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Observations of very high energy emission from B2 1215+30 with VERITAS
Abstract
We present results of 45 hours of data taken from January to June 2011 with
VERITAS above 200 GeV in the direction of B2 1215+30. The blazar was clearly
detected with a constant flux of F(E>200 GeV) = (8.0 +- 0.9) \times 10^{-12}
cm^{-2} s^{-1}. The energy spectrum can be fitted using a power-law with index
-3.6 +- 0.5. Simultaneous multi-wavelength data are also presented, and are
used to model the spectral energy distribution with a one-zone leptonic model.
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The procedures in current use for analyzing the results of gamma-ray
astronomy experiments are assessed. Two formulas are proposed for
estimating the significance of positive observations in searching
gamma-ray sources or lines. The correctness of the formulas is tested
with the aid of Monte Carlo simulations. One formula is derived by
immediately estimating the standard deviation of the observed signal
NS; the other is derived by applying the method of the
statistical hypotheses test.
Context. We present the discovery of very high energy (VHE, E > 100GeV)
gamma-ray emission from the BL Lac object 1ES 1215+303 by the MAGIC telescopes
and simultaneous multi-wavelength data in a broad energy range from radio to
gamma-rays. Aims. We study the VHE gamma-ray emission from 1ES 1215+303 and its
relation to the emissions in other wavelengths. Methods. Triggered by an
optical outburst, MAGIC observed the source in January-February 2011 for 20.3
hrs. The target was monitored in the optical R-band by the KVA telescope that
also performed optical polarization measurements. We triggered target of
opportunity observations with the Swift satellite and obtained simultaneous and
quasi-simultaneous data from the Fermi Large Area Telescope and from the
Mets\"ahovi radio telescope. We also present the analysis of older MAGIC data
taken in 2010. Results. The MAGIC observations of 1ES 1215+303 carried out in
January-February 2011 resulted in the first detection of the source at VHE with
a statistical significance of 9.4 sigma. Simultaneously, the source was
observed in a high optical and X-ray state. In 2010 the source was observed in
a lower state in optical, X-ray, and VHE, while the GeV gamma-ray flux and the
radio flux were comparable in 2010 and 2011. The spectral energy distribution
obtained with the 2011 data can be modeled with a simple one zone SSC model,
but it requires extreme values for the Doppler factor or the electron energy
distribution.
A 2,695 MHz survey of radio sources in a narrow strip of declination has recently been completed with the Jodrell Bank Mk II radio telescope (full details will be published later). Accurate positions for all the 74 sources have been measured with the RRE Malvern 2,695 MHz interferometer. Using these positions and a transparent overlay technique of identification the relative positions of radio and optical objects were located on the National Geographic‐Palomar Sky Survey prints with an r.m.s. error of 4 arc s. In the 24 h of right ascension covered by the survey, twenty-three identifications with stellar objects have been made; among these are two remarkably bright (14.5 magnitude) objects, separated by less than two degrees in the sky, which may be worthy of special attention. Finding charts for these two objects are shown in Fig. 1.
We present the results of optical spectroscopic identifications of a bright subsample of 2-10keV hard X-ray selected sources from the ASCA Medium Sensitivity Survey in the northern sky. The flux limit of the subsample is 3*10^-13 erg s^-1 cm^-2 in the 2-10keV band. All but one of the 87 hard X-ray selected sources are optically identified, with AGNs, 7 clusters of galaxies, and 1 galactic star. It is the largest complete sample of hard X-ray selected AGNs at the bright flux limit. Amounts of absorption to their nuclei are estimated to be hydrogen column densities (N_H) of up to ~3*10^23 cm^-2 from their X-ray spectra. Optical properties of X-ray absorbed AGNs with N_H > 1*10^22 cm^-2 indicate the effects of dust absorption: at redshifts, z<0.6, AGNs without broad H\beta emission lines have significantly larger N_H value than AGNs with broad H\beta emission lines. At z>0.6, the X-ray absorbed AGNs have a large hard X-ray to optical flux ratio (log f2-10keV/fR > +1). However, three X-ray absorbed z>0.6 AGNs show strong broad lines. In combination with hard X-ray selected AGN samples from the ASCA Large Sky Survey, the ASCA Deep Survey in the Lockman Hole and Chandra Deep Field North, the luminosity distributions of absorbed and less-absorbed AGNs are compared. Comment: 56 pages with 14 figures. Accepted to ApJS. Fig.1 and tables are available from http://www.subaru.naoj.org/staff/akiyama/papers/papers.html
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