Cork Institute of Technology SWORD - South West Open Research Deposit Physical Sciences Articles Physical Sciences 2009-06-01 Detection Of The Bl Lacertae Object H1426+428 At Tev Gamma- Ray Energies P. T. Reynolds Cork Institute of Technology Et. al. Follow this and additional works at: https://sword.cit.ie/dptphysciart Part of the Astrophysics and Astronomy Commons Recommended Citation Horan, D. et al., 2002. Detection of the BL Lacertae Object H1426+428 at TeV Gamma‐Ray Energies. The Astrophysical Journal, 571(2), pp.753–762. Available at: http://dx.doi.org/10.1086/340019. This Article is brought to you for free and open access by the Physical Sciences at SWORD - South West Open Research Deposit. It has been accepted for inclusion in Physical Sciences Articles by an authorized administrator of SWORD - South West Open Research Deposit. For more information, please contact [email protected]. The Astrophysical Journal, 571:753–762, 2002 June 1 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A. DETECTION OF THE BL LACERTAE OBJECT H1426+428 AT TeV GAMMA-RAY ENERGIES D. Horan,1,2 H. M. Badran,1,3 I. H. Bond,4 S. M. Bradbury,4 J. H. Buckley,5 M. J. Carson,2 D. A. Carter-Lewis,6 M. Catanese,1 W. Cui,7 S. Dunlea,2 D. Das,8 I. de la Calle Perez,4 M. D’Vali,4 D. J. Fegan,2 S. J. Fegan,1,9 J. P. Finley,7 J. A. Gaidos,7 K. Gibbs,1 G. H. Gillanders,11 T. A. Hall,6,12 A. M. Hillas,4 J. Holder,4 M. Jordan,5 M. Kertzman,13 D. Kieda,14 J. Kildea,2 J. Knapp,4 K. Kosack,5 F. Krennrich,6 M. J. Lang,11 S. LeBohec,6 R. Lessard,7 J. Lloyd-Evans,4 B. McKernan,2 P. Moriarty,15 D. Muller,10 R. Ong,16 R. Pallassini,4 D. Petry,6 J. Quinn,2 N. W. Reay,8 P. T. Reynolds,17 H. J. Rose,4 G. H. Sembroski,7 R. Sidwell,8 N. Stanton,8 S. P. Swordy,10 V. V. Vassiliev,14 S. P. Wakely,10 and T. C. Weekes1 Received 2001 November 29; accepted 2002 February 4 ABSTRACT A very high energy -ray signal has been detected at the 5.5 level from H1426+428, an X-ray–selected BL Lacertae object at a redshift of 0.129. The object was monitored from 1995 to 1998 with the Whipple 10 m imaging atmospheric Cerenkov telescope as part of a general blazar survey; the results of these observations, although not statistically significant, were consistently positive. X-ray observations of H1426+428 during 1999 with the BeppoSAX instrument revealed that the peak of its synchrotron spectrum occurs at greater than 100 keV, leading to the prediction of observable TeV emission from this object. H1426+428 was monitored extensively at the Whipple Observatory during the 1999, 2000, and 2001 observing seasons. The strongest TeV signals were detected in 2000 and 2001. During 2001, an integral flux of 2:04 Æ 0:35 Â 10À11 cmÀ2 sÀ1 above 280 GeV was recorded from H1426+428. The detection of H1426+428 supports the idea that, as also seen in Mrk 501 and 1ES 2344+514, BL Lacertae objects with extremely high synchrotron peak frequencies produce -rays in the TeV range. Subject headings: BL Lacertae objects: individual (1ES 1426+42.8) — gamma rays: observations 1. INTRODUCTION excess (or ‘‘ blue bump ’’), and a core-dominated radio mor- phology. BL Lacertae (BL Lac) objects are a subclass of Blazars are the main class of active galactic nuclei blazars that are notable for their lack of prominent emission (AGNs) detected above 100 MeV by the EGRET experi- lines. The broadband double-humped spectral energy distri- ment on the Compton Gamma Ray Observatory (CGRO) and by ground-based -ray observatories (Mukherjee et al. butions (SEDs) of BL Lac objects identified in X-ray sur- veys differ significantly from the SEDs of those identified in 1997; Weekes 2002). They comprise a subclass of AGN and radio surveys. This led to the subclassification of BL Lac are characterized by a highly variable nonthermal contin- uum, strong variable optical polarization, the lack of a UV objects into high-frequency peaked BL Lac objects (HBLs) and low-frequency peaked BL Lac objects (LBLs) based on the ratio of their X-ray to radio flux densities (Padovani & 1 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center Giommi 1995). The first ‘‘ hump,’’ generally assumed to be for Astrophysics, P.O. Box 97, Amado, AZ 85645; the peak of the synchrotron emission (in a F representa- [email protected]. tion), is in the IR-optical for LBLs and in the EUV–soft X- 2 Experimental Physics Department, National University of Ireland, Belfield, Dublin 4, Ireland. ray band for HBLs. BL Lac objects make up a significant 3 Physics Department, Tanta University, Tanta, Egypt. fraction of the 70 blazars in the Third EGRET Catalog 4 Department of Physics, University of Leeds, Leeds, LS2 9JT (Hartman et al. 1999), and most of them are classified as Yorkshire, England, UK. LBLs. It has been shown that there is not a sharp division 5 Department of Physics, Washington University, St. Louis, MO 63130. between these two classes of objects (see, e.g., Fossati et al. 6 Department of Physics and Astronomy, Iowa State University, Ames, IA 50011. 1998; Ghisellini 1999). 7 Department of Physics, Purdue University, West Lafayette, IN 47907. Recent studies with ground-based -ray telescopes have 8 Department of Physics, Kansas State University, Manhattan, KS produced evidence for TeV -ray emission from seven BL 66506. Lac objects, five of which are classified as HBL and two as 9 Department of Physics, University of Arizona, Tucson, AZ 85721. 10 Enrico Fermi Institute, University of Chicago, Chicago, IL 60637. LBL (Weekes et al. 2002). The most prominent of these are 11 Physics Department, National University of Ireland, Galway, Mrk 421, which has been detected by five ground-based Ireland. imaging atmospheric Cerenkov -ray observatories, and 12 Physics and Astronomy Department, University of Arkansas at Little Mrk 501, which has been detected by six such observatories. Rock, Little Rock, AR 72204. The emission from both of these objects can be explained by 13 Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135. Compton-synchrotron models, although detailed modeling 14 High Energy Astrophysics Institute, University of Utah, Salt Lake is still fraught with many uncertainties. Both objects are City, UT 84112. characterized by rapid variability on timescales from hours 15 School of Science, Galway-Mayo Institute of Technology, Galway, to months. In the TeV energy range, the energy spectrum of Ireland. 16 Department of Physics, University of California, Los Angeles, CA Mrk 501 and, more recently, that of Mrk 421 have been 90095. shown to exhibit absorption-like features (Krennrich et al. 17 Department of Physics, Cork Institute of Technology, Cork, Ireland. 1999, 2001; Aharonian et al. 1999). The temporal and spec- 753 754 HORAN ET AL. Vol. 571 tral properties of the other TeV BL Lac objects are less well tron peak frequencies and are therefore possible TeV emit- defined. ters. These four candidates for TeV emission were 1ES Since 1992, the Whipple Gamma Ray collaboration, 0120+340, PKS 0548À322, 1ES 1426+42.8 (i.e., using the 10 m imaging atmospheric Cerenkov telescope H1426+428), and H2356À309. on Mount Hopkins, has been searching for TeV -ray The spectra for three of these objects (1ES 0120+340, emission from AGNs. Initially the search was concen- PKS 0548À322, and H2356À309) were well fitted by a con- trated on blazars detected by EGRET at any redshift; vex broken power law with a break energy, and hence the these observations led to the detection of Mrk 421 peak of the synchrotron emission, occurring at about 1.4 (Punch et al. 1992) and upper limits on some 30 other keV for 1ES 0120+340, 4.4 keV for PKS 0548À322, and 1.8 blazars (Kerrick et al. 1995). More recently, the search keV for H2356À309. In the case of H1426+428, however, has concentrated on nearby BL Lac objects, leading to no evidence for a spectral break up to 100 keV was found. the detection of Mrk 501 (Quinn et al. 1996) and 1ES Instead, its spectrum was well fitted by a single power law, 2344+514 (Catanese et al. 1998). Between 1995 and 1998, with a flat spectral index of 0.92–100 keV, thus constraining the survey included 24 objects (17 HBLs, 7 LBLs) rang- the peak of the synchrotron emission to lie near or above ing in redshift from 0.046 to 0.44; the results for these this value. The best fit of a pure homogeneous synchrotron objects will be published shortly (Horan et al. 2000). self-Compton (SSC) model for H1426+428 (Costamante et Although none of these observations resulted in a detec- al. 2001) predicted detectable -ray emission at TeV ener- tion, the observations of H1426+428 yielded the highest gies. At the time of the BeppoSAX observation, the observed consistently positive statistical significances. X-ray flux in the 2–10 keV band was at one of the lowest lev- In this paper, evidence is presented for the detection of els ever recorded from H1426+428, indicating that it was very high energy (VHE) -rays from H1426+428. The not in a flaring state (Costamante et al. 2001). This implies H1426+428 observations are divided into two categories— that, in the event of a flare, the synchrotron peak could shift those taken as part of the general blazar survey between to even higher values, as was observed for both Mrk 501 1995 and 1998, and the subsequent concentrated observa- and 1ES 2344+514.