arXiv subjects

S. Sahayanathan

Publications and source records attributed to S. Sahayanathan.

43 records · Page 3Linked to original sources

arXiv2009

Boundary Shear Acceleration in the Jet of MKN501

The high resolution image of the jet of the BL Lac object MKN501 in radio, show a limb-brightened feature. An explanation of this feature as an outcome of differential Doppler boosting of jet spine and jet boundary due to transverse velocity structure of the jet requires large viewing angle. However this inference contradicts with the constraints derived from the high energy $γ$-ray studies unless the jets bends over a large angle immediately after the $γ$-ray zone (close to the central engine). In this letter we propose an alternate explanation to the limb-brightened feature of MKN501 by considering the diffusion of electrons accelerated at the boundary shear layer into the jet medium and this consideration does not require large viewing angle. Also the observed difference in the spectral index at the jet boundary and jet spine can be understood within the frame work of shear acceleration.

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arXiv2008

A Two Zone Synchrotron Model for the Knots in the M87 Jet

The flux and the spectral index in X-ray energy band from the knots of M87 jet as observed by {\it{Chandra}} indicate a possible synchrotron origin but cannot be explained by simple one zone models with continuous injection of non-thermal electrons. In this letter we propose a two-zone model to explain the observed spectra of the knots of M87 jet. We consider the synchrotron emission from a region with tangled magnetic field where relativistic non-thermal electrons are continuously injected in from an associated acceleration region. The acceleration region is assumed to be compact zone possibly around a shock front. A power-law distribution of electron is injected into the acceleration region and are accelerated to a maximum energy determined by the acceleration time scale and the loss processes. With the present model we are able to explain the overall broadband features of the knots of M87 jet. Also the present model predicts a change in spectral index at ultraviolet energies and future observations at these energies can be used to constrain the parameters involved in the model.

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arXiv2007

The TACTIC atmospheric Cherenkov Imaging telescope

The TACTIC $γ$-ray telescope, equipped with a light collector of area $\sim$9.5m$^2$ and a medium resolution imaging camera of 349-pixels, has been in operation at Mt.Abu, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11$\times$11 pixels ($\sim$ 3.4$^\circ$$\times3.4^\circ$), the telescope records a cosmic ray event rate of $\sim$2.5 Hz at a typical zenith angle of 15$^\circ$. Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above $\sim$1.2 TeV energy, at a sensitivity level of $\sim5.0σ$ in $\sim$25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong $γ$-ray signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during 2001 and 2005-2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV $γ$-ray emission activity from other active galactic nuclei on a long term basis.

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arXiv2007

Observations of TeV gamma-rays from Mrk 421 during Dec. 2005 to Apr. 2006 with the TACTIC telescope

The TACTIC $γ$-ray telescope has observed Mrk 421 on 66 clear nights from Dec. 07, 2005 to Apr. 30, 2006, totalling $\sim$ 202 hours of on-source observations. Here, we report the detection of flaring activity from the source at $\geq$ 1 TeV energy and the time-averaged differential $γ$-ray spectrum in the energy range 1-11 TeV for the data taken between Dec. 27, 2005 to Feb. 07, 2006 when the source was in a relatively higher state as compared to the rest of the observation period. Analysis of this data spell, comprising about $\sim$97h reveals the presence of a $\sim 12.0 σ$ $γ$-ray signal with daily flux of $>$ 1 Crab unit on several days. A pure power law spectrum with exponent $-3.11\pm0.11$ as well as a power law spectrum with an exponential cutoff $(Γ= -2.51\pm0.26$ and $E_0=(4.7\pm2.1) TeV)$ are found to provide reasonable fits to the inferred differential spectrum within statistical uncertainties. We believe that the TeV light curve presented here, for nearly 5 months of extensive coverage, as well as the spectral information at $γ$-ray energies of $>$ 5 TeV provide a useful input for other groups working in the field of $γ$-ray astronomy.

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arXiv2005

Interpretation of the Radio/X-ray Knots of AGN Jets within the Internal Shock Model Framework

The dynamics of relativistically moving blobs ejected out of a central AGN, are considered. It is assumed that the collision between two blobs are completely inelastic, such that the bulk kinetic energy lost in the collision is used to energize electrons to relativistic energies via acceleration in internal shocks which are formed due to the collision. These high energy electrons which are produced on a time-scale corresponding to the collision time-scale, cool by radiative losses due to synchrotron and Inverse Compton processes. The model is applied to the radio/X-ray knots of several AGN. For three of these sources we have analyzed long (> 40 ksec) Chandra observations and report on constrains on the X-ray spectral indices. In the framework of this model the AGN are inferred to sporadically eject relativistic blobs on time-scales ranging from 10^{11} to 10^{12} secs for different sources. It is shown that the collision time-scales can be longer than the age of the knot and, hence, non-thermal electrons are continuously being injected into the system. This continuous injection, in contrast to an instantaneous one time injection, gives rise to a characteristic spectral break, rather than a high-energy cutoff in the spectrum.

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arXiv2003

A Continuous Injection Plasma Model for the X-Ray/Radio Knots in Kpc-Scale Jets of AGN

We consider the evolution of a spherically expanding plasma cloud, where there is continuous injection of non-thermal electrons. We compute the time dependent electron distribution and resultant photon spectra taking into account synchrotron, adiabatic and inverse Compton cooling. This model is different from previous works where, instead of a continuous injection of particles, a short injection period was assumed. We apply this model to the radio/optical knots in the large scale jets of AGN, detected in X-rays by {\it Chandra} and find that the overall broadband spectral features can be reproduced. It is shown that for some sources, constraints on the X-ray spectral index (by a longer {\it Chandra} observation) will be able to differentiate between the different models. This in turn will put a strong constraint on the acceleration mechanism active in these sources.

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arXiv1999

Observations of the Crab Nebula, Mkn 501 and Mkn 421 using the TACTIC Imaging Element

The results of our observational campaigns on the two extragalactic sources Mkn 501 and Mkn 421, carried out with the Imaging Element of the TACTIC array, during March-May, 1998 and April-May, 1999, are presented. The results indicate that the two BL Lac objects (Mkn 501 and Mkn 421) were in a `low' gamma-ray emission state during both epochs of our observations.

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