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Added by
Krishna Kumar Singh
Publication date
2014
fields
Physics
and research's language is
English
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In this paper, we present results of TeV $gamma$--ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the $TACTIC$ (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are primarily motivated by the unusually hard GeV-TeV spectrum of the source despite its relatively large redshift. The source is observed in the TeV energy range with the $TACTIC$ from March 1, 2013 to April 15, 2013 (MJD 56352--56397) for a total observation time of 39.62 h and no evidence of TeV $gamma$--ray activity is found from the source. The corresponding 99$%$ confidence level upper limit on the integral flux above a threshold energy of 1.1 TeV is estimated to be 3.41 $times10^{-12}$ photons cm$^{-2}$ s$^{-1}$ (i.e $<23%$ Crab Nebula flux) assuming a power law differential energy spectrum with photon index 3.0, as previously observed by the $MAGIC$ and $VERITAS$ telescopes. For the study of multi-wavelength emission from the source, we use nearly simultaneous optical, UV and and X--ray data collected by the UVOT and XRT instruments on board the emph{Swift} satellite and high energy $gamma$--ray data collected by the Large Area Telescope on board the emph{Fermi} satellite. We also use radio data at 15 GHz from OVRO 40 m telescope in the same period. No significant increase of activity is detected from radio to TeV $gamma$--rays from 1ES1218+304 during the period from March 1, 2013 to April 15, 2013.
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The BL Lac object H1426+428 ($zequiv 0.129$) is an established source of TeV $gamma$-rays and detections of these photons from this object also have important implications for estimating the Extragalactic Background Light (EBL) in addition to the understanding of the particle acceleration and $gamma$-ray production mechanisms in the AGN jets. We have observed this source for about 244h in 2004, 2006 and 2007 with the TACTIC $gamma$-ray telescope located at Mt. Abu, India. Detailed analysis of these data do not indicate the presence of any statistically significant TeV $gamma$-ray signal from the source direction. Accordingly, we have placed an upper limit of $leq1.18times10^{-12}$ $photons$ $cm^{-2}$ $s^{-1}$ on the integrated $gamma$-ray flux at 3$sigma$ significance level.
In March 2013, a flaring episode from the Crab Nebula lasting ~2 weeks was detected by the Fermi-LAT (Large Area Telescope on board the Fermi Gamma-ray Space Telescope). VERITAS provides simultaneous observations throughout this period. During the flare, the Fermi-LAT detected a 20-fold increase in flux above the average synchrotron flux >100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very-high-energy flux change >1 TeV and the flux change seen in the Fermi-LAT band >100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 * 10^-3 with 95% confidence.
We present optical photopolarimetric observations of the BL Lac object S4 0954+658 obtained with the 70-cm telescope in Crimea, 40-cm telescope in St.Petersburg, and 1.8-m Perkins telescope at Lowell Observatory (Flagstaff, Az). After a faint state with a brightness level R ~17.6 mag registered in the first half of January 2011, the optical brightness of the source started to rise and reached ~14.8 mag during the middle of March, showing flare-like behavior. The most spectacular case of intranight variability was observed during the night of 2011 March 9, when the blazar brightened by ~0.7 mag within ~7 hours. During the rise of the flux the position angle of optical polarization rotated smoothly over more than 200 degrees. S4 0954+658 is a gamma-ray blazar with gamma-ray flux of (5{pm}3)x10^{-10} phot/cm^2/s according to the Fermi 11-month Catalog Extragalactic Sources. Our analysis of contemporaneous Fermi LAT data does not show any sign of increased gamma-ray activity above the detection threshold except for an elevated flux on 2011 March 5, JD2455626, coincident with the local optical maximum.
We report on a bright flare in the Crab Nebula detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The period of significantly increased luminosity occurred in 2013 March and lasted for approximately 2 weeks. During this period, we observed flux variability on timescales of approximately 5,hours. The combined photon flux above 100 MeV from the pulsar and its nebula reached a peak value of $(12.5pm 0.8)cdot 10^{-6}$,cm$^{-2}$,s$^{-1}$ on 2013 March 6. This value exceeds the average flux by almost a factor of 6 and implies a $sim20$ times higher flux for the synchrotron component of the nebula alone. This is the second brightest flare observed from this source. Spectral and temporal analysis of the LAT data collected during the outburst reveal a rapidly varying synchrotron component of the Crab Nebula while the pulsar emission remains constant in time.
Context. On March 4, 2013, the Fermi-LAT and AGILE reported a flare from the direction of the Crab Nebula in which the high-energy (HE; E > 100 MeV) flux was six times above its quiescent level. Simultaneous observations in other energy bands give us hints about the emission processes during the flare episode and the physics of pulsar wind nebulae in general. Aims. We search for variability of the emission of the Crab Nebula at very-high energies (VHE; E > 100 GeV), using contemporaneous data taken with the H.E.S.S. array of Cherenkov telescopes. Methods. Observational data taken with the H.E.S.S. instrument on five consecutive days during the flare were analysed concerning the flux and spectral shape of the emission from the Crab Nebula. Night-wise light curves are presented with energy thresholds of 1 TeV and 5 TeV. Results. The observations conducted with H.E.S.S. on 2013 March 6 to March 10 show no significant changes in the flux. They limit the variation on the integral flux above 1 TeV to less than 63% and the integral flux above 5 TeV to less than 78% at a 95% confidence level.
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