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Register a new userVERITAS Observations of the BL Lac Object PG 1553+113
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We present results from VERITAS observations of the BL Lac object PG 1553+113 spanning the years 2010, 2011, and 2012. The time-averaged spectrum, measured between 160 and 560,GeV, is well described by a power law with a spectral index of $4.33 pm 0.09$. The time-averaged integral flux above $200,$GeV measured for this period was $(1.69 pm 0.06) times 10^{-11} , mathrm{ph} , mathrm{cm}^{-2} , mathrm{s}^{-1}$, corresponding to 6.9% of the Crab Nebula flux. We also present the combined $gamma$-ray spectrum from the Fermi Large Area Telescope and VERITAS covering an energy range from 100~MeV to 560~GeV. The data are well fit by a power law with an exponential cutoff at $rm {101.9 pm 3.2 , mathrm{GeV}} $. The origin of the cutoff could be intrinsic to PG~1553+113 or be due to the $gamma$-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of $rm z egthinspace > egthinspace 0.395$ based on optical/UV observations of PG~1553+113, the cutoff would be dominated by EBL absorption. Conversely, the small statistical uncertainties of the VERITAS energy spectrum have allowed us to provide a robust upper limit on the redshift of PG 1553+113 of $z egthinspace leq egthinspace 0.62$. A strongly-elevated mean flux of $(2.50 pm 0.14) times 10^{-11} , mathrm{ph} , mathrm{cm}^{-2} , mathrm{s}^{-1}$ (10.3% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as $(4.44 pm 0.71) times 10^{-11} , mathrm{ph} , mathrm{cm}^{-2} , mathrm{s}^{-1}$ (18.3% of the Crab Nebula flux) on MJD 56048. The light curve measured during the 2012 observing season is marginally inconsistent with a steady flux, giving a $chi^2$ probability for a steady flux of 0.03%.
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VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent sensitivity at these energies, and ever-deepening source exposures, VERITAS is in a position to probe distant blazars for clear absorption signatures in their very-high-energy gamma-ray spectra due to interactions with the extragalactic background light (EBL). We discuss results from recent VERITAS observations of PG 1553+113 (z > 0.4) which have resulted in the most significant very-high-energy detection ever obtained for this source. The most recent VERITAS spectral measurements are used to place an upper limit on the source redshift of z < 0.5 at the 95% confidence level. Also discussed are the prospects of using these observations, along with those of other hard- spectrum blazars, to place constraints on the EBL.
A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous HST-COS and IUE observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30 eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: i) orientation effects, ii) additional absorption, iii) multiple emission components, and iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet model.
On 2017 September 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event icnu, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar txs~(3FGL J0509.4+0541), which was in an elevated gamma-ray emission state as measured by the emph{Fermi} satellite. VERITAS observations of the neutrino/blazar region started on 2017 September 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E $>$ 100 GeV) emission was observed from the blazar by VERITAS in the two-week period immediately following the IceCube alert, TXS 0506+056 was detected by VERITAS with a significance of 5.8 standard deviations ($sigma$) in the full 35-hour data set. The average photon flux of the source during this period was $(8.9 pm 1.6) times 10^{-12} ; mathrm{cm}^{-2} , mathrm{s}^{-1}$, or 1.6% of the Crab Nebula flux, above an energy threshold of 110 GeV, with a soft spectral index of $4.8 pm 1.3$.
We report simultaneous multi-frequency observations of the blazar PG 1553+113, that were carried out in March-April 2008. Optical, X-ray, high-energy (HE; greater than 100 MeV) gamma-ray, and very-high- energy (VHE; greater than 100 GeV) gamma-ray data were obtained with the KVA, REM, RossiXTE/ASM, AGILE and MAGIC telescopes. This is the first simultaneous broad-band (i.e., HE+VHE) gamma-ray observation of a blazar. The source spectral energy distribution derived combining these data shows the usual double-peak shape, and is interpreted in the framework of a synchrotron-self-Compton model.
The MAGIC telescope has observed very-high energy gamma-ray emission from the BL Lac object PG 1553+113 in 2005 and 2006 at an overall significance is 8.8 sigma. The light curve shows no significant flux variations on a daily timescale. The flux level during 2005 was, however, significantly higher as compared to 2006. The differential energy spectrum between approx. 90 GeV and 500 GeV is well described by a power law with a spectral index of -4.2+-0.3. The photon energy spectrum and spectral modeling allow to pose upper limits of z=0.74 and z=0.56, respectively, on the yet undetermined redshift of PG 1553+113. Recent VLT observations of this blazar show featureless spectra in the near-IR, thus no direct redshift could be determined from these measurements.
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