Rukaiya Khatoon

Bio: Rukaiya Khatoon is an academic researcher from Tezpur University. The author has contributed to research in topics: Physics & Flare. The author has an hindex of 2, co-authored 8 publications receiving 33 citations.

The flux distribution of individual blazars as a key to understand the dynamics of particle acceleration

Abstract: The observed log-normal flux distributions in the high energy emission from blazars have been interpreted as being due to variability stemming from non-linear multiplicative processes generated dynamically from the accretion disc. On the other hand, rapid minute scale variations in the flux point to a compact emitting region inside the jet, probably disconnected from the disc. In this work, we show that linear Gaussian variations of the intrinsic particle acceleration or escape time-scales can produce distinct non-Gaussian flux distributions, including log-normal ones. Moreover, the spectral index distributions can provide confirming evidence for the origin of the variability. Thus, modelling of the flux and index distributions can lead to quantitative identification of the micro-physical origin of the variability in these sources. As an example, we model the X-ray flux and index distribution of Mkn 421 obtained from over 9 years of MAXI observations and show that the variability in the X-ray emission is driven by Gaussian fluctuations of the particle acceleration process rather than that of the escape rate.

Broad-band study of OQ 334 during its flaring state

Abstract: The blazar OQ 334 displayed a {\gamma}-ray flare in 2018, after being in the long quiescent {\gamma}-ray state since 2008 Subsequent to the flare, the source was in a higher {\gamma}-ray flux state and again flared in 2020 We present here the first spectral and timing analysis of the source at its various flaring states During the higher {\gamma}-ray state, we found four major peaks identified as P1, P2, P3, and P4 From timing analysis, we found the rise and decay time of the order of hours with the fastest variability time of 901+/-078 hr We found the highest {\gamma}-ray photon of 77 GeV during P4, which suggests the location of the {\gamma}-ray emitting region at the outer edge of the broad-line region or the inner edge of the torus The {\gamma}-ray spectral analysis of the source indicates that during P4, the {\gamma}-ray spectrum clearly deviates from the power-law behavior From cross-correlation analysis of the {\gamma}-ray and radio lightcurves, we found that the two emission regions are separated by about 11 pc Our broadband spectral energy distribution modeling of the source during quiescent and active phases indicates that more electron and proton power are required to change the source from low flux to high flux state The Anderson-Darling test and histogram fitting results suggest that the three days binned {\gamma}-ray fluxes follow a lognormal distribution

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

Abstract: We present a detailed study of flux and index distributions of three blazars [one flat-spectrum radio quasar (FSRQ) and two BL Lacertae objects (BL Lacs)] by using 16 yr of Rossi X-ray Timing Explorer (RXTE) archival data. The three blazars were chosen such that their flux and index distributions have sufficient number of data points (≥90) with relatively less uncertainty |$\left(\overline{\sigma _{\rm err}^{2}}/\sigma ^{2} < 0.2\right)$| in light curves. Anderson–Darling (AD) test and histogram fitting show that flux distribution of FSRQ 3C 273 is lognormal, while its photon index distribution is Gaussian. This result is consistent with linear Gaussian perturbation in the particle acceleration time-scale, which produces lognormal distribution in flux. However, for two BL Lacs, viz. Mrk 501 and Mrk 421, AD test shows that their flux distributions are neither Gaussian nor lognormal, and their index distributions are non-normal. The histogram fitting of Mrk 501 and Mrk 421 suggests that their flux distributions are more likely to be a bimodal, and their index distributions are double Gaussian. Since, Sinha et al. had shown that Gaussian distribution of index produces a lognormal distribution in flux, double Gaussian distribution of index in Mrk 501 and Mrk 421 indicates that their flux distributions are probably double lognormal. Observation of double lognormal flux distribution with double Gaussian distribution in index reaffirms two flux states hypothesis. Further, the difference observed in the flux distribution of FSRQ (3C 273) and BL Lacs (Mrk 501 and Mrk 421) at X-rays suggests that the low-energy emitting electrons have a single lognormal flux distribution, while the high-energy ones have a double lognormal flux distribution.

A comprehensive study of the 2019–2020 flare of OJ 287 using AstroSat, Swift and NuSTAR

Abstract: OJ 287 is a well-studied binary black hole system, that occasionally exhibits bright X-ray and optical flares. Here we present a detailed spectral study of its second brightest X-ray flare observed during 2019-2020 using archival Swift and NuSTAR observations along with ToO observations from AstroSat. The entire flaring period is divided into three states, defined as low, intermediate, and high states. The variation of hardness ratio (HR) with 0.3-10.0 keV integrated flux suggests a \enquote{softer-when-brighter} behavior, as also previously reported based on flux-index variations. Simultaneous high state X-ray spectra obtained using Swift, NuSTAR and AstroSat are very steep with a power-law index $>$2. A significant spectral change is observed in AstroSat-SXT and LAXPC spectrum which is consistent with Swift-XRT and NuSTAR spectrum. Together, optical-UV and X-ray spectrum during the high flux state, suggesting the emergence of a new high BL Lacertae (HBL) component. We have modeled the synchrotron peak with publicly available code named GAMERA for low, intermediate, and high flux state. Our modeling suggests the need of high magnetic field to explain the high state under the leptonic scenarios. Increase in the magnetic field value inside the jet could be linked to the increase in accretion rate as expected in the BH-disk impact scenario. The color-magnitude diagram reveals a \enquote{bluer-when-brighter} spectral energy distribution chromatism during the flaring period. Different chromatism or no chromatism at various occasions suggests a complex origin of optical emission, which is believed to be produced by disc impact or through synchrotron emission in the jet.

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

Abstract: 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 ({\xi} 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 {\xi} 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.

Studies in Astronomical Time Series Analysis: VII. An Enquiry Concerning Non-Linearity, the RMS-Mean Flux Relation, and log-Normal Flux Distributions

Abstract: A broad and widely used class of stationary, linear, additive time series models can have statistical properties which many authors have asserted imply that the underlying process must be non-linear, non-stationary, multiplicative, or inconsistent with shot noise. This result is demonstrated with exact and numerical evaluation of the model flux distribution function and dependence of flux standard deviation on mean flux (here and in the literature called the \emph{rms-flux relation}). These models can: (1) exhibit normal, log-normal or other flux distributions; (2) show linear or slightly non-linear rms-mean flux dependencies; as well as (3) match arbitrary second order statistics of the time series data. Accordingly the above assertions cannot be made on the basis of statistical time series analysis alone. Also discussed are ambiguities in the meaning of terms relevant to this study -- \emph{linear}, \emph{stationary} and \emph{multiplicative} -- and functions that can transform observed fluxes to a normal distribution as well or better than the logarithm.

The Nature of $\gamma$-ray Variability in Blazars.

Abstract: We present an in-depth and systematic variability study of a sample of 20 powerful blazars, including 12 BL Lacs and 8 flat spectrum radio quasars, applying various analysis tools such as flux distribution, symmetry analysis, and time series analysis on the decade-long Fermi/LAT observations. The results show that blazars with steeper \gama-ray spectral indexes are found to be more variable; and the \gama-ray flux distribution closely resembles lognormal probability distribution function. The statistical variability properties of the sources as studied by power spectral density analysis are consistent with \emph{flicker noise} ($P( u)\propto1/ u$) -- an indication of long-memory processes at work. Statistical analysis of the distribution of flux rise and decay rates in the light curves of the sources, aimed at distinguishing between particle acceleration and energy dissipation timescales, counter-intuitively suggests that both kinds of rates follow a similar distribution and the derived mean variability timescales are in the order of a few weeks. The corresponding emission region size is used to constrain location of \gama-ray production sites in the sources to be a few parsecs. Additionally, using Lomb-Scargle periodogram and weighted wavelet z-transform methods and extensive Monte Carlo simulations, we detected year timescale quasi-periodic oscillations in the sources S5 0716+714, Mrk 421, ON +325, PKS 1424-418 and PKS 2155-304. The detection significance was computed taking proper account of the red-noise and other artifacts inherent in the observations. We explain the results in the light of current blazar models with relativistic shocks propagating down the jet viewed close to the line of sight.

Gamma-Ray Astrophysics in the Time Domain

Abstract: The last few years have seen gamma-ray astronomy maturing and advancing in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter, depending on the source. This review focuses on some of the key science issues and conceptual developments concerning the timing characteristics of active galactic nuclei (AGN) at gamma-ray energies. It highlights the relevance of adequate statistical tools and illustrates that the developments in the gamma-ray domain bear the potential to fundamentally deepen our understanding of the nature of the emitting source and the link between accretion dynamics, black hole physics, and jet ejection.

Evaluating Quasi-Periodic Variations in the $\gamma$-ray Lightcurves of Fermi-LAT Blazars

Abstract: The detection of periodicities in light curves of active galacticnuclei (AGN) could have profound consequences for our understanding of the nature and radiation physics of these objects. At high energies (HE; E>100 MeV) 5 blazars (PG 1553+113,PKS 2155-304, 0426-380, 0537-441, 0301-243) have been reported to show year-like quasi-periodic variations (QPVs) with significance >3 sig. As these findings are based on few cycles only, care needs to be taken to properly account for random variations which can produce intervals of seemingly periodic behaviour. We present results of an updated timing analysis for 6 blazars (adding PKS 0447-439), utilizing suitable methods to evaluate their long term variability properties and to search for QPVs in their light curves. We generate gamma-ray light curves covering almost 10 years, study their timing properties and search for QPVs using the Lomb-Scargle Periodogram and the Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the statistical significance. Comparing their probability density functions (PDFs), all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian distribution, but are consistent with being log-normal, suggesting that the underlying variability is of a non-linear, multiplicative nature. Apart from PKS 0301-243 the power spectral density for all investigated blazars is close to flicker noise (PL slope -1). Possible QPVs with a local significance ~ 3 sig. are found in all light curves (apart from PKS 0426-380 and 0537-441), with observed periods between (1.7-2.8) yr. The evidence is strongly reduced, however, if evaluated in terms of a global significance. Our results advise caution as to the significance of reported year-like HE QPVs in blazars. Somewhat surprisingly, the putative, redshift-corrected periods are all clustering around 1.6 yr. We speculate on possible implications for QPV generation.

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

Abstract: Context. The detection of periodicities in the light curves of active galactic nuclei (AGNs) could have profound consequences for our understanding of the nature and radiation physics of these objects. At high energies (HE; E > 100 MeV), five blazars (PG 1553+113, PKS 2155−304, PKS 0426−380, PKS 0537−441 and PKS 0301−243) have been reported to show year-like quasi-periodic variations (QPVs) with significance > 3σ . As these findings are based on only a few cycles, care needs to be taken to properly account for random variations that can produce intervals of seemingly periodic behavior.Aims. We present results of an updated timing analysis for six blazars (adding PKS 0447−439 to the above), using suitable methods to evaluate their long-term variability properties and to search for QPVs in their light curves.Methods. We generate γ -ray light curves covering almost ten years, study their timing properties, and search for QPVs using the Lomb-Scargle Periodogram and the Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the statistical significance.Results. (1) Comparing their probability density functions, all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian distribution, but are consistent with being log-normal, suggesting that the underlying variability is of a nonlinear, multiplicative nature. (2) Apart from PKS 0301−243, the power spectral density for all investigated blazars is close to flicker noise (power-law slope −1). (3) Possible QPVs with a local significance ≳3σ are found in all light curves (apart from PKS 0426−380 and PKS 0537−441), with observed periods in the range (1.7 − 2.8) yr. The evidence is strongly reduced however if evaluated in terms of a global significance.Conclusions. Our results advise caution as to the significance of reported year-like HE QPVs in blazars. Somewhat surprisingly, the putative redhift-corrected period all cluster around ∼1.6 yr. We speculate on possible implications for QPV generation.