATA REDUCTION
The
Fermi -LAT
Pass7 event and spacecraft data ( ft1 and ft2 files, respectively) were downloaded from theLAT Data Server at the Fermi
Science Support Centerwebpage. We took the ft1 photon event data file spanningMission Elapsed Time (MET, measured in seconds from2001 Jan. 1) 239557417 (2008 August 4 15:43:36 UT) to389039485 (2013 April 30 18:31:23 UT) and chose a 10 ◦ radius for the region of interest (ROI) centered at the radioposition of PKS 0426 - v9r27p1 of the Fermi Science Tools . Only the
CLEAN class events from 100 MeV to 300 GeV were selected. Themaximum zenith angle was set to 100 ◦ to avoid contami-nation from Earth limb γ rays. The good time interval wasgenerated by applying a recommended filter expression of (DATA_QUAL==1)&&(LAT_CONFIG==1)&&ABS(ROCK_ANGLE) < and ROI-based zenith angle cut( roicut=yes ). We used the P7CLEAN_V6
InstrumentResponse Functions (IRFs).We first performed unbinned maximum likelihood( gtlike ) analysis for the 4.7-year LAT data by using anXML source model in which the spectral parameters ofall the sources included in the Second Fermi -LAT Catalog(2FGL; Nolan et al. 2012) within 10 ◦ radius were set free,while those within an annulus from 10 ◦ to 15 ◦ were fixedto their 2FGL values. For PKS 0426 -
380 we assumed alog-parabola spectral shape, as measured in the 2FGL cata-log. The template files gal_2yearp7v6_v0.fits and iso_p7v6clean.txt were used to represent the Galacticand isotropic diffuse emission components, respectively .To allow for potential small errors in the flux and spectrumof the Galactic diffuse emission model we multiplied it bya power law in energy whose normalization and index were http://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi Models are defined using the XML language. Available from the
Fermi
Science Support Center (FSSC),http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html free during the fit. Using the output XML file obtained afterrunning gtlike , we ran the gtsrcprob tool to calculatea probability that each detected VHE event originates fromthe direction of PKS 0426 - -
380 with asingle power-law with both the normalization and the photonindex allowed to vary. Then, we ran gtlike and generateda weekly (7-day binned) light curve for PKS 0426 - - gtlike for the entire 4.7-year dataset to avoid unreasonably largeerrors.To construct the γ -ray spectrum of the source, we first se-lected energy intervals chosen as 11 octaves ranging from0.1 to 204.8 GeV (namely, 0.1–0.2, 0.2–0.4, ..., 102.4–204.8 GeV). In the XML source model, the normalizations ofthe surrounding sources within 10 ◦ and of the Galactic and ex-tragalactic diffuse emission components were set free, whileall the other spectral parameters were fixed to the values de-rived for the LAT data accumulated for the selected period.PKS 0426 -
380 was modeled using a broken power-law modelwith the photon indices and break energy fixed to the valuesthat we derived for the entire dataset, but with the normaliza-tion set free. Using this XML file, we ran gtlike for eachenergy bin and generated the spectrum of the source. RESULTS
In Figure 1 we present the LAT count map of
ULTRACLEAN events (a subset of the CLEAN class with thehighest probability of being γ rays) with 5–300 GeV ener-gies around PKS 0426 - ≤ . ◦ ) and are well inside the point spread func-tion (PSF) of Fermi -LAT. A detailed summary of the VHEevents, including their precise localizations and arrival times,is given in Table 1. Based on the gtsrcprob results (seeTable 1), we calculated the null hypothesis probability thatboth events originate from foreground/background (Galac-tic/extragalactic) diffuse emission components, or other sur-rounding 2FGL sources rather than PKS 0426 - . × - . We can thereforereject the null hypothesis at the 5 . σ confidence level (CL),and claim robustly that at least one of the two detected VHE γ rays originates from PKS 0426 - ULTRACLEAN ones, we further visually inspected thetracks in the LAT tracker subsystem after the point ofpair conversion. The first event converted at the top layerof a tower and the long straight paths of the convertedelectron-positron pairs were nicely tracked. The secondevent converted at the third layer of the tracker and similarlyshowed no unusual signatures. The showers in the calorime-ters for both events were also well-behaved. In conclusion,we did not find any problematic features in the case of theanalyzed VHE detections.Figure 2 presents the weekly (7-day binned)
Fermi -LATlight curve of PKS 0426 -
380 within the energy range 0 . - HE γ rays from PKS 0426 -
380 3
TABLE 1D
ETAILED DESCRIPTION OF THE TWO
VHE
EVENTS DETECTED BY
Fermi -LATEnergy ∗ MET R.A. (J2000) Dec. (J2000) Angular separation † gtsrcprob ‡ [GeV] (UT) [deg] [deg] [deg] probability134 285043901.724 67.182 - - OTE . — Both of the events are
ULTRACLEAN class and
FRONT converting. ∗ The energy resolution is of the order of 10% (Ackermann et al. 2012a). † Angular separation is calculated from the radio position of PKS 0426 - ◦ and Dec.= - ◦ (J2000) (Johnston et al. 1995). ‡ The probability that the event belongs to PKS 0426 - gtsrcprob . - . - . - . - . - . - . PKS 0426-380 radio position134 GeV122 GeV
R. A. (J2000) D ec . ( J ) F IG . 1.— Fermi -LAT count map of 5–300 GeV
ULTRACLEAN events (scale= 0.05 ◦ pixel - ) centered on the radio position of PKS 0426 -
380 (red dia-mond). The magenta and yellow crosses indicate the positions of the twoVHE and five 50–100 GeV events, respectively. The dashed circle indi-cates the 68% containment radius (0.12 deg) of
Fermi -LAT PSF for front-converting events above 100 GeV (Ackermann et al. 2012a).
300 GeV, along with the arrival times of γ rays with energiesabove 50 GeV. It is clear that the two VHE events were de-tected during high states of the source. Interestingly however,the VHE detections did not coincide with the flux maximaof the flaring epochs. Complex structures of the source lightcurve, with multiple peaks within each flaring state, precludeus however from speculating if the VHE photons preceded orfollowed the flux maxima at lower photon energies. To furtherinvestigate the temporal and spectral variations within ± . -
300 GeV flux dramatically increaseon the day of the VHE detection. On the other hand, a signif-icant spectral hardening can be noticed on the day of the firstVHE detection, but not on the day of the second one.Figure 4 shows the γ -ray spectrum of PKS 0426 -
380 dur-ing the most energetic flare, derived from the accumulated
Fermi -LAT data between MET 280000000 and 302000000(see Figure 2). No high-energy cutoff expected from theEBL-related attenuation of the γ -ray continuum seems to bepresent up to several tens of GeV, although the highest energy(102 . - . Γ low = P ho t on E n e r gy [ G e V ] . - G e V F l ux [ - pho t on s c m - s - ] F IG . 2.— top panel: Arrival times (MJD) and energies of the seven mostenergetic E >
50 GeV
ULTRACLEAN events detected from the vicinity of thedirection of PKS 0426 -
380 (see Figure 1). bottom panel:
Fermi -LAT weeklybinned light curve of PKS 0426 -
380 (0 . -
300 GeV energy range). Blacktriangles denote the 95% CL flux upper limits (when TS < Fermi -LAT spectrum was constructed(see Figure 4). . ± . Γ high = 2 . ± .
17, and E break = 8 . ± . Fermi -LAT data accumulated over ∼ γ rays, one has to be very careful in interpreting spectralfeatures apparent in the figure, like for example the disconti-nuity around 10 GeV (see Abdo et al. 2010, for other exam-ples). Based on the observed SED, we generated the EBL-corrected spectra of the source (Figure 4) using two differ-ent EBL models (Franceschini et al. 2008; Inoue et al. 2013).The de-absorbed spectra seem to reveal an additional high-energy flat-spectrum component above several tens of GeV(see also ¸Sentürk et al. 2013). However, the significance ofthis feature is marginal and, more importantly, model de-pendent. Nonetheless, the results obtained do suggest thatPKS 0426 -
380 is a very promising target for future follow-up studies with Imaging Atmospheric Cherenkov Telescopes(IACTs) such as H.E.S.S. II and CTA (Actis et al. 2011). DISCUSSION
Three blazars classified as FSRQs have previously beendetected in the VHE range by IACTs, 3C 279 ( z =0 . -
089 ( z = 0 . . - G e V F l ux [ - pho t on s c m - s - ] P o w e r- l a w i nd e x . - G e V F l ux [ - pho t on s c m - s - ] P o w e r- l a w i nd e x F IG . 3.— Fermi -LAT daily binned light curve and power-law indices (0.1–300 GeV energy range) derived for PKS 0426 -
380 around the times ( ± <
10. Note the4.7-year average 0.1–300 GeV photon flux was (1 . ± . × - photonscm - s - . Abramowski et al. 2013), and 4C +21.35 ( z = 0 . - z = 0 .
61 (Becherini et al.2012, though the redshift is still tentative) and PKS 1424+240at z ≥ . - z = 1 . -
380 is just around the horizonfor ≃
100 GeV γ rays (namely, EBL-related optical depth ofthe Universe τ ∼
1) as recently determined by
Fermi -LAT (Ackermann et al. 2012b, and Figure 1 therein), hencethe VHE detection from the z = 1 . - L Mg II ≃ . × erg s - (Sbarufatti et al. 2005), imply-ing a BLR luminosity L BLR ≃ . × erg s - using thescaling relation L BLR ≃ . × L Mg II (Wang et al. 2004).Spectral modeling of the accretion-related continuum in thesource (Ghisellini et al. 2011; Sbarrato et al. 2012) yields M BH ≃ × M ⊙ . Based on the [Mg II] line FWHMof 4 ,
700 km s - , and the observed V -band magnitude of18.6 (assuming negligible starlight contamination), we de-rived a slightly larger value of M BH ≃ (0 . - . × M ⊙ ,using the scaling relations from Wang et al. (2009) andVestergaard & Osmer (2009). This corresponds to the Ed-dington luminosity L Edd ≃ erg s - , the ratio L BLR / L Edd ≃ - , and the accretion rate at the level of Λ acc = L disk / L Edd ∼
30% (assuming the standard bolometric correction factor L disk ≃ × L B for the B -band source luminosity L B ≃ . × erg s - ). The derived high accretion rate is consistent -11 -10 -9 E d N / d E [ e r g c m - s - ] F IG . 4.— SED of PKS 0426 -
380 derived from the
Fermi -LAT data accu-mulated during the most energetic flaring state spanning MET 280000000(17:46:38 UT on 2009 November 15) to 302000000 (08:53:18 UT on 2010July 28; see also the black horizontal line in Figure 2). The observed spectrumis denoted by black squares and the highest energy bin is a 95% confidencelevel upper limit. A broken power-law model which maximizes the likeli-hood for the 0.1–300 GeV
Fermi -LAT data is indicated with black dashedline. The spectra corrected for the EBL-related attenuation, using the EBLmodels of Franceschini et al. (2008) and Inoue et al. (2013), are representedby red circles and blue triangles, respectively. with PKS 0426 -
380 being a FSRQ.The intense γ -ray emission of FSRQs is widely thoughtto arise due to inverse-Compton up-scattering of low-energyphotons generated outside of a jet by ultra-relativistic elec-trons accelerated within the innermost parts of the relativisticoutflows (Sikora et al. 2009; Ghisellini & Tavecchio 2009). Ifthis blazar emission zone is located at sub-parsec distancesfrom the central black hole, as is often anticipated in the litera-ture, then the abundant circum-nuclear photon fields providedby the BLR and/or hot dust are expected to attenuate the VHEblazar emission substantially due to the photon-photon pairproduction, leading to the formation of breaks and cut-offs inthe γ -ray continua of FSRQs (see in this context the discus-sion in Poutanen & Stern 2010; Tanaka et al. 2011). The ob-served sharp break ( Γ high - Γ low ∼ .
8) at ∼ γ -ray spec-trum when corrected for the cosmological absorption showeda flattened shape at energies above 10 GeV. This flat compo-nent, if connected to the sub-TeV range, should come fromanother emission region outside the BLR to avoid γγ attenu-ation.Care must be taken not to over-interpret such high-energyfeatures (flattening) in unfolded blazar spectra constructed us-ing Fermi -LAT data accumulated over longer periods of time,since those may simply arise due to averaging over differ-ent activity states characterized by different spectral prop-erties (see the related discussion and analysis in Abdo et al.2011, concerning the well-known BL Lac object Mrk 501).Still, the results presented here for PKS 0426 -
380 are inprinciple consistent with the emergence of an additionalvery high-energy flat-spectrum component during the flar-ing states of the source. One possibility for the produc-tion of such a component could be electron pile-up at thehighest energies due to the efficient and continuous ac-celeration processes limited only by the radiative losses(Stawarz & Petrosian 2008; Lefa et al. 2011). Another pos-sibility could be an additional hadronic emission componentdominating occasionally the source spectrum in the VHErange (e.g., Böttcher et al. 2009; Dermer et al. 2012). InHE γ rays from PKS 0426 -
380 5particular, assuming that PKS 0426 -
380 generates cosmicrays with energies above 10 EeV, or γ rays above 30 TeV,the induced intergalactic cascade emission may provide anon-negligible contribution within the VHE range of thesource spectrum (Essey & Kusenko 2010; Murase et al. 2012;Takami et al. 2013). The time scale of the expected delay be-tween the production of the primary ultra-high energy cosmicrays/ γ -ray photons and the observed re-processed VHE signalis ∼ E /
100 GeV) - ( B / - G) ( λ γγ /
100 Mpc) days and ∼
30 ( E /
100 GeV) - ( B / - G) ( λ BH / z =1 .
1, respectively, where B is the intergalactic magnetic fieldstrength, λ γγ is the mean free path of the pair creation,and λ BH is the mean free path of the Bethe-Heitler process(Murase 2012). Rather speculatively, if there are axion-like particles, photon and axion mixing in the intergalacticmedium may in addition enhance the EBL-absorbed photonflux (e.g., Sánchez-Conde et al. 2009). Lorentz invariance vi-olation, on the other hand, could inhibit pair production (e.g.,Protheroe & Meyer 2000).In summary, with Fermi -LAT we have detected two VHE γ rays from close to the direction of a high-redshift FSRQ,PKS 0426 -
380 at z = 1 .
1. Both of the events were detectedduring high γ -ray states of the source, although the specific VHE arrival times did not coincide with any particularly large0 . -
300 GeV source flux. Only in the first case was theobserved LAT spectrum harder than usual. The very com-plex γ -ray variability patterns revealed by the Fermi -LATdata for PKS 0426 -
380 calls for sensitive follow-up studieswith simultaneous VHE coverage provided by IACTs such asH.E.S.S. II or future CTA.We appreciate the referee’s critical reading and valuablecomments. Y.T.T. is supported by Kakenhi 24840031. Workby C.C.C. at NRL is supported in part by NASA DPR S-15633-Y. Ł.S. was supported by Polish NSC grant DEC-2012/04/A/ST9/00083.The
Fermi -LATCollaboration acknowledges support froma number of agencies and institutes for both development andthe operation of the LAT as well as scientific data analysis.These include NASA and DOE in the United States, CEA/Irfuand IN2P3/CNRS in France, ASI and INFN in Italy, MEXT,KEK, and JAXA in Japan, and the K. A. Wallenberg Foun-dation, the Swedish Research Council and the National SpaceBoard in Sweden. Additional support from INAF in Italy andCNES in France for science analysis during the operationsphase is also gratefully acknowledged.
Facilities: