Highlights of Results

Active Galactic Nuclei

One class of prime targets for HAGAR is blazar class AGNs. All AGNs are presumed to be powered by accretion of matter onto the supermassive black hole at the centre. In case of blazars, relativistic jets emanating from the centre, are aligned with our line of sight within few degrees. Emission from blazars is dominated by Doppler boosted emission from jets. Multiwaveband spectral energy distributions (SEDs) of blazars show two humps and are explained in terms of emission arising from leptonic or hadronic beams from jets.

                         Image Credit: Wikipedia

Flare from Mkn 421

Mkn 421 is a near by blazar (redshift : 0.031) known to show frequent flares in various wavebands. We detected a huge flare from this object in February 2010 using HAGAR.

HAGAR detected maximum flux of about 6-7 Crab units on 17th February with significance level of 12.7 sigma at energies above 250 GeV.

Multiwaveband SED of Mkn 421 during flare is fitted in terms of one zone State 3 Synchotron Self-Compton model. According this model electrons in jet emit synchrotron photons giving rise to first peak in SED. These photons are then Compton upscattered by same population of electrons giving rise to second peak in SED. Entire flaring episode is explained in terms of passing shock.

(Ref : A. Shukla et al. A&A, 541, A140, 2012)

Long term study of Mkn 421

Mkn 421 has been observed with HAGAR on several occasions in 21 observation seasons spanning seven years period.

Multiwaveband light curve of Mkn 421 during 2009-2015 from radio to VHE gamma rays. Panels are arranged in increasing order of energy from top to bottom. Last panel shows HAGAR light curve.

SEDs for 21 observation seasons are fitted with one zone SSC model. Flux variations during these seasons are found to be mainly due to changes in underlying particle distributions rather than changes in jet parameters like magnetic field or Doppler factor.

(Ref. A. Sinha et al., A&A, 591,83,2016)

Mkn 501: Hints for two Component Nature of the Emission

Mkn 501 is another nearby blazar (redshift : 0.034). VHE gamma ray emission was detected by HAGAR during observations carried out in 2010-11. Source was detected in moderately high state in April-May 2011 with the flux level of about 1.5 Crab units at energies above 250 GeV with significance level of 5 sigma. Multiwaveband SEDs of this object at various epochs indicated complex nature than single zone SSC. SEDs at various flux levels were successfully reproduced with two-zone SSC model indicating multi-component nature of the emission.

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HAGAR light curve from Mkn 501 during March-June 2011.

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Multiwaveband SED for April-May 2011 fitted with two-zone SSC model.

(Ref: A. Shukla et al., ApJ, 798, 2, 2015)

1ES 1011+496 : Investigation of Curvature in Particle Spectrum

Another blazar, 1ES 1011+496, was observed with HAGAR in February 2014 just after Fermi-LAT and some VHE experiments reported a flare. HAGAR observations corresponded to the epoch when flare had decayed and there was no statistically significant detection of signal. Multiwaveband SEDs were studied which were successfully reproduced with SSC model. Curvature seen in X-ray spectrum was attributed to underlying particle distribution and energy dependent escape rate of particles from main emission region was inferred.

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Multiwaveband SED of 1ES 1011+496. Blue line : energy independent escape, black line : energy dependent escape of particles with escape rate varying as E0.5

(Ref: A. Sinha et al., ApJ, 836, 83, 2017)

Estimation of Extragalactic Background Light using VHE Observations of Blazars

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EBL causes redshift dependent attenuation and distortion of TeV spectrum.

Examining spectral measurements of a sample of VHE blazars from literature, VHE spectral indices were found to be redshift dependent. No such correlation with redshift was found in X-ray or GeV bands. Attributing this to effect of Extragalactic Background Light (EBL), the spectral shape of EBL was estimated in a model independent way. Estimated spectrum is consistent with upper and lower limits imposed by different observations. Also it also agrees closely with the theoretical estimates from cosmological evolution models.

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Black line : best fit EBL spectrum estimate from this work along with 1σ and 2σ limits. Various theoretical model estimates are shown. Gray region : upper and lower limits from various observations.

(Ref : A. Sinha et al., ApJ, 795, 91, 2014)

Supernova Remnants and Pulsars

Massive stars end their life through supernova explosions. This explosion blows off outer layers of star forming supernova remnant. Supernova remnants are important sources of VHE gamma rays. They are thought to be the sites for acceleration of cosmic rays and possible mechanism is diffusive shock acceleration.

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In many cases supernova explosion results in rapidly rotating neutron star (pulsar) along with the bright core or nebula within the shell. These neutron stars are highly magnetised with rotation axis misaligned with magnetic field axis. As neutron star spins, beam of radiation sweeps through our line of sight and we see pulsations.

                                                                                                                                                          Pulsar (Image credit : NASA)

Nebula is powered by pulsar wind consisting of electrons and positrons.

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Crab nebula is a remnant of a supernova observed by Chinese astronomers in 1054 AD. It is a steady, bright source, considered as a standard candle. It emits non-thermal emission spanning 21 decades of frequencies from radio to gamma rays. Pulsar at the centre of Crab nebula spins with a period of about 33 ms. Pulsations are detected at various frequencies from radio to gamma rays.

       Crab nebula image from Hubble telescope

HAGAR observations of Crab nebula and pulsar

Crab nebula is detected successfully at a significance level of 19σ from 188 hours of HAGAR data at energies above 230 GeV.

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Seasonwise average count rates from Crab nebula for various trigger conditions during 2008-2014

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Spectrum of Crab nebula measured by HAGAR agrees with the spectral measurements from other experiments.

Pulsations are detected from Crab pulsar with period of about 33 ms at energies above 230 GeV with significance level of about 6 sigma based on 260 hours of data from HAGAR.

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Phasogram showing Crab data from HAGAR folded with period of 33 ms. Excess seen at phases marked as P1 and P2. These are consistent with the phases where Fermi-LAT and other experiments have seen the excess.

(Ref : B. B. Singh et al., in preparation)

Multiwaveband Studies of a Gamma Ray Binary LSI+61 303

Gamma-ray binaries are composed of a massive star and a compact object. There are two models fro binary star systems emitting gamma rays: Left a microquasar with a stellar-mass black hole accreting material from a massive companion star. Right: a binary system with a pulsar and a Be star.

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                                                           (Image Credit : F. Mirabel)

LSI+61 303 is a gamma ray binary consisting of a B0 main sequence star with circumstellar disk (Be star) and a compact object which could be a neutron star or a black hole. Two periodicities have been seen in this system : orbital period of 26.496 days and superorbital period of 1626 days. Using multiwaveband data from radio to gamma rays superorbital modulation was studied as a function of orbital phase and its wavelength and its wavelength dependence was investigated.

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Multiwaveband SEDs were generated for various states and these were explained in terms of one zone microquasar jet model consisting of synchrotron emission from electrons, its inverse Compton and Comptonization of external photons from companion star.

          (Ref : L. Saha et al., ApJ, 823, 134, 2016)





HAGAR Group: TIFR ,Room No:- C115,D307,C123,C125,D414,Homi Bhabha Road,Colaba,Mumbai, (India)
Phone : 022-22782675 Email : mano.ranjan@tifr.res.in

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