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Register a new userPG 1553+113: five years of observations with MAGIC
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MAGIC Collaboration
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We present the results of five years (2005-2009) of MAGIC observations of the BL Lac object PG 1553+113 at very high energies (VHEs, E > 100 GeV). Power law fits of the individual years are compatible with a steady mean photon index Gamma = 4.27 $pm$ 0.14. In the last three years of data, the flux level above 150 GeV shows a clear variability (probability of constant flux < 0.001%). The flux variations are modest, lying in the range from 4% to 11% of the Crab Nebula flux. Simultaneous optical data also show only modest variability that seems to be correlated with VHE gamma ray variability. We also performed a temporal analysis of (all available) simultaneous Fermi/LAT data of PG 1553+113 above 1 GeV, which reveals hints of variability in the 2008-2009 sample. Finally, we present a combination of the mean spectrum measured at very high energies with archival data available for other wavelengths. The mean spectral energy distribution can be modeled with a one-zone Synchrotron Self Compton (SSC) model, which gives the main physical parameters governing the VHE emission in the blazar jet.
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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.
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.
Very high energy (VHE; >100 GeV) gamma-ray observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. A strong signal, ~10 standard deviations, is detected by HESS during the 2 years of observations (24.8 hours live time). The time-averaged energy spectrum, measured between 225 GeV to ~1.3 TeV, is characterized by a very steep power law (photon index of Gamma = (4.5 +- 0.3 {stat} +- 0.1 {syst}). The integral flux above 300 GeV is ~3.4% of the Crab Nebula flux and shows no evidence for any variations, on any time scale. H+K (1.45-2.45 micron) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI, an integral field spectrometer of the ESO Very Large Telescope (VLT) in Chile. The redshift of PG 1553+113 is still unknown, as no absorption or emission lines were found.
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%.
Very high energy (VHE, $E>$100 GeV) $gamma$-ray flaring activity of the high-frequency peaked BL Lac object pg has been detected by the hess telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with hint of intra-night variability. No counterpart of this event has been detected in the fla data. This pattern is consistent with VHE $gamma$ ray flaring being caused by the injection of ultrarelativistic particles, emitting $gamma$ rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at bestz using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz Invariance Violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are $textrm{E}_{rm QG,1}>4.10times 10^{17}$ GeV and $textrm{E}_{rm QG,2}>2.10times 10^{10}$ GeV for linear and quadratic LIV effects, respectively.
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