arXiv subjects

Zahir Shah

Publications and source records attributed to Zahir Shah.

34 records · Page 2Linked to original sources

arXiv2023

Multi-wavelength variability and broadband SED modeling of BL Lac during a bright flaring period MJD 59000-59943

We carried a detailed temporal and spectral study of the BL\,Lac by using the long-term \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT observations, during the period MJD\,59000-59943. The daily-binned $γ$-ray light curve displays a maximum flux of $1.74\pm 0.09\times 10^{-5} \rm ph\,cm^{-2}\,s^{-1}$ on MJD\,59868, which is the highest daily $γ$-ray flux observed from BL\,Lac. The $γ$-ray variability is characterised by power-spectral-density (PSD), r.m.s-flux relation and flux-distribution study. We find that power-law model fits the PSD with index $\sim 1$, which suggest for long memory process at work. The observed r.m.s.-flux relation exhibits a linear trend, which indicates that the $γ$-ray flux distribution follows a log-normal distribution. The skewness/Anderson-Darling test and histogram-fit reject the normality of flux distribution, and instead suggest that the flux distribution is log-normal distribution. The fractional-variability amplitude shows that source is more variable in X-ray band than in optical/UV/$γ$-ray bands. In order to obtain an insight into the underlying physical process, we extracted broadband spectra from different time periods of the lightcurve. The broadband spectra are statistically fitted with the convolved one-zone leptonic model with different forms of the particle energy distribution. We found that spectral energy distribution during different flux states can be reproduced well with the synchrotron, synchrotron-self-Compton and external-Compton emissions from a broken power-law electron distribution, ensuring equipartition condition. A comparison between the best fit physical parameters show that the variation in different flux-states are mostly related to increase in the bulk-Lorentz factor and spectral hardening of the particle distribution.

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arXiv2023

Understanding the Broadband Emission Process of 3C 279 through Longterm Spectral Analysis

The long term broadband spectral study of Flat Spectrum Radio Quasars during different flux states has the potential to infer the emission mechanisms and the cause of spectral variations. To scrutinize this, we performed a detailed broadband spectral analysis of 3C 279 using simultaneous Swift-XRT/UVOT and Fermi-LAT observations spanning from August 2008 to June 2022. We also supplement this with the simultaneous NuSTAR observations of the source. The optical/UV, X-ray, and gamma-ray spectra were individually fitted by a power-law to study the long-term variation in the flux and the spectral indices. A combined spectral fit of simultaneous optical/UV and X-ray spectra was also performed to obtain the transition energy at which the spectral energy distribution is minimum. The correlation analysis suggests that the long-term spectral variations of the source are mainly associated with the variations in the low energy index and the break energy of the broken power-law electron distribution which is responsible for the broadband emission. The flux distribution of the source represents a log-normal variability while the gamma-ray flux distribution showed a clear double log-normal behavior. The spectral index distributions were again normal except for gamma-ray which showed a double-Gaussian behavior. This indicates that the log-normal variability of the source may be associated with the normal variations in the spectral index. The broadband spectral fit of the source using synchrotron and inverse Compton processes indicates different emission processes are active at optical/UV, X-ray, and gamma-ray energies.

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arXiv2023

Understanding the Very High Energy γ-ray excess in nearby blazars using leptonic model

The availability of simultaneous X-ray and Very High Energy (VHE) observations of blazars helps to identify the plausible radiative contributors to the VHE emission. Under leptonic scenario, the VHE emission from BL Lacs are attributed to the synchrotron self Compton (SSC) emission. However, many BL Lacerate (BL Lacs) have shown significant hardening at VHE after correction for the Extra Galactic Background Light (EBL) attenuation. We study the spectral hardening of two nearby BL Lac objects, Mkn 421 and Mkn 501 having most number of simultaneous X-ray and VHE observations available among all the blazars. These BL Lacs are relatively close and the effect of EBL attenuation is relatively minimal/negligible. We study the scatter plot between the X-ray spectral indices and intrinsic VHE indices to identify the plausible origin of the VHE emission. For Mkn 501, the VHE spectral indices are steeper than X-ray spectra, suggesting the scattering process happening at extreme Klein-Nishina regime. On the other hand, for Mkn 421, the VHE spectra is remarkably harder than the X-ray spectra, which suggests an additional emission mechanism other than the SSC process. We show this hard VHE spectrum of Mkn 421 can be explained by considering the inverse Compton (IC) emission from a broken power law electron distribution with Maxwellian pileup. The possibility of the hadronic contribution at VHE γ-rays is also explored by modeling the hard spectrum under photomeson process.

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arXiv2022

Very High Energy Flat Spectral Radio Quasar Candidates

The attenuation of very high energy (VHE) photons by the extragalactic background light (EBL) prevents the observation of high redshift flat spectrum radio quasars (FSRQs). However, the correlation of VHE spectral index with source redshift suggests that EBL intensity may be less than what is predicted. This deviation can draw new constraints on opacity of the universe to VHE gamma-rays. Therefore, more FSRQs may fall above the sensitivity of the forthcoming VHE telescopes than the ones predicted by the existing EBL models. In order to account for the lower EBL intensity predicted by the index-redshift correlation, we introduce a redshift dependent correction factor to the opacity estimated from commonly used cosmological EBL model. Considering this modified opacity, we identify the plausible VHE FSRQ candidates by linearly extrapolating the \emph{Fermi} gamma-ray spectrum at 10 GeV to VHE regime. Our study suggests among 744 FSRQs reported in \emph{Fermi} fourth catalog-Data release 2 (4FGL-DR2), 32 FSRQs will be detectable by Cherenkov Telescope Array Observatory (CTAO). Since the FSRQS are proven to be highly variable, we assume a scenario where the average \emph{Fermi} gamma-ray flux increases by a factor of 10 and this predicts additional 90 FSRQs that can be detected by CTAO.

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arXiv2022

Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data

We have performed a detailed analysis of the X-ray spectra of the blazar Mkn 421 using Swift-XRT observations taken between 2005 and 2020, to quantify the correlations between spectral parameters for different models. In an earlier work, it has been shown that such spectral parameter correlations obtained from a single short flare of duration $\sim$ 5-days of Mkn 421, can be used to distinguish spectrally degenerate models and provide estimates of physical quantities. In this work, we show that the results from the long-term spectral parameter correlations are consistent with those obtained from the single flare. In particular, that the observed spectral curvature is due to maximum cutoff energy in the particle distribution is ruled out. Instead, models where the curvature is due to the energy dependence of escape or acceleration time-scale of the particles are favored. The estimated values of the physical parameters for these models are similar to the ones obtained from the single flare analysis and are somewhat incompatible with the physical assumption of the models, suggesting that more complex physical models are required. The consistency of the results obtained from the long and short-term evolution of the source, underlines the reliability of the technique to use spectral parameter correlations to distinguish physical models.

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arXiv2022

Temporal and spectral study of PKS 0208-512 during the 2019-2020 flare

We present the temporal and spectral study of blazar PKS 0208-512, using recent flaring activity from November 2019 to May 2020, as detected by the Fermi-LAT observatory. The contemporaneous X-ray, optical/UV observations from Swift-XRT/UVOT are also used. During the activity state, 2-days binned $γ$-ray lightcurve shows multiple peaks indicating sub-flares. To understand the possible physical mechanisms behind flux enhancement, we divided the activity state of the source into several flux-states and carried out detailed temporal and spectral studies. Timing analysis of lightcurves suggest that peaks of sub-flares have rise and decay time on the order of days, with flux-doubling time $\sim$ 2-days. The 2-days binned $γ$-ray lightcurve shows double-lognormal flux distribution. The broadband spectral-energy-distribution (SED) for three selected flux states can be well fitted under synchrotron, synchrotron self-Compton (SSC) and external-Compton (EC) emission mechanisms. We obtained the physical parameters of the jet by the SED modeling and their confidence intervals through $χ^2$-statistics. Our SED modeling results suggest that during quiescent-state, $γ$-ray spectrum can be well explained by considering the EC-scattering of infra-red (IR) photons from dusty-torus. However, $γ$-ray spectra corresponding to flares demand additional target photons from broad-line-region (BLR) along with IR. These suggest that during flares, the emission-region is close to the edge of BLR, while for quiescent-state the emission-region is away from BLR. The best fit results suggest that marginal increase in magnetic-field during the flaring-episode can result in the flux enhancement. This is possibly associated with the efficiency of particle acceleration during flaring-states as compared to quiescent-state.

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arXiv2021

Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations

We present a time-resolved X-ray spectral study of the high energy peaked blazar Mkn 421 using simultaneous broadband observations from the LAXPC and SXT instruments on-board AstroSat. The ~ 400 ksec long observation taken during 3-8 January, 2017 was divided into segments of 10 ksecs. Each segment was fitted using synchrotron emission from particles whose energy distribution was represented by a log-parabola model. We also considered particle energy distribution models where (i) the radiative cooling leads to a maximum energy (ξ max model), (ii) the system has energy dependent diffusion (EDD) and (iii) has energy dependent acceleration (EDA). We found that all these models describe the spectra, although the EDD and EDA models were marginally better. Time resolved spectral analysis allowed for studying the correlation between the spectral parameters for different models. In the simplest and direct approach, the observed correlations are not compatible with the predictions of the ξ max model. While the EDD and EDA models do predict the correlations, the values of the inferred physical parameters are not compatible with the model assumptions. Thus, we show that spectrally degenerate models, can be distinguished based on spectral parameter correlations (especially those between the model normalization and spectral shape ones) making time-resolved spectroscopy a powerful tool to probe the nature of these systems.

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arXiv2021

The long-term X-ray flux distribution of Cygnus X-1 using RXTE-ASM and MAXI observations

We studied the long term flux distribution of Cygnus X-1 using RXTE-ASM lightcurves in two energy bands B (3-5 keV) & C (5-12.1 keV) as well as MAXI lightcurves in energy bands B (4-10 keV) & C (10-20 keV). The flux histograms were fitted using a two component model. For MAXI data, each of the components is better fitted by a log-normal distribution, rather than a Gaussian one. Their best fit centroids and fraction of time the source spends being in that component are consistent with those of the Hard and Soft spectral states Thus, the long term flux distribution of the states of Cygnus X-1 have a log-normal nature which is the same as that found earlier for much shorter time-scales. For RXTE-ASM data, one component corresponding approximately to the Hard state is better represented by a log-normal but for the other one a Gaussian is preferred and whose centroid is not consistent with the Soft state.This discrepancy, could be due to the larger fraction of the Intermediate state (~11.25%) in the RXTE-ASM data as compared to the MAXI one (~4%). Fitting the flux distribution with three components did not provide an improvement for either RXTE-ASM or MAXI data, suggesting that the data corresponding to the Intermediate state may not represent a separate spectral state, but rather represent transitions between the two states.

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arXiv2021

AstroSat observation of the HBL 1ES 1959+650 during its October 2017 flaring

We present the results of the X-ray flaring activity of 1ES 1959+650 during October 25-26, 2017 using AstroSat observations. The source was variable in the X-ray. We investigated the evolution of the X-ray spectral properties of the source by dividing the total observation period ($\sim 130$ ksecs) into time segments of 5 ksecs, and fitting the SXT and LAXPC spectra for each segment. Synchrotron emission of a broken power-law particle density model provided a better fit than the log-parabola one. The X-ray flux and the normalised particle density at an energy less than the break one, were found to anti-correlate with the index before the break. However, a stronger correlation between the density and index was obtained when a delay of $\sim 60$ ksec was introduced. The amplitude of the normalised particle density variation $|Δn_γ/n_γ| \sim 0.1$ was found to be less than that of the index $ΔΓ\sim 0.5$. We model the amplitudes and the time delay in a scenario where the particle acceleration time-scale varies on a time-scale comparable to itself. In this framework, the rest frame acceleration time-scale is estimated to be $\sim 1.97\times10^{5}$ secs and the emission region size to be $\sim 6.73\times 10^{15}$ cms.

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arXiv2021

Correlation between optical and $γ$-ray flux variations in BL Lacs

We report here results on the analysis of correlated flux variations between the optical and GeV $γ$-ray bands in three bright BL Lac objects, namely AO\, 0235+164, OJ 287 and PKS 2155$-$304. This was based on the analysis of about 10 years of data from the {\it Fermi} Gamma-ray Space Telescope covering the period between 08 August 2008 to 08 August 2018 along with optical data covering the same period. For all the sources, during the flares analysed in this work, the optical and $γ$-ray flux variations are found to be closely correlated. From broad band spectral energy distribution modelling of different epochs in these sources using the one zone leptonic emission model, we found that the optical-UV emission is dominated by synchrotron emission from the jet. The $γ$-ray emission in the low synchrotron peaked sources AO\, 0235+164 and OJ 287 are found to be well fit with external Compton (EC) component, while, the $γ$-ray emission in the high synchrotron peaked source PKS 2155$-$304 is well fit with synchrotron self Compton component. Further we note that the $γ$-ray emission during the high flux state of AO 0235+164 (epochs A and B) requires seed photons from both the dusty torus and broad line region, while the $γ$-ray emission in OJ 287 and during epochs C and D of AO\,0235+164 can be modelled by EC scattering of infra-red photons from the torus.

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arXiv2021

Unveiling the broadband spectral and temporal properties of PKS\,0903-57 during its brightest flare

We carried a detailed spectral and temporal study of blazar, \mbox{PKS\,0903-57} using the \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT observations, during its brightest flaring period MJD\,58931--58970. During this period, the maximum daily averaged $γ$-ray flux ($\rm F_{0.1-500\,GeV}$) of $\rm9.42\times10^{-6}\,ph\,cm^{-2}\,s^{-1}$ is observed on MJD\,58951.5, the highest $γ$-ray flux detected from \mbox{PKS\,0903-57} till now. Several high-energy (HE) photons ($>\,10$\,GeV) consistent with the source location at high probability (>\,99\%) are detected, and the $γ$-ray lightcurve in the active state shows multiple substructures with asymmetric profile. In order to understand the possible physical scenario responsible for the flux enhancement, we carried a detailed broadband spectral study of \mbox{PKS\,0903-57} by choosing different flux states from its active period. Neglecting the multi-band variability in each of the selected time intervals, we could reproduce their averaged broadband SEDs with a one-zone leptonic model whose parameters were derived with a $χ^2$-fit. We found that the broadband SED during different flux states can be reproduced by the synchrotron, synchrotron-self-Compton (SSC) and External-Compton (EC) processes. The seed photons for EC process from BLR or IR torus provide acceptable fits to the GeV spectrum in all the flux states; however, the detection of HE photons together with the equipartition condition suggest that the EC/IR process is a more likely scenario. Further, a detailed comparison between the fit parameters shows that the flux enhancement from quiescent-state to the flaring-state is mostly related to increase in the bulk Lorentz factor of the emission region and change in the break energy of the source spectrum.

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arXiv2020

On the determination of log-normal flux distributions for astrophysical systems

Determining whether the flux distribution of an Astrophysical source is a Gaussian or a log-normal, provides key insight into the nature of its variability. For lightcurves of moderate length ($< 10^3$), a useful first analysis is to test the Gaussianity of the flux and logarithm of the flux, by estimating the skewness and applying the Anderson-Darling (AD) method. We perform extensive simulations of lightcurves with different lengths, variability, Gaussian measurement errors and power spectrum index $β$ (i.e. $P(f) \propto f^{-β}$), to provide a prescriptionand guidelines for reliable use of these two tests. We present empirical fits for the expected standard deviation of skewness and tabulated AD test critical values for $β= 0.5$ and $1.0$, which differ from the values given in the literature which are for white noise ($β= 0$). Moreover, we show that for white noise, for most practical situations, these tests are meaningless, since binning in time alters the flux distribution. For $β\gtrsim 1.5$, the skewness variance does not decrease with length and hence the tests are not reliable. Thus, such tests can be applied only to systems with $β\gtrsim 0.5$ and $β\lesssim 1.0$. As an example of the prescription given in this work, we reconfirm that the Fermi data of the blazar, 3FGL\,J0730.2-1141, shows that its $γ$ ray flux is consistent with a log-normal distribution and not with a Gaussian one.

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arXiv2019

Study of long-term flux and photon Index distributions of blazars using RXTE observations

We present a detailed study of flux and index distributions of three blazars (one FSRQ and two BL Lacs) by using 16 years of Rossi X-ray Timing Explorer archival data. The three blazars were chosen such that their flux and index distributions have sufficient number of data points ($\geq$90) with relatively less uncertainty ($\bar{σ_{err}^{2}}/σ^{2}$<0.2) in light curves. Anderson-Darling (AD) test and histogram fitting shows that flux distribution of FSRQ 3C273 is log-normal, while its photon index distribution is Gaussian. This result is consistent with linear Gaussian perturbation in the particle acceleration time-scale, which produces log-normal distribution in flux. However, for two BL Lacs viz. Mkn501 and Mkn421, AD test shows that their flux distributions are neither Gaussian nor log-normal, and their index distributions are non-normal. The histogram fitting of Mkn501 and Mkn421, suggests that their flux distributions are more likely to be a bi-modal, and their index distributions are double Gaussian. Since, Sinha et al. (2018) had shown that Gaussian distribution of index produces a log-normal distribution in flux, double Gaussian distribution of index in Mkn501 and Mkn421 indicates that their flux distributions are probably double log-normal. Observation of double log-normal flux distribution with double Gaussian distribution in index reaffirms two flux states hypothesis. Further, the difference observed in the flux distribution of FSRQ (3C273) and BL Lacs (Mkn501 and Mkn421) at X-rays, suggest that the low energy emitting electrons have a single log-normal flux distribution while the high energy ones have a double log-normal flux distribution.

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arXiv2019

Study on Temporal and Spectral behavior of 3C 279 during 2018 January flare

We present a detailed temporal and spectral study of the blazar 3C\,279 using multi-wavelength observations from Swift-XRT, Swift-UVOT and Fermi-LAT during a flare in 2018 January. The temporal analysis of $γ$-ray light curve indicates a lag of $\sim 1$ d between the 0.1--3 GeV and 3--500 GeV emission. Additionally, the $γ$-ray light curve shows asymmetry with slow rise--fast decay in energy band 0.1--3 GeV and fast rise--slow decay in the 3--500 GeV band. We interpret this asymmetry as a result of shift in the Compton spectral peak. This inference is further supported by the correlation studies between the flux and the parameters of the log-parabola fit to the source spectra in the energy range 0.1--500 GeV. We found that the flux correlates well with the peak spectral energy and the log-parabola fit parameters show a hard index with large curvature at high flux states. Interestingly, the hardest index with large curvature was synchronous with a very high energy flare detected by H.E.S.S. Our study of the spectral behavior of the source suggests that $γ$-ray emission is most likely to be associated with the Compton up-scattering of IR photons from the dusty environment. Moreover, the fit parameters indicate the increase in bulk Lorentz factor of emission region to be a dominant cause for the flux enhancement.

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arXiv2018

Log-normal flux distribution of bright Fermi blazars

We present the results of the $γ$-ray flux distribution study on the brightest blazars which are observed by the \emph{Fermi}-LAT. We selected 50 brightest blazars based on the maximum number of detection reported in the LAT third AGN catalog. We performed standard unbinned maximum likelihood analysis on the LAT data during the period between August 2008 and December 2016, in order to obtain the average monthly flux. After quality cuts, blazars for which at least 90\% of the total flux was survived were selected for the further study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and $χ^2$ tests suggest that the integrated monthly flux follow a log-normal distribution for all sources, except for three FSRQs for which neither a normal nor a log-normal distribution was preferred. A double log-normal flux distribution tendency were observed in these sources, though it has to be confirmed with improved statistics. We also found that, the standard deviation of the log-normal flux distribution increases with the mean spectral index of the blazar, and can be fitted with a line of slope 0.24$\pm$0.04. We repeat our study on three additional brightest unclassified blazars to identify their flux distribution properties. Based on the features of their log-normal flux distribution, we infer these unclassified blazars may be closely associated with FSRQs. We also highlight that considering the log-normal behavior of the flux distribution of blazars, averaging their long term flux in linear scale can largely under estimate the nominal flux and this discrepancy can propagate down to the estimation of source parameters through spectral modeling.

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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.

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