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1. INTRODUCTION of the Beam (See, E.G., Mannheim 1993) View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by MURAL - Maynooth University Research Archive Library THE ASTROPHYSICAL JOURNAL, 511:149È156, 1999 January 20 ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. MEASUREMENT OF THE MULTI-TeV GAMMA-RAY FLARE SPECTRA OF MARKARIAN 421 AND MARKARIAN 501 F. KRENNRICH,1 S. D. BILLER,2 I. H. BOND,3 P. J. BOYLE,4 S. M. BRADBURY,3 A. C. BRESLIN,4 J. H. BUCKLEY,5 A. M. BURDETT,3 J. BUSSONS GORDO,4 D. A. CARTER-LEWIS,1 M. CATANESE,1 M. F. CAWLEY,6 D. J. FEGAN,4 J. P. FINLEY,7 J. A. GAIDOS,7 T. HALL,7 A. M. HILLAS,3 R. C. LAMB,8 R. W. LESSARD,7 C. MASTERSON,4 J. E. MCENERY,9 G. MOHANTY,1,10 P. MORIARTY,11 J. QUINN,12 A. J. RODGERS,3 H. J. ROSE,3 F. W. SAMUELSON,1 G. H. SEMBROSKI,6 R. SRINIVASAN,6 V. V. VASSILIEV,12 AND T. C. WEEKES12 Received 1998 July 22; accepted 1998 August 27 ABSTRACT The energy spectrum of Markarian 421 in Ñaring states has been measured from 0.3 to 10 TeV using both small and large zenith angle observations with the Whipple Observatory 10 m imaging telescope. The large zenith angle technique is useful for extending spectra to high energies, and the extraction of spectra with this technique is discussed. The resulting spectrum of Markarian 421 is Ðtted reasonably well by a simple power law: J(E) \ E~2.54B0.03B0.10 photons m~1 s~1 TeV~1, where the Ðrst set of errors is statistical and the second set is systematic. This is in contrast to our recently reported spectrum of Markarian 501, which over a similar energy range has substantial curvature. The di†erences in TeV energy spectra of gamma-ray blazars reÑect both the physics of the gamma-ray production mechanism and possibly di†erential absorption e†ects at the source or in the intergalactic medium. Since Markarian 421 and Markarian 501 have almost the same redshift (0.031 and 0.033, respectively), the di†erence in their energy spectra must be intrinsic to the sources and not due to intergalactic absorption, assuming the intergalactic infrared background is uniform. Subject headings: BL Lacertae objects: individual (Markarian 421, Markarian 501) È galaxies: active È gamma rays: observations 1. INTRODUCTION of the beam (see, e.g., Mannheim 1993). The inverse Three blazars, Markarian 421 (Punch et al. 1992), Mar- ] Compton models predict typical blazar gamma-ray energy karian 501 (Quinn et al. 1996), and 1ES 2344 514 cuto†s from 10 GeV to about 30 TeV, whereas proton beam (Catanese et al. 1998), have been detected at TeV energies. models allow gamma-ray energies exceeding 100 TeV. Spec- At GeV energies, the EGRET instrument aboard the trum measurements at both the low-energy end (perhaps Compton Gamma Ray Observatory found only upper probing the energy threshold of the second component) and limits for Markarian 501 (Catanese et al. 1997) and 1ES ] the high-energy end (perhaps showing an energy cuto†) are 2344 514 (D. J. Thompson 1996, private communication). important for constraining models. Markarian 421 was detected by EGRET, but only very The TeV gamma-ray spectra of extragalactic objects can weakly (Thompson et al. 1995). While blazar emission for be modiÐed by di†erential absorption due to photon- X-rayÈselected objects at lower energies (up to about 1È100 photon collisions with intergalactic IR radiation (Gould & keV) is almost certainly due to synchrotron emission from a Schre` der 1967; Stecker, De Jager, & Salamon 1992). Indeed, beam of highly relativistic electrons, the GeV/TeV emission TeV observations of Markarian 421 and Markarian 501 forms a second component usually attributed to inverse (Zweerink et al. 1997; Krennrich et al. 1997; Aharonian et Compton scattering of relatively low-energy photons by the al. 1997) have been used to set upper limits on the density of electron beam (see, e.g., Sikora, Begelman, & Rees 1994) or intergalactic IR radiation (Stanev & Franceschini 1998; perhaps to pion photoproduction by a proton component Biller et al. 1998). These upper limits provide the best con- straints on infrared densities in the 0.02È0.2 eV regime and 1 Department of Physics and Astronomy, Iowa State University, Ames, do not su†er from local galactic background contributions IA 50011-3160. 2 Department of Physics, Oxford University, Oxford, England, UK. as are present in direct measurements. At this time no 3 Department of Physics, University of Leeds, Leeds, LS2 9JT, England, unambiguous evidence has been found for an IR absorption UK. spectral cuto†. In order to infer the magnitude of inter- 4 Experimental Physics Department, University College, BelÐeld, galactic IR background radiation from absorption e†ects Dublin 4, Ireland. 5 Department of Physics, Washington University, St. Louis, MO 63130. on TeV spectra, it is necessary to have a good model for 6 Physics Department, National University of Ireland, Maynooth, intrinsic spectra or to have spectra from several objects and Ireland. assume that the intrinsic TeV spectra of the objects are 7 Department of Physics, Purdue University, West Lafayette, IN 47907. identical (or at least very similar), or to have detections from 8 Space Radiation Laboratory, California Institute of Technology, many sources and assume that they are similar in a sta- Pasadena, CA 91125. \ 9 Present address: Department of Physics, University of Utah, Salt tistical sense. For the redshift range (z 0.031È0.044) of the Lake City, UT 84112. detected TeV blazars Markarian 421, Markarian 501, and 10 Present address: LPNHE Ecole Polytechnique, 91128 Palaiseau 1ES 2344]514, a recognizable cuto† (optical depth D1È5) CEDEX, France. is expected to occur between 5 and 20 TeV (Stecker & De 11 School of Science, Galway-Mayo Institute of Technology, Galway, Ireland. Jager 1997; Stecker et al. 1998). 12 Fred Lawrence Whipple Observatory, Harvard-Smithsonian CfA, We have recently published a brief report giving a P.O. Box 97, Amado, AZ 85645-0097. detailed spectrum of Markarian 501 spanning the energy 149 150 KRENNRICH ET AL. Vol. 511 range 260 GeVÈ10 TeV derived from observations with the we have taken additional care in the treatment of energy Whipple Observatory gamma-ray imaging Cerenkov tele- threshold e†ects in order to obtain Ñux values at lower scope (Samuelson et al. 1998). This spectrum was derived energies. The second detection (27 minutes on source) had from observations made during ““ high ÏÏ states of the active an average Ñux of 2.8 crab units and exhibited a remarkably galactic nucleus (AGN) that give good statistical precision. short rise and fall time of 15 minutes. It occurred 8 days Markarian 501 (Quinn et al. 1998) is variable in the TeV after the Ðrst detection. The LZA observations consisted of energy range showing changes on a timescale of several 1.5 hr (1995 June) of on-source data at zenith angles of hours. The data were taken at both standard small zenith 55¡È60¡ with an average Ñux of 3.3 crab units. The new angles (SZA) of less than 25¡ and at large zenith angles spectrum is consistent with our previously published result (LZA) in the range of 55¡È60¡. The SZA observations are but spans a larger energy range (comparable to that sensitive to relatively low energies (E \ 5 TeV), and the published for Markarian 501) and has better statistics at LZA observations yield better statistics at high energies. high energies. The spectrum appears less curved than the The details of the analysis were not explained in the brief Markarian 501 spectrum. We show that the spectra for the report but are given here. In particular, the characteristics two equidistant AGN (Markarian 421 and Markarian 501) of Cerenkov imaging telescopes relevant for spectral deter- clearly di†er, and this reÑects intrinsic spectral di†erences mination change substantially from SZA to LZA obser- near the sites of gamma-ray production. Combined with vations. In addition, uncertainties in the spectrum, e.g., due data for the two AGN at X-ray energies, these spectra con- to corresponding uncertainties in atmospheric absorption, strain models of physical processes in the jets (see Hillas et are also given here. Finally, we have used the Ñux from the al. 1999b). The di†erences also point toward a major diffi- Crab Nebula (which often serves as a standard candle in culty in inferring intergalactic background radiation inten- TeV gamma-ray astronomy) to check spectra extracted sities via TeV photon attenuation. from LZA observations against our standard spectrum OBSERVATIONS (Hillas et al. 1999a). 2. These recent results from Markarian 501 indicate a spec- The observations presented here were made with the trum that is not consistent with a simple power law in which Whipple Observatory 10 m imaging Cerenkov telescope. the Ñux, J, is proportional to E~c, but is more accurately The camera consisted of 109 (until 1996 December 4) or 151 described by a three-parameter curve (parabolic in a plot of (after 1996 December 4) photomultiplier tubes placed on a log J vs. log E given by J D E [ 2.22 ^ 0.04 ^ 0.05 [ 1/4¡ hexagonal matrix. These cameras covered Ðelds of view ^ (0.47 0.07) log10 E, where the Ðrst set of errors is sta- of about2¡.7 and 3¡.2, respectively. Pulses from each of the tistical and the second systematic and E is in TeV. photomultiplier tubes were ampliÐed and sent to gated inte- (Previously published spectra of Markarian 501 covered a grating analog-to-digital converters, and, in addition, those smaller energy span and were consistent with a simple from the inner 91 tubes were sent to discriminators with power law; see, e.g., Bradbury et al.
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