Upper Limits from Five Years of Blazar Observations with the VERITAS Telescopes PDF Print

 

 

Stacked significance distribution of VERITAS blazar observations. For details, see Figure 1 below.

Reference: S. Archambault et al. (The VERITAS Collaboration), Astronomical Journal 151: 142, 2016

Full text version

ArXiv: ArXiV:1603.02410

Contacts: Matteo Cerruti

Blazars are the most common extragalactic sources of very-high-energy (VHE, E > 100 GeV) photons and VERITAS has already detected emission from 32 of them. However, much larger number of blazars were observed by VERITAS without being detected, and only an upper limit on their VHE flux can be determined.  In this work, we present the analysis results from a sample of 114 blazars observed, but non-detected, by VERITAS from 2007 to 2012. The sample corresponds to about 570 hours of observing time, and also includes all the sources from the second Fermi-LAT (2FGL) catalog which fall in the field of view of the VERITAS observations. For every source in our sample, we provide the details of the observations and the upper limit on its VHE flux as measured with the VERITAS telescopes. We performed a variety of stacked analyses of the individual results (e.g. by blazar type).  One of these shows evidence for an excess of gamma rays at the 4 sigma level in the overall sample, which can also be seen as a shift in the observed significance distributions (see Figure 1) .

A critical aspect of blazar studies is that a significant portion of them do not have a spectroscopic redshift estimation. We performed optical spectroscopy measurements to determine the redshifts of 18 of the blazars included in the VERITAS sample. We present new results from the Kast Dual Spectrograph on the Shane telescope at the Lick observatory, resulting in three redshift determinations, and a lower limit. The new optical spectra are shown in Figures A1 through A6.

 

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Figures from paper (click to get full size image):

 


Figure 1: Left: stacked significance distribution of the sources included in our sample, classified according to their redshift. Sources with unknown z are in blue, sources with z > 0.6 are in red and sources with z < 0.6 are in grey. The Gaussian function represents the expectation from a randomly distributed sample, with mean equal to zero, and variance equal to 1. Right: same as left panel, but for sources classified according to the AGN type. Unidentified sources are in blue, IBL/LBL/FSRQ in red and HBL in grey.
Figure A1: Spectra shown from top to bottom: RGB J0250+172 (August 15, 2010), 1ES 1118+424 (February 14, 2013), RBS 1366 (May 30, 2014), 1ES 2321+419 (October 28, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N. Solid gray lines indicate features at non-zero redshift.

Figure A2: Spectra shown from top to bottom: RBS 0082 (August 13, 2010), 1ES 0033+595 (December 4, 2013), 1RXS J0045.3+2127 (October 28, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

Figure A3: Spectra shown from top to bottom: 1ES 0446+449 (February 14, 2013), RGB J0505+612 (February 14, 2013), 2FGL J0540.4+5822 (October 28,2014). The spectrum of 1ES 0446+449 (February 14, 2013) only includes the red arm data (5500-8000 Angstroms) because there were complications in reducing the blue arm data. Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

Figure A4: Spectra shown from top to bottom: B2 0912+29 (January 4, 2013), RBS 0929 (April 7, 2013), RGB J1037+571 (February 14, 2013). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

Figure A5: Spectra shown from top to bottom: PG 1246+586 (May 30, 2014), 1RXS J144053.2+061013 (January 4, 2013), RGB J1725+118 (May 30, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

Figure A6: Spectra shown from top to bottom: RGB J1903+556 (June 13, 2013), BZB J2243+2021 (August 13, 2010). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

Last Updated on Sunday, 13 November 2016 17:03
 

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