arXiv subjects

Nijil Mankuzhiyil

Publications and source records attributed to Nijil Mankuzhiyil.

12 recordsLinked to original sources

arXiv2023

Prospects of additional contribution at Optical-NIR band of EBL in the light of VHE spectra

The Extragalactic Background Light (EBL) that spans the UV-IR band originates from direct and dust-reprocessed starlight integrated over the history of the Universe. EBL measurements are very challenging due to foreground emission like the zodiacal light and interplanetary dust emission. Indeed, some optical/NIR direct measurements overpredict EBL models based on galaxy counts. On the other hand, there is some debate on possible additional components of the Optical-NIR photon density: e.g., population-III stars, axion-photon decay, direct collapse of black holes, intra-halo light etc. Owing to the absorption of Very High Energy (VHE) $γ$ rays by interaction with EBL photons, we study the prospects of accommodating an additional population of EBL sources in the Optical-NIR band on top of the standard galaxy-count--based component. To this aim we use 105 VHE spectra of 37 blazars with known redshifts, $0.03 $1.5, we estimate, at different wavelengths, upper limits to the additional low energy photon fields which would contribute to the absorption of $γ$-rays. Considering these limits we suggest that there is room for photons from Pop III stars and axion-like particle (ALP) annihilation. However, these additional hypothetical photon fields are bound to fall significantly below direct published EBL measurements by several instruments, and therefore our limits are either in tension or even inconsistent with such measurements.

astro-ph.HE↗

arXiv2017

Clues on High Energy Emission Mechanism from Blazar 3C 454.3 during 2015 August Flare

We perform a detailed spectral study of a recent flaring activity from the Flat Spectrum Radio Quasar (FSRQ), 3C\,454.3, observed simultaneously in optical, UV, X-ray and $γ$-ray energies during 16 to 28 August, 2015. The source reached its peak $γ$-ray flux of $(1.9\pm0.2)\times\,10^{-05} \; {\rm ph\,cm^{-2}\,s^{-1}}$ on 22 August. The time averaged broadband spectral energy distribution (SED) is obtained for three time periods, namely "flaring state"; covering the peak $γ$-ray flux, "post flaring state"; immediately following the peak flare and "quiescent state"; separated from the flaring event and following the post flaring state. The SED corresponding to the flaring state is investigated using different emission models involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) mechanisms. Our study suggests that the X-ray and $γ$-ray emission from 3C\,454.3 cannot be attributed to a single emission mechanism and instead, one needs to consider both SSC and EC mechanisms. Moreover, the target photon energy responsible for the EC process corresponds to an equivalent temperature of 564 K, suggesting that the flare location lies beyond the broad line emitting region of the FSRQ. SED fitting of the other two flux states further supports these inferences.

astro-ph.HE↗

arXiv2014

Polarimetric Tomography of Blazar Jets

In this paper we propose a way to use optical polarisation observations to provide independent constraints and guide to the modelling of the spectral energy distribution (SED) of blazars, which is particularly useful when two-zone models are required to fit the observed SED. As an example, we apply the method to the 2008 multiwavelength campaign of PKS 2155-304, for which the required polarisation information was already available. We find this approach succesful in being able to simultaneously describe the SED and variability of the source, otherwise difficult to interpret. More generally, by using polarisation data to disentangle different active regions within the source, the method reveals otherwise unseen correlations in the multiwavelength behaviour which are key for the SED modelling.

astro-ph.HE↗

arXiv2012

Emitting electrons and source activity in Markarian 501

We study the variation of the broad-band spectral energy distribution (SED) of the BL Lac object Mrk 501 as a function of source activity, from quiescent to flaring. Through chi-square-minimization we model eight simultaneous SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model, and examine how model parameters vary with source activity. The emerging variability pattern of Mrk 501 is complex, with the Compton component arising from gamma-electron scatterings that sometimes are (mostly) Thomson and sometimes (mostly) extreme Klein-Nishina. This can be seen from the variation of the Compton to synchrotron peak distance according to source state. The underlying electron spectra are faint/soft in quiescent states and bright/hard in flaring states. A comparison with Mrk 421 suggests that the typical values of the SSC parameters are different in the two sources: however, in both jets the energy density is particle dominated in all states.

astro-ph.HE↗

arXiv2011

BL Lac Objects: Laboratories to study the environment and properties of emitting particles in relativistic jets

We report the variation of the spectral energy distribution (SED) of blazars as a function of source activity, based on available, simultaneous multi-wavelength (MWL) observations of BL Lac objects. We use a fully automatized \c{hi}2 minimization procedure, instead of the commonly used eye-ball fit, to model the data sets with a one-zone Synchrotron-Self-Compton (SSC) model. The obtained SSC parameters are then analyzed as a function of source luminosity, and the correlation between parameters is shown. Possibilities of improving the present observational and modeling status of BL Lac objects are also discussed.

astro-ph.CO↗

arXiv2011

Emission models and EBL as a tool to measure the redshift of BL Lac objects

We introduce a new method to determine the redshift of unknown-redshift BL Lac Objects. The method relies on simultaneous multi-wavelength (MWL) observations of BL Lac objects in optical, X-ray, HE (E>100 MeV) gamma-rays and VHE (E>100 GeV)gamma-rays. It involves best-fitting spectral energy distribution (SED) from optical through HE gamma-rays with a Synchrotron-Self-Compton (SSC) model. We extrapolate such best fitting model into VHE regime, and assume that it represents the intrinsic emission of the object. We then compare the observed VHE flux which has been affected by the interaction with Extragalactic Background Light (EBL). Constraining the measured vs intrinsic emission leads to the determination of gamma-gamma opacity. Comparing the obtained opacity with the predicted opacity based on EBL model, we obtain the redshift of the photon source.

astro-ph.CO↗

arXiv2011

Environment and properties of emitting electrons in blazar jets: Mrk 421 as a laboratory

Here we report our recent study on the spectral energy distribution (SED) of the high frequency BL Lac object Mrk 421 in different luminosity states. We used a full-fledged chi2-minimization procedure instead of more commonly used "eyeball" fit to model the observed flux of the source (from optical to very high energy), with a Synchrotron-Self-Compton (SSC) emission mechanism. Our study shows that the synchrotron power and peak frequency remain constant with varying source activity, and the magnetic field decreases with the source activity while the break energy of electron spectrum and the Doppler factor increase. Since a lower magnetic field and higher density of electrons result in increased electron-photon scattering efficiency, the Compton power increases, so does the total emission.

astro-ph.HE↗

arXiv2011

The environment and distribution of emitting electrons as a function of source activity in Markarian 421

For the high-frequency peaked BL Lac object Mrk 421 we study the variation of the spectral energy distribution (SED) as a function of source activity, from quiescent to active. We use a fully automatized chi-squared minimization procedure, instead of the "eyeball" procedure more commonly used in the literature, to model nine SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model and examine how the model parameters vary with source activity. The latter issue can finally be addressed now, because simultaneous broad-band SEDs (spanning from optical to VHE photon energies) have finally become available. Our results suggest that in Mrk 421 the magnetic field decreases with source activity, whereas the electron spectrum's break energy and the Doppler factor increase -- the other SSC parameters turn out to be uncorrelated with source activity. In the SSC framework these results are interpreted in a picture where the synchrotron power and peak frequency remain constant with varying source activity, through a combination of decreasing magnetic field and increasing number density of electrons below the break energy: since this leads to an increased electron-photon scattering efficiency, the resulting Compton power increases, and so does the total (= synchrotron plus Compton) emission.

astro-ph.HE↗

arXiv2010

High-frequency-peaked BL Lacertae objects as spectral candles to measure the Extragalactic Background Light in the Fermi and air Cherenkov telescopes era

The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of measured VHE gamma-ray spectra: neither the intrinsic spectrum, nor the EBL, are separately known -- only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of BL Lac objects in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the BL Lacs' intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the photon-photon opacity to VHE photons. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the high-frequency-peaked BL Lac object PKS 2155-304 and discuss how similar observations can more accurately probe the EBL.

astro-ph.CO↗

arXiv2010

Blazars as beamlights to probe the Extragalactic Background Light in the Fermi and Cherenkov telescopes era

The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of the measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are separately known - only their combination is. In this paper we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of blazars in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution (SED) from optical through HE gamma-rays (the latter being largely unaffected by EBL attenuation as long as z<1) with a Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represent the blazars' intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the gamma-gamma opacity to VHE photons - hence, upon assuming a specific cosmology, we derive the EBL photon number density. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multi-frequency observations of the blazar PKS2155-304 and discuss how similar observations can more accurately probe the EBL.

astro-ph.CO↗

arXiv2009

Simultaneous multi-frequency observations of PG 1553+113

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.

astro-ph.HE↗

arXiv2009

A new upper limit on the redshift of PG 1553+113 from observations with the MAGIC Telescope

Very high energy gamma ray emission from the active galactic nucleus PG 1553+113 was observed during 2005 and 2006 by the MAGIC collaboration, for a total observation time of 18.8 hours. Here we present the results of follow up observations: more than 20 hours of good quality data collected by the MAGIC Telescope during the 2007 and 2008 campaigns. The obtained spectra are compared and combined with previous measurements, and corrected for absorption adopting different EBL models. Upper limits on the unknown source redshift are derived by assuming the absence of a break in the intrinsic spectrum, or alternatively by constraining the hardness of the intrinsic source spectrum.

astro-ph.HE↗