arXiv subjects

Bagmeet Behera

Publications and source records attributed to Bagmeet Behera.

6 recordsLinked to original sources

arXiv2014

PKS 1510-089: a rare example of a flat spectrum radio quasar with a very high-energy emission

The blazar PKS 1510-089 is an example of flat spectrum radio quasars. High-energy emissions from this class of objects are believed to have been produced by inverse Compton radiation with seed photons originating from the broad line region. In such a paradigm, a lack of very high-energy emissions is expected because of the Klein-Nishina effect and strong absorption in the broad line region. Recent detection of at least three such blazars by Cherenkov telescopes has forced a revision of our understanding of these objects. We have aimed to model the observed spectral energy distribution of PKS 1510-089 from the high-energy flares in March 2009, during which very high-energy emission were also detected by H.E.S.S. We have applied the single-zone internal shock scenario to reproduce the multiwavelength spectrum of PKS~1510-089. We have followed the evolution of the electrons as they propagate along the jet and emit synchrotron and inverse Compton radiation. We have considered two sources of external photons: the dusty torus and the broad line region. We have also examined the effects of the gamma-gamma absorption of the high-energy photons both in the AGN environment (the broad line region and the dusty torus), as well as while traveling over cosmological distances: the extragalactic background light. We have successfully modeled the observed spectrum of PKS 1510-089. In our model, the highest energy emission is the result of the Comptonization of the infrared photons from the dusty torus, thus avoiding Klein-Nishina regime, while the bulk of the emissions in the GeV range may still be dominated by the Comptonization of radiation coming from the broad line region.

astro-ph.HE↗

arXiv2013

Pointing Calibration for the Cherenkov Telescope Array Medium Size Telescope Prototype

Pointing calibration is an offline correction applied in order to obtain the true pointing direction of a telescope. The Cherenkov Telescope Array (CTA) aims to have the precision to determine the position of point-like as well as slightly extended sources, with the goal of systematic errors less than 7 arc seconds in space angle. This poster describes the pointing calibration concept being developed for the CTA Medium Size Telescope (MST) prototype at Berlin-Adlershof, showing test results and preliminary measurements. The MST pointing calibration method uses two CCD cameras, mounted on the telescope dish, to determine the true pointing of the telescope. The "Lid CCD" is aligned to the optical axis of the telescope, calibrated with LEDs on the dummy gamma-camera lid; the "Sky CCD" is pre-aligned to the Lid CCD and the transformation between the Sky and Lid CCD camera fields of view is precisely modelled with images from special pointing runs which are also used to determine the pointing model. During source tracking, the CCD cameras record images which are analysed offline using software tools including Astrometry.net to determine the true pointing coordinates.

astro-ph.IM↗

arXiv2013

Potential of EBL and cosmology studies with the Cherenkov Telescope Array

Very high energy (VHE, E >100 GeV) gamma-rays are absorbed via interaction with low-energy photons from the extragalactic background light (EBL) if the involved photon energies are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with redshift, distorts the VHE spectra observed from distant objects. The observed energy spectra of the AGNs carry, therefore, an imprint of the EBL. The detection of VHE gamma-ray spectra of distant sources (z = 0.11 - 0.54) by current generation Imaging Atmospheric Cherenkov Telescopes (IACTs) enabled to set strong upper limits on the EBL density, using certain basic assumptions about blazar physics. In this paper it is studied how the improved sensitivity of the Cherenkov Telescope Array (CTA) and its enlarged energy coverage will enlarge our knowledge about the EBL and its sources. CTA will deliver a large sample of AGN at different redshifts with detailed measured spectra. In addition, it will provide the exciting opportunity to use gamma ray bursts (GRBs) as probes for the EBL density at high redshifts.

astro-ph.CO↗

arXiv2009

A new strategy to select candidate blazars for VHE observation

A new strategy to select candidate blazars for VHE observations, that gives an inherently complete sample from an all-sky compilation (excluding a narrow region around the galactic plane) of GeV-bright blazars is presented. The strategy is based on selecting candidate VHE blazars from a GeV-detected blazar sample, namely the LAT-bright-AGN-sample (LBAS) of the Fermi Gamma-ray Space Telescope (FGST). The intrinsic spectra in the VHE regime is estimated based on the extrapolation of the GeV spectra measured with FGST. This is corrected for the unavoidable extinction due to the Extragalactic Background Light (EBL) in the UV to mid-IR. The resulting observable spectra ranked by the integrated flux is used to extract candidates for observations in the VHE regime with Imaging Atmospheric Cherenkov Telescopes (IACTs). The predictions are cross-checked using the sample of known VHE blazars.

astro-ph.HE↗

arXiv2009

Correlation between the synchrotron and the inverse-Compton components, in the longterm light curve of GeV blazars

To explore the correlation in the GeV band (i.e. the inverse-Compton component) with the optical band (i.e. the synchrotron component) of blazars that are bright in high energies, the light curves of a number of blazars monitored with the Fermi Gamma-ray Space Telescope (FGST), were compared to the corresponding light curves recorded with the Automatic Telescope for Optical Monitoring (ATOM). The results are presented.

astro-ph.HE↗

arXiv2009

Constraints on cosmological parameters from the EBL attenuation of VHE spectra of extragalactic sources

The Extragalactic Background Light (EBL) causes attenuation of the VHE spectra of extragalactic sources, via pair-production. The amount of attenuation depends on the distance of the source, i.e. on the integration over the line of sight path taken by the VHE photons. The line of sight path calculation, depends on the values of the cosmological parameters and the evolution of these parameters. With the recent indications of a lower EBL level from observations and the greater sensitivity of the proposed future Cherenkov experiments it will be possible to detect higher redshift sources. We study whether the EBL attenuation of extragalactic sources is sensitive enough to put constrains on the cosmological parameters.

astro-ph.CO↗