Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on its redshift
VERITAS Collaboration, V. A. Acciari, Fermi Collaboration, A. A. Abdo, S. D. Barber, D. M. Terndrup
aa r X i v : . [ a s t r o - ph . C O ] D ec Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on its redshift VERITAS collaboration:
V. A. Acciari v , E. Aliu v , T. Arlen v , T. Aune v , M. Bautista v , M. Beilicke v , W. Benbow v , M. B¨ottcher v , D. Boltuch v , S. M. Bradbury v , J. H. Buckley v , V. Bugaev v , K. Byrum v , A. Cannon v , A. Cesarini v , Y. C. Chow v , L. Ciupik v , P. Cogan v , W. Cui v , C. Duke v , A. Falcone v , J. P. Finley v , G. Finnegan v , L. Fortson v , A. Furniss v , ∗ , N. Galante v , D. Gall v , G. H. Gillanders v , S. Godambe v , J. Grube v , R. Guenette v , G. Gyuk v , D. Hanna v , J. Holder v , C. M. Hui v , T. B. Humensky v , P. Kaaret v , N. Karlsson v , M. Kertzman v , D. Kieda v , A. Konopelko v , H. Krawczynski v , F. Krennrich v , M. J. Lang v , S. LeBohec v , G. Maier v , S. McArthur v , A. McCann v , M. McCutcheon v , J. Millis v ,v , P. Moriarty v , T. Nagai v , R. A. Ong v , A. N. Otte v , ∗ , D. Pandel v , J. S. Perkins v , A. Pichel v , M. Pohl v , J. Quinn v , K. Ragan v , L. C. Reyes v , P. T. Reynolds v , E. Roache v , H. J. Rose v , M. Schroedter v , G. H. Sembroski v , G. Demet Senturk v , A. W. Smith v , D. Steele v , S. P. Swordy v , M. Theiling v , S. Thibadeau v , A. Varlotta v , V. V. Vassiliev v , S. Vincent v , R. G. Wagner v , S. P. Wakely v , J. E. Ward v , T. C. Weekes v , A. Weinstein v , T. Weisgarber v , D. A. Williams v , S. Wissel v , M. Wood v , B. Zitzer v , Fermi LAT collaboration:
A. A. Abdo , , M. Ackermann , M. Ajello , L. Baldini , J. Ballet , G. Barbiellini , , D. Bastieri , , B. M. Baughman , K. Bechtol , R. Bellazzini , B. Berenji , R. D. Blandford , E. D. Bloom , E. Bonamente , , A. W. Borgland , J. Bregeon , A. Brez , M. Brigida , , P. Bruel , T. H. Burnett , G. A. Caliandro , , R. A. Cameron , P. A. Caraveo , J. M. Casandjian , E. Cavazzuti , C. Cecchi , , ¨O. C¸ elik , , , A. Chekhtman , , C. C. Cheung , J. Chiang , ∗ , S. Ciprini , , R. Claus , J. Cohen-Tanugi , J. Conrad , , , S. Cutini , C. D. Dermer , A. de Angelis , F. de Palma , , E. do Couto e Silva , P. S. Drell , A. Drlica-Wagner , R. Dubois , , , C. Farnier , C. Favuzzi , , S. J. Fegan , W. B. Focke , P. Fortin , M. Frailis , Y. Fukazawa , P. Fusco , , F. Gargano , D. Gasparrini , N. Gehrels , , S. Germani , , B. Giebels , N. Giglietto , , P. Giommi , F. Giordano , , T. Glanzman , G. Godfrey , I. A. Grenier , J. E. Grove , L. Guillemot , , S. Guiriec , Y. Hanabata , E. Hays , R. E. Hughes , M. S. Jackson , , , G. J´ohannesson , A. S. Johnson , W. N. Johnson , T. Kamae , H. Katagiri , J. Kataoka , , N. Kawai , , M. Kerr , J. Kn¨odlseder , M. L. Kocian , M. Kuss , J. Lande , L. Latronico , F. Longo , , F. Loparco , , B. Lott , , M. N. Lovellette , P. Lubrano , , G. M. Madejski , A. Makeev , , M. N. Mazziotta , J. E. McEnery , C. Meurer , , P. F. Michelson , W. Mitthumsiri , T. Mizuno , A. A. Moiseev , , C. Monte , , M. E. Monzani , A. Morselli , I. V. Moskalenko , S. Murgia , P. L. Nolan , J. P. Norris , E. Nuss , T. Ohsugi , N. Omodei , E. Orlando , J. F. Ormes , D. Paneque , D. Parent , , V. Pelassa , M. Pepe , , M. Pesce-Rollins , F. Piron , T. A. Porter , S. Rain`o , , R. Rando , , M. Razzano , A. Reimer , , O. Reimer , , T. Reposeur , , A. Y. Rodriguez , M. Roth , F. Ryde , , H. F.-W. Sadrozinski , D. Sanchez , A. Sander , P. M. Saz Parkinson , J. D. Scargle , C. Sgr`o , M. S. Shaw , E. J. Siskind , P. D. Smith , G. Spandre , P. Spinelli , , M. S. Strickman , D. J. Suson , H. Tajima , H. Takahashi , T. Tanaka , J. B. Thayer , J. G. Thayer , D. J. Thompson , L. Tibaldo , , , D. F. Torres , , G. Tosti , , A. Tramacere , , Y. Uchiyama , , T. L. Usher , V. Vasileiou , , , N. Vilchez , V. Vitale , , A. P. Waite , P. Wang , B. L. Winer , K. S. Wood , T. Ylinen , , , M. Ziegler , and S. D. Barber o , D. M. Terndrup o ,o v1 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics,Amado, AZ 85645, USA v2 Department of Physics and Astronomy and the Bartol Research Institute, University ofDelaware, Newark, DE 19716, USA v3 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095,USA v4 Santa Cruz Institute for Particle Physics and Department of Physics, University of Cal-ifornia, Santa Cruz, CA 95064, USA v5 Physics Department, McGill University, Montreal, QC H3A 2T8, Canada v6 Department of Physics, Washington University, St. Louis, MO 63130, USA v7 Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Athens,OH 45701 v8 School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK v9 Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA v10
School of Physics, University College Dublin, Belfield, Dublin 4, Ireland v11
School of Physics, National University of Ireland, Galway, Ireland v12
Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605,USA v13
Department of Physics, Purdue University, West Lafayette, IN 47907, USA v14
Department of Physics, Grinnell College, Grinnell, IA 50112-1690, USA v15
Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State Uni-versity, University Park, PA 16802, USA v16
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112,USA v17
Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA v18
Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, 4 –IA 52242, USA v19
Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037,USA v20
Department of Physics, Pittsburg State University, 1701 South Broadway, Pittsburg, KS66762, USA v21
Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA v22 now at Department of Physics, Anderson University, 1100 East 5th Street, Anderson,IN 46012 v23
Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, DublinRoad, Galway, Ireland v24
Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 - Sucursal 28,(C1428ZAA) Ciudad Autnoma de Buenos Aires, Argentina v25
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA v26
Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bish-opstown, Cork, Ireland v27
Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA National Research Council Research Associate, National Academy of Sciences, Wash-ington, DC 20001, USA W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astro-physics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory,Stanford University, Stanford, CA 94305, USA Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy Laboratoire AIM, CEA-IRFU/CNRS/Universit´e Paris Diderot, Service d’Astrophysique,CEA Saclay, 91191 Gif sur Yvette, France Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy 5 – Dipartimento di Fisica, Universit`a di Trieste, I-34127 Trieste, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy Dipartimento di Fisica “G. Galilei”, Universit`a di Padova, I-35131 Padova, Italy Department of Physics, Center for Cosmology and Astro-Particle Physics, The OhioState University, Columbus, OH 43210, USA Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy Dipartimento di Fisica, Universit`a degli Studi di Perugia, I-06123 Perugia, Italy Dipartimento di Fisica “M. Merlin” dell’Universit`a e del Politecnico di Bari, I-70126Bari, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy Laboratoire Leprince-Ringuet, ´Ecole polytechnique, CNRS/IN2P3, Palaiseau, France Department of Physics, University of Washington, Seattle, WA 98195-1560, USA INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Center for Research and Exploration in Space Science and Technology (CRESST), NASAGoddard Space Flight Center, Greenbelt, MD 20771, USA University of Maryland, Baltimore County, Baltimore, MD 21250, USA George Mason University, Fairfax, VA 22030, USA Laboratoire de Physique Th´eorique et Astroparticules, Universit´e Montpellier 2,CNRS/IN2P3, Montpellier, France Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm,Sweden Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. 6 –Wallenberg Foundation Dipartimento di Fisica, Universit`a di Udine and Istituto Nazionale di Fisica Nucleare,Sezione di Trieste, Gruppo Collegato di Udine, I-33100 Udine, Italy Universit´e de Bordeaux, Centre d’ ´Etudes Nucl´eaires Bordeaux Gradignan, UMR 5797,Gradignan, 33175, France CNRS/IN2P3, Centre d’ ´Etudes Nucl´eaires Bordeaux Gradignan, UMR 5797, Gradignan,33175, France Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima739-8526, Japan University of Maryland, College Park, MD 20742, USA University of Alabama in Huntsville, Huntsville, AL 35899, USA Department of Physics, Royal Institute of Technology (KTH), AlbaNova, SE-106 91Stockholm, Sweden Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551,Japan Waseda University, 1-104 Totsukamachi, Shinjuku-ku, Tokyo, 169-8050, Japan Cosmic Radiation Laboratory, Institute of Physical and Chemical Research (RIKEN),Wako, Saitama 351-0198, Japan Centre d’ ´Etude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F-30128 ToulouseCedex 4, France Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma,Italy Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA Max-Planck Institut f¨ur extraterrestrische Physik, 85748 Garching, Germany Santa Cruz Institute for Particle Physics, Department of Physics and Department of 7 –Received ; accepted Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064,USA Institut f¨ur Astro- und Teilchenphysik and Institut f¨ur Theoretische Physik, Leopold-Franzens-Universit¨at Innsbruck, A-6020 Innsbruck, Austria Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000,USA NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN46323-2094, USA Instituci´o Catalana de Recerca i Estudis Avan¸cats, Barcelona, Spain Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara,Kanagawa 229-8510, Japan Dipartimento di Fisica, Universit`a di Roma “Tor Vergata”, I-00133 Roma, Italy School of Pure and Applied Natural Sciences, University of Kalmar, SE-391 82 Kalmar,Sweden o1 Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma,440 W. Brooks St., Norman, OK 73019, USA o2 Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus,OH 43210, USA o3 National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230, USA * Corresponding author, [email protected], [email protected], [email protected] 8 –
ABSTRACT
We report the first detection of very-high-energy (VHE) gamma-ray emissionabove 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift.The photon spectrum above 140 GeV measured by VERITAS is well described bya power law with a photon index of 3 . ± . stat ± . syst and a flux normalizationat 200 GeV of (5 . ± . stat ± . syst ) × − TeV − cm − s − , where stat and systdenote the statistical and systematical uncertainty, respectively. The VHE flux issteady over the observation period between MJD 54881 and 55003 (2009 Febru-ary 19 to June 21). Flux variability is also not observed in contemporaneous highenergy observations with the Fermi
Large Area Telescope (LAT). Contempora-neous X-ray and optical data were also obtained from the
Swift
XRT and MDMobservatory, respectively. The broadband spectral energy distribution (SED) iswell described by a one-zone synchrotron self-Compton (SSC) model favoring aredshift of less than 0.1. Using the photon index measured with
Fermi in com-bination with recent extragalactic background light (EBL) absorption models itcan be concluded from the VERITAS data that the redshift of PKS 1424+240 isless than 0.66.
Subject headings:
BL Lacertae objects: individual ( PKS 1424+240 = VER J1427+237); gamma rays: observations γ -ray emission above 100 GeV 9 –
1. Introduction PKS 1424+240 was detected as a radio source by Condon et al. (1977). It was classified as a blazar by Impey & Tapia (1988) from optical polarization studies. Fleming et al. (1993) verified the polarization results and also reported non-thermal X-ray radiation, further strengthening the classification. Blazar emission is dominated by non-thermal radiation, which is thought to be related to charged particle acceleration near a massive compact object in the center of the host galaxy, or in outflowing relativistic jets. The SED is characterized by two peaks. The lower peak is widely accepted to be synchrotron radiation from relativistic electrons and occurs in the IR to X-ray bands. The higher energy peak is in the gamma-ray band, sometimes at energies as high as a few TeV, and can be created via either inverse-Compton scattering by relativistic electrons or hadronic interactions (for a review see B¨ottcher 2007, and references therein). The position of the synchrotron peak of PKS 1424+240 has not been measured, but it can be constrained from optical and X-ray data to be between 10 Hz and 10 Hz. Depending on the definition used, PKS 1424+240 is either an intermediate-frequency-peaked BL Lac (IBL) (Nieppola et al. 2006) or a high-frequency-peaked BL Lac (HBL) (Padovani & Giommi 1996; Abdo et al. 2009a). Gamma-ray emission from PKS 1424+240 was not detected by EGRET (Fichtel et al. Fermi
LAT pair-conversion telescope (Abdo et al. 2009b,c). The reported flux above 100 MeV of (6 . ± . × − cm − s − and hard spectral index Γ = 1 . ± .
07 ( dN/dE ∝ E − Γ ) triggered VERITAS observations. The redshift of PKS 1424+240 is not known. Scarpa & Falomo (1995) have derived a lower limit on the redshift of z > z > both assuming a minimum luminosity of the host galaxy. The latter authors also reported evidence that the ratio of the nucleus to host luminosity is much larger than 100, which is
10 –typical for BL Lac objects but complicates photometric determination of the redshift. We report the detection of PKS 1424+240 in VHE gamma rays and contemporaneous observations with Fermi , Swift , and the MDM observatory. Shortly after the VHE discovery (Ong 2009), it was confirmed by the MAGIC collaboration (Teshima 2009). This discovery marks the first Fermi -motivated VHE discovery.
2. Observations and Analysis of VERITAS Data The VERITAS observatory, located in southern Arizona at 1.3 km a.s.l., is described in detail in Weekes et al. (2002) and Holder et al. (2006). PKS 1424+240 was observed with VERITAS between 2009 February 19 and June 21 at zenith angles between 7 ◦ and 30 ◦ . The observations were performed in wobble mode (Fomin et al. 1994) with a 0 . ◦ offset, enabling simultaneous background estimation. About one third of the data were taken during low levels of moonlight. About 65% of the observations were conducted with only three telescopes due to the relocation of one telescope, which began in May and was completed in August 2009. Of the 37.3 hours of data, 28.5 hours survive standard data quality selection. Events are reconstructed following the procedure in Acciari et al. (2008). The recorded shower images are parameterized by their principal moments, giving an efficient suppression of the far more abundant cosmic-ray background. Two separate sets of cuts are applied to reject background events, hereafter called soft and medium . These cuts are applied to the parameters mean scaled width (MSW), and mean scaled length (MSL), apparent altitude of the maximum Cherenkov emission (shower maximum), and θ , the squared direction between the position of PKS 1424+240 and the reconstructed origin of the event. Studies on independent data sets show that a shower-maximum cut significantly improves
11 –the low energy sensitivity.
Soft cuts have a higher sensitivity for sources with soft photon spectra because of a lower energy threshold resulting from a minimum size cut of 50 photoelectrons. In the medium cuts a minimum size cut of 100 photoelectrons is applied.
Size is a measure of the recorded photoelectrons from a shower and a good indicator of the energy of the primary. For the soft -cuts analysis the remaining cuts are MSW < . MSL < .
30, shower maximum > θ < (0 . ◦ ) , and MSW < .
04, MSL < . shower maximum > θ < (0 . ◦ ) for the medium cuts. The cuts have been optimized a priori to yield the highest sensitivity for a source with 5% of the Crab Nebula gamma-ray flux. The results are independently reproduced with two different analysis packages explained in Cogan (2008) and Daniel (2008). In the soft -cuts analysis, 1907 on-source events remain out of 1 . × triggered events. The background calculated with the reflected-region method (Berge et al. 2007) is θ distribution. The statistical significance of the observed excess is 8.5 standard deviations, σ , calculated with Equation 17 of Li & Ma (1983), and including a trials factor of two for the two sets of cuts. In the medium -cuts analysis the post-trials significance is 4 . σ (329 on-source events with an estimated background of 244). The angular distribution of the excess events is consistent with a point source. The center of gravity of the excess is h m , s ± sstat , 23 ◦ ′ ± ′ stat coinciding with the position of PKS 1424+240 in radio (Fey et al. 2004). The VERITAS source name is VER J1427+237. Figure 2 shows the light curve of PKS 1424+240 in different energy bands for the time period overlapping the VERITAS observations. The flux measured by VERITAS above
140 GeV is ∼
5% of the Crab Nebula flux. The VERITAS data from each dark period are combined into a single bin to produce a light curve, which is consistent with a constant flux, The ∼ χ =0.3 for 3 degrees of freedom (d.o.f.). However, even a doubling in flux would have been difficult to detect. There is no evidence for strong flaring episodes on shorter timescales. Figure 3 shows the differential photon spectra derived with the soft -cuts and medium -cuts analyses, with one overlapping flux point at 260 GeV. The fraction of events that are used both in the last bin in the soft -cuts analysis and in the second bin in the medium -cuts analysis is about 2%, small enough to allow a combined fit of the flux points from the two analyses, with the more significant soft -cuts result at 260 GeV used in the fit. The combined spectrum is well parameterized ( χ =2.2 for 4 d.o.f.) by a power law dN/dE = F · ( E/E ) − Γ , where the photon index Γ is 3 . ± . stat ± . syst and F is (5 . ± . stat ± . syst ) × − TeV − cm − s − for E = 200 GeV. The combined spectrum is consistent with the fit of the soft -cuts points alone, albeit with half the uncertainty on the photon index.
3. Multiwavelength Observations
Gamma-ray observations with
Fermi -LAT (100 MeV to 300 GeV), X-ray and optical observations with
Swift
XRT (0.2–10 keV) and UVOT (170–650 nm), and optical observations in the R, V and I bands at the MDM observatory were obtained simultaneously or quasi-simultaneously with the VERITAS observations.
The LAT pair-conversion telescope on board the
Fermi
Gamma-ray Space Telescope continuously monitors the entire sky between 100 MeV and several hundred GeV (Atwood et al. 2009). The LAT data overlapping with the VERITAS observations were analyzed by selecting “diffuse” class events that have the highest probability of being photons. Further event selection was done by only accepting events that come within a 15 ◦ radius from PKS 1424+240 and have energies between 0.1 and 300 GeV. Events with zenith angles above
13 –105 ◦ were excluded to limit contamination by gamma rays coming from the Earth’s albedo. The analysis of the photon spectrum and light curve were performed with the standard likelihood analysis tools available from HEASARC
ScienceTools v9r15p2 . Accidental coincidences with charged cosmic rays in the detector were accounted for using instrument response functions
P6 V3 DIFFUSE . The background model used to extract the gamma-ray signal includes a Galactic diffuse emission component and an isotropic component . The isotropic component includes contributions from the extragalactic diffuse emission as well as from residual charged particle backgrounds. The spectral shape of the isotropic component was derived from residual high latitude events after the Galactic contribution had been modeled. The background model also takes into account unresolved gamma-ray sources in the region of interest, thus avoiding a bias in the spectral reconstruction. To further reduce systematic uncertainties in the analysis, the normalization and spectral parameters in the background model were allowed to vary freely during the spectral point fitting. The
Fermi -LAT flux measurements are shown in the broadband SED in Figure 4. The flux values are unfolded by assuming an underlying power-law, giving an integrated flux over the 0.1–300 GeV band (7 . ± . stat ± . sys ) × − cm − s − , and a differential photon spectral index Γ LAT = 1 . ± . stat ± . sys . The light curve of the integral flux above 100 MeV is plotted with 10-day bins in Figure 2. A fit with a constant yields a χ = 11 . Target of opportunity observations of nearly 16 ksec, distributed over ten observing periods, were obtained with
Swift (Gehrels et al. 2004) following the detection of VHE emission from PKS 1424+240. The data reduction and calibration of the XRT data were completed with the HEASoft v6.6.3 standard tools. The XRT data were taken in http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html 14 –photon-counting mode and contained modest pile up for nine of the observations, which was taken into account by masking a region with 3-6 pixel radius around the source. The outer radius chosen for the signal region was 20 pixels and a background region of similar size was chosen about 5 arcminutes off source. X-ray energy spectra could be extracted from all observing periods and are well described by an absorbed power law using the fixed Galactic column density of neutral hydrogen from Dickey & Lockman (1990) ( N h = 0 . × cm − ). The fit spectral index varies between 2.1 and 2.9 (photon index between 3.1 and 3.9) with a typical statistical uncertainty of 0.1, while the normalization changes between 1 . × − and 0 . × − photons keV − cm − s − at 1 keV with a typical uncertainty of 0 . × − keV − cm − s − . For the modeling of the SED we use the average spectrum shown in Figure 4. The light curve shows that the X-ray flux is variable over the ten days of observation. A fit to a constant flux yields a χ of 60 for 9 d.o.f.. UVOT observations were taken in the six V, B U, W1, M2 and W2 bands and were calibrated using standard techniques (Poole et al.
Schlegel et al. (1998) with a galactic spectral extinction model (Fitzpatrick 1999) obtaining of z=0. The corresponding light curves are shown in Figure 2.
Data in the optical bands were also obtained with the 1.3 m telescope and 4K imager of the MDM observatory located on the west ridge of Kitt Peak near Tucson,
Arizona. The CCD was operated in unbinned mode, which produces an image scale of observation. Physical magnitudes were computed from differences in the instrumental magnitudes from the three standard stars in Fiorucci & Tosti (1996), assuming that the magnitudes quoted in that paper are exact. The magnitudes were then corrected for
15 –Galactic extinction using extinction coefficients calculated following Schlegel et al. (1998), taken from NED , and were then converted into νF ν fluxes. During the 14-day span of the optical photometry, the visual brightness increased by 14% and colors became slightly bluer.
4. Redshift Upper Limit
The observed gamma-ray spectrum above 100 GeV is affected by the absorption of gamma rays via pair conversion with EBL photons (Nikishov A. I. 1962; Gould & Schr´eder spectrum. We estimate an upper limit of the redshift of PKS 1424+240 by assuming an intrinsic VHE spectrum and making use of the recent advances in extragalactic background light (EBL) modeling.
We assume that the intrinsic spectrum above 140 GeV can be described by a power law.
The hardest photon index that we consider is 1.7, which is the value from the simultaneous
Fermi observations. The use of Fermi observations allows a model independent estimate of the hardest possible intrinsic spectrum (see also Abdo et al. 2009d). The power law with an index of 1.7 is absorbed using recent EBL models from Franceschini et al. (2008),
Gilmore et al. (2009), and Finke et al. (2009). After absorption the shape of the spectrum is fit to the VERITAS spectrum with the normalization as a free parameter, and the best estimate of the redshift is determined by minimizing χ . For an intrinsic index of 1.7 this best fit redshift is z= 0 . ± . stat ± . syst with a χ =4 and 5 d.o.f. . The systematic uncertainty is estimated from the differences in the EBL models. Instead of assuming no break in the photon spectrum, a more likely scenario is that the intrinsic spectrum softens with increasing energy. In this case an index of 1.7 is an upper z < . with a 95% confidence level.
5. Spectral Modeling
The spectral energy distribution, comprising data from all of the observations, is shown in Figure 4. We model the SED using an improved version of the leptonic one-zone jet model of B¨ottcher & Chiang (2002). These calculations include time-dependent particle injection and evolution, and they allow for quasi-equilibrium solutions in which a slowly varying broken power-law electron distribution arises from a single power-law injection function, dn inj /dγ ∝ γ − q with a low- and high-energy cutoff γ and γ , respectively. All model fits presented here are in the fast-cooling regime, with the cooling break at γ . We define the magnetic-field equipartition ǫ B as ǫ B ≡ L B /L e with L B the Poynting flux derived from the magnetic energy density and L e the energy flux of the electrons propagating along the jet. The corresponding partition fraction for an electron-proton plasma assuming L p = 10 × L e of cold protons would be one order of magnitude lower. For an in-depth description of this quasi-equilibrium jet model, see Acciari et al. (2009). There are few observational constraints on the model parameters for PKS 1424+240 and the redshift is unknown. No superluminal motion has been resolved in this object, and it has not been monitored well enough to firmly establish a minimum variability timescale to constrain the size of the emitting region. The different sizes of the emission region R B assumed here are compatible with the X-ray variability timescale of about a day. We therefore consider a range of plausible redshifts and adopt model parameters which were typically adequate for modeling other VHE blazars. The redshifts investigated range from z = 0 .
05, similar to the redshift of the nearby HBLs Mrk 421 and Mrk 501, to z = 0 .
7. This covers the redshift range determined in the previous section and is just above the lower
17 –limit set by Sbarufatti et al. (2005), z > . The shape of the high-energy part of the electron spectrum is well constrained by the rather steep slope of the X-ray spectrum, which has an average photon index Γ X − ray ∼ . In all fits, the relativistic electrons are injected into the emission region with a fixed q = 5 . Lacking direct constraints on the viewing angle θ obs , it was chosen such that the Doppler factor D = (Γ[1 − β Γ cos θ obs ]) − = Γ, where Γ is the bulk Lorentz factor of the emitting material, and β Γ c is the velocity. The model parameters that were varied are shown in Table
1. Figure 4 shows the fits, after EBL absorption using the model of Gilmore et al. (2009).
The SED modeling shows that a reasonable fit can in principle be achieved for any redshift in the considered range. However, the inset in Figure 4 illustrates that above z ∼ .
2, the model VHE gamma-ray spectrum becomes increasingly too steep compared with the observed VERITAS spectrum. Furthermore, for redshifts z > . require unreasonably large Doppler factors of D >
50. We note that in particular for the lowest redshift considered, z = 0 .
05, a good fit can be achieved with almost equipartition between magnetic-field and electron energy densities.
An attempt to improve the fit in the gamma-ray bands, by including an external
Compton component, results in a steeper VHE gamma-ray spectrum. This is in conflict with the VERITAS spectrum and a worse representation of the
Fermi spectrum. We therefore conclude that a leptonic fit to the SED of PKS 1424+240 during the VERITAS observation is possible with a pure SSC model very close to equipartition, in particular if the redshift of the source is z < .
18 –
6. Summary
We report the detection of PKS 1424+240 in VHE gamma-rays. The observation with
VERITAS was motivated by the release of the first
Fermi source lists (Abdo et al. 2009b,c) and this is the first time that
Fermi observations have led to the discovery of a new source in the adjacent VHE band.
The VHE spectrum of PKS 1424+240 has a photon index of 3 . ± . stat ± . sys , whereas the spectrum in the Fermi energy range has a photon index of 1 . ± . stat ± . sys , indicating a break in the spectrum at several tens of GeV. The break can be explained by a one-zone SSC model assuming a wide range of redshifts or could result from EBL absorption if the redshift is about 0.5 and the intrinsic photon index is 1.7, from which a redshift upper limit of 0.66 is inferred. The modeling favors a lower redshift but cannot exclude that PKS 1424+240 is among the most distant sources detected in the VHE regime. PKS 1424+240 is the third extragalactic source detected in the VHE regime with an unknown or uncertain redshift. It is evident that increased efforts are needed to determine the redshifts of VHE detected blazars. A redshift measurement will allow a better understanding of the source-intrinsic mechanisms and the absorption effects which go along with the gamma-ray propagation.
VERITAS is supported by grants from the US Department of Energy, the US National
Science Foundation, and the Smithsonian Institution, by NSERC in Canada, by Science
Foundation Ireland, and by STFC in the UK. We acknowledge the excellent work of the technical support staff at the FLWO and the collaborating institutions in the construction and operation of the instrument. N. O. acknowledges the receipt of a Feodor Lynen fellowship of the Alexander von Humboldt Foundation.
The
Fermi
LAT Collaboration acknowledges support from a number of agencies
19 –and institutes for both development and the operation of the LAT as well as scientific data analysis. These include NASA and DOE in the United States, CEA/Irfu and
IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space
Board in Sweden. Additional support from INAF in Italy and CNES in France for science analysis during the operations phase is also gratefully acknowledged.
This research has made use of the SIMBAD database, operated at CDS, Strasbourg,
France.
Facilities:
VERITAS, Swift, Fermi.
20 –
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This manuscript was prepared with the AAS L A TEX macros v5.2. 23 – ] [ deg θ E ve n t s Fig. 1.— Distribution of θ for VERITAS events selected with soft cuts. The points witherror bars denote the on-source events. The background is shown by the shaded histogram.The dashed vertical line shows the applied θ -cut. The expected distribution for a pointsource is given by the dotted line. 24 – -12 X 10
MJD 54880 54900 54920 54940 54960 54980 55000 × Fermi LAT (0.1 - 300 GeV) -8 X 10
XRT (2 - 10 KeV) -3 X 10
UVOT
U V B W1 M2 W2
MDM Observatory I R V
MJD - s - c m F l u x Fig. 2.— Light curves of PKS 1424+240 in VHE gamma rays (VERITAS), HE gamma rays(
Fermi -LAT), X-rays (
Swift
XRT), UV (
Swift
UVOT) and optical (
Swift
UVOT, MDM).The X-ray, UV and optical light curves cover the time period indicated in the upper twolight curves by the shaded region. The horizontal bars in the VHE and HE light curvesgive the range over which the flux has been integrated. The HE upper limit is at the 95%confidence level. 25 –
Energy [TeV]0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 - s - c m - T e V d N / d E -14 -13 -12 -11 -10 -9 Fig. 3.— The time averaged differential photon spectrum of PKS 1424+240 measured byVERITAS between February 19 and June 21, 2009. The triangles are from the soft -cutsanalysis and the squares from the medium -cuts analysis. The flux point at 260 GeV isreconstructed in both analysis. The solid lines shows the fit with a power law. The shadedarea shows the systematic uncertainty of the fit, which is dominated by a 20% uncertaintyon the energy scale. 26 –Table 1: SSC fit parameters for PKS 1424+240 as a function of assumed redshift.Parameter z = 0 . z = 0 . z = 0 . z = 0 . z = 0 . z = 0 . z = 0 . L e [10 erg s − ] 1.60 4.12 10.7 18.9 29.2 47.1 88.8 L B [10 erg s − ] 1.66 5.47 16.9 31.1 45.9 49.8 66.2 γ [10 ] 3.7 3.7 3.6 3.4 3.2 3.6 3.7 γ [10 ] 4.0 4.0 4.0 4.0 4.5 4.0 4.0 D