arXiv subjects

Haritma Gaur

Publications and source records attributed to Haritma Gaur.

At least 19 recordsLinked to original sources

arXiv2026

Detection of quasi-periodic oscillations in the 37 GHz radio light curve of the blazar Ton 599 during 1990-2020

Blazars are a subclass of radio-loud active galactic nuclei (AGNs) that display strong multi-wavelength variability on diverse timescales ranging from years down to minutes. In the last 1.5 decades, there have been occasional detections of quasi-periodic oscillations in several blazars in their time series data. We search for quasi-periodic oscillations (QPOs) in the 37 GHz radio band light curve of the flat-spectrum radio quasar Ton~599 made at the RT-22 radio telescope in Simeiz, Crimea, from 1990 to 2020. To identify and quantify the QPO nature of this radio light curve of Ton 599, we used the Lomb-Scargle periodogram (LSP), REDFIT, and weighted wavelet Z-transform (WWZ) analyses. We report the detection of a likely QPO of about 2.4 years in the 37 GHz radio light curves of Ton 599. We briefly discuss possible emission models for radio-loud active galactic nuclei that could explain such QPOs with periods of a few years.

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arXiv2025

Signature of Seyfert-like component in a blazar 3C 273 and its reflection-based explanation

We present the results of blazar 3C 273 obtained from simultaneous observations obtained using XMM-Newton and NuSTAR satellites during the period 2015-2019 in five epochs. When the spectra are modeled with a power-law, significant residuals arise below 2 keV and in the energy range of 30-78 keV in NuSTAR data. Residuals in the lower energy band represent soft X-ray excess while at higher energies it likely represents Compton reflection hump which might be a weak component arising from dense and cold material. The presence of a faint iron line is present in XMM-Newton observations. We interpret such features as attributed to the coronal emission plus those arising from reflection from an accretion disk. We model the SEDs with the single zone inverse Compton jet model based on Synchrotron Self Compton and External Compton phenomena. It is found that a one-zone synchrotron plus IC model explains quite well the SEDs but the jet component alone fails to fit the multiband X-ray emission for the low state of this object in 2018 and 2019 which arises due to spectral flattening at low energy X-rays, indicating that an additional Seyfert-like thermal component must be present at X-rays. This is further supported by a big blue bump present in the optical/ultraviolet band in all SEDs. Finally, we analyzed all the epochs using relxill model to incorporate relativistic reflection to model those residuals of soft excess and Compton hump in the X-ray bands.

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arXiv2023

Optical intra-day variability of the blazar S5 0716+714

We present an extensive recent multi-band optical photometric observations of the blazar S5 0716+714 carried out over 53 nights with two telescopes in India, two in Bulgaria, one in Serbia, and one in Egypt during 2019 November -- 2022 December. We collected 1401, 689, 14726, and 165 photometric image frames in B, V, R, and I bands, respectively. We montiored the blazar quasi-simultaneously during 3 nights in B, V, R, and I bands; 4 nights in B, V, and R; 2 nights in V, R, and I; 5 nights in B and R; and 2 nights in V and R bands. We also took 37 nights of data only in R band. Single band data are used to study intraday flux variability and two or more bands quasi-simultaneous observations allow us to search for colour variation in the source. We employ the power-enhanced F-test and the nested ANOVA test to search for genuine flux and color variations in the light curves of the blazar on intraday timescales. Out of 12, 11, 53, and 5 nights observations, intraday variations with amplitudes between ~3% and ~20% are detected in 9, 8, 31 and 3 nights in B, V, R, and I bands, respectively, corresponding to duty cycles of 75%, 73%, 58% and 60%. These duty cycles are lower than those typically measured at earlier times. On these timescales color variations with both bluer-when-brighter and redder-when-brighter are seen, though nights with no measurable colour variation are also present. We briefly discuss possible explanations for this observed intraday variability.

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arXiv2023

Multi-band optical variability on diverse timescales of the TeV blazar TXS 0506+56, the first cosmic neutrino source

We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506+056 on intra-night to long-term timescales using BVRI data collected over 220 nights between January 21, 2017 to April 9, 2022 using 8 optical ground-based telescopes. In our search for intraday variability (IDV), we have employed two statistical analysis techniques, the nested ANOVA test and the power enhanced F-test. We found the source was variable in 8 nights out of 35 in the R-band and in 2 of 14 in the V-band yielding Duty Cycles (DC) of 22.8% and 14.3%, respectively. Clear colour variation in V - R was seen in only 1 out of 14 observing nights, but no IDV was found in the more limited B, I, and B - I data. During our monitoring period the source showed a 1.18 mag variation in the R-band and similar variations are clearly seen at all optical wavelengths. We extracted the optical (BVRI) SEDs of the blazar for 44 nights when observations were carried out in all four of those wavebands. The mean spectral index (α) was determined to be 0.897+-0.171

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arXiv2023

Quasi-simultaneous Optical Flux and Polarization Variability of the Binary Super Massive Black Hole Blazar OJ 287 from 2015 to 2023: Detection of an Anticorrelation in Flux and Polarization Variability

We study the optical flux and polarization variability of the binary black hole blazar OJ 287 using quasi-simultaneous observations from 2015 to 2023 carried out using telescopes in the USA, Japan, Russia, Crimea, and Bulgaria. This is one of the most extensive quasi-simultaneous optical flux and polarization variability studies of OJ 287. OJ 287 showed large amplitude, ~3.0 mag flux variability, large changes of ~37% in degree of polarization, and a large swing of ~215 degrees in the angle of the electric vector of polarization. During the period of observation, several flares in flux were detected. Those flares are correlated with a rapid increase in the degree of polarization and swings in electric vector of polarization angle. A peculiar behavior of anticorrelation between flux and polarization degree, accompanied by a nearly constant polarization angle, was detected from JD 2,458,156 to JD 2,458,292. We briefly discuss some explanations for the flux and polarization variations observed in OJ 287.

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arXiv2023

X-ray Studies of Blazar 1ES 1959+650 Using SWIFT & XMM-NEWTON Satellite

High synchrotron energy peaked blazar 1ES 1959+650 is studied with Swift and XMM-Newton satellite in total 127 observations during the period June 2018-December 2020. We extensively studied its flux and spectral variability on intra-day and long-term timescales. Discrete correlation function analysis between soft and hard X-ray bands indicates soft as well as hard lags. The results are used to constrain the magnetic field of the emitting region which is found to be 0.64 (0.05) Gauss. On long-term timescales, distribution of fluxes shows lognormality behaviour which could be attributed to minijets-in-a-jet model or might be due to the propagation of relativistic shocks down the jet. The spectral energy distribution around the synchrotron peak is well described by the log parabola model. Spectral parameters like peak energy E$_{p}$, curvature $β$ and the peak luminosity L$_{p}$ are derived from spectral analysis. Their correlations are studied to constrain the acceleration processes of the emitting particles. E$_{p}$ shows strong correlation with L$_{p}$ during the high state of the source which indicates spectral changes might be caused by the variations of the average electron energy. Low values of curvature parameter $β$ and a weak correlation between E$_{p}$ and $β$ indicates co-existence of stochastic/statistical acceleration of electrons in the emitting region. Implications of other results are also discussed.

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arXiv2023

Intra-night optical flux and polarization variability of BL~Lacertae during its 2020 $-$ 2021 high state

In this work, we report the presence of rapid intra-night optical variations in both -- flux and polarization of the blazar BL Lacertae during its unprecedented 2020--2021 high state of brightness. The object showed significant flux variability and some color changes, but no firmly detectable time delays between the optical bands. The linear polarization was also highly variable in both -- polarization degree and angle (EVPA). The object was observed from several observatories throughout the world, covering in a total of almost 300 hours during 66 nights. Based on our results, we suggest, that the changing Doppler factor of an ensemble of independent emitting regions, travelling along a curved jet that at some point happens to be closely aligned with the line of sight can successfully reproduce our observations during this outburst. This is one of the most extensive variability studies of the optical polarization of a blazar on intra-night timescales.

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arXiv2022

Multi-band Optical Variability of the TeV Blazar PG 1553+113 in 2019

We report the flux and spectral variability of PG 1553+113 on intra-night (IDV) to short-term timescales using BVRI data collected over 91 nights from 28 February to 8 November 2019 employing ten optical telescopes: three in Bulgaria, two each in India and Serbia, and one each in Greece, Georgia, and Latvia. We monitored the blazar quasi-simultaneously for 16 nights in the V and R bands and 8 nights in the V, R, I bands and examined the light curves (LCs) for intra-day flux and colour variations using two powerful tests: the power-enhanced F-test and the nested ANOVA test. The source was found to be significantly (> 99%) variable in 4 nights out of 27 in R-band, 1 out of 16 in V-band, and 1 out of 6 nights in I-band. No temporal variations in the colours were observed on IDV timescale. During the course of these observations the total variation in R-band was 0.89 mag observed. We also investigated the spectral energy distribution (SED) using B, V, R, and I band data. We found optical spectral indices in the range of 0.878+-0.029 to 1.106+-0.065 by fitting a power law to these SEDs of PG 1553+113. We found that the source follows a bluer-when-brighter trend on IDV timescales. We discuss possible physical causes of the observed spectral variability.

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arXiv2022

Study of Intra-Day Flux Distributions of Blazars Using XMM-Newton Satellite

We present a study of the flux distribution of a sample of 15 Intermediate and Low-energy peaked blazars using XMM-Newton observations in a total of 57 epochs on short-term timescales. We characterise the X-ray variability of all of the light curves using excess fractional variability amplitude and found that only 24 light curves in 7 sources are significantly variable. In order to characterise the origin of X-ray variability in these blazars, we fit the flux distributions of all these light curves using Gaussian and lognormal distributions, as any non-Gaussian perturbation could indicate the imprints of fluctuations in the accretion disc, which could be Doppler boosted through the relativistic jets in blazars. However, intra-day variability, as seen in our observations, is difficult to reconcile using disc components as the emissions in such sources are mostly dominated by jets. We used Anderson-Darling (AD) and $χ^{2}$ tests to fit the histograms. In 11 observations of 4 blazars, namely, ON 231, 3C 273, PKS 0235+164 and PKS 0521-365, both models equally fit the flux distributions. In the rest of the observations, we are unable to model them with any distribution. In two sources, namely, BL Lacertae and S4 0954+650, the lognormal distribution is preferred over the normal distribution, which could arise from non-Gaussian perturbations from relativistic jets or linear Gaussian perturbation in the particle time scale leading to such flux distributions.

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arXiv2022

X-ray intraday variability of the TeV blazar Mrk 421 with {\it XMM-Newton}

Highly variable Markarian 421 is a bright high synchrotron energy peaked blazar showing wide featureless non-thermal spectrum making it a good candidate for our study of intraday flux and spectral variations over time. We analyse its X-ray observations of over 17 years taken with the EPIC-PN instrument to probe into the intraday variability properties. The photon energy band of 0.3 - 10.0 keV, and its sub-bands, soft 0.3-2.0 keV and hard 2.0-10.0 keV. To examine flux variability, fractional variability amplitude and the minimum variability timescale have been calculated. We also probed into the spectral variability by studying hardness ratio for each observation and the correlation between the two energy bands using discrete correlation function and inspecting the normalized light curves. The parameters obtained from these methods have been studied for any correlation or non-random trends. From this work, we speculate on the constraints for possible particle acceleration and emission processes in the jet, for better understanding of the processes involving a turbulent behaviour except of shocks. A positive discrete correlation function between the two sub-bands indicates the role of the same electron population in the emission of photons in the two bands. The correlation between the parameters of flux variability and parameters of spectral variation and lags in sub-energy bands provide the constraints to be considered for any modelling of emission processes.

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arXiv2022

X-ray Intraday Variability and Power Spectral Density Profiles of the Blazar 3C 273 with XMM-Newton during 2000-2021

We present X-ray intraday variability and power spectral density (PSD) analyses of the longest 23 pointed XMM-Newton observations of the blazar 3C 273 that were taken during 2000 -- 2021. These good time intervals contained between 5 and 24.6 hours of data. Variability has been estimated in three energy bands: 0.2 -- 2 keV (soft), 2 -- 10 keV (hard), and 0.2 -- 10 keV (total). Nine of the 23 observations exhibited some variability, though no major variations exceeding 5 per cent were detected. Typical timescales for variability were $\sim 1$ ks. For those variable light curves we find that a power-law model provides good fits to each PSD, with most of the slopes between $-1.7$ and $-2.8$. Although no variations of hardness ratio could be measured in any individual observation, an anti-correlation in flux and hardness ratio is found in long term data that indicates a harder when brighter trend. Our flux and spectral analyses indicate that both particle acceleration and synchrotron cooling processes make an important contribution to the emission from this blazar.

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arXiv2021

Multi-band Variability of the TeV Blazar PG 1553+113 with XMM-Newton

We present variability analyses of twenty pointed XMM-Newton observations of the high energy peaked TeV blazar PG 1553+113 taken during 2010 to 2018. We found intraday variability in the total X-ray energy range (0.3 -- 10 keV) in 16 out of 19 light curves or a duty cycle of ~84%. A discrete correlation function analysis of the intraday light curves in the soft and hard X-ray bands peaks on zero lag, showing that the emission in hard and soft bands are co-spatial and emitted from the same population of leptons. Red-noise dominates the power spectral density (PSD) of all the LCs although the PSDs have a range of spectral slopes from -2.36 to -0.14. On longer timescales, the optical and UV variability patterns look almost identical and well correlated, as are the soft and hard X-ray bands, but the optical/UV variations are not correlated to those in the X-ray band, indicating that the optical/UV and X-ray emissions are emitted by two different populations of leptons. We briefly discuss physical mechanisms which may be capable of explaining the observed flux and spectral variability of PG 1553+113 on these diverse timescales.

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arXiv2021

Breaks in the X-ray spectra of high redshift blazars and the intervening medium

The flat spectrum radio quasars (FSRQs) are a sub-class of blazars characterised by prominent optical emission lines and a collimated large-scale jet along the observer line of sight. An X-ray spectral flattening has been reported in FSRQs (at relatively high redshifts), attributable to either absorption from gas along the line of sight or intrinsic jet based radiative processes. We study a sample of 16 high redshift FSRQs (z of 1.1 -- 4.7; rest frame energy upto 50 keV) observed with XMM-Newton and Swift satellites spanning 29 epochs. The X-ray spectra are fit with a power law including free excess absorption and one multiplied by an exponential roll off to account for the intrinsic jet based processes. A statistical analysis is used to distinguish between these models to understand the origin of the spectral flattening. The model selection is unable to distinguish between them in ten of the sixteen FSRQs. Intrinsic jet based radiative processes are indicated in four FSRQs where we infer energy breaks consistent with their expectation from the external Compton scattering of low energy ambient photons. Two of the FSRQs indicate mixed results, supportive of either scenario, illustrating the difficulty in identifying X-ray absorption signatures. A clear detection can be employed to disentangle the relative contributions from the inter-galactic medium and the intra-cluster medium, the methodology of which is outlined and applied to the latter two sources.

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arXiv2021

X-ray Intraday Variability of the TeV Blazar PKS 2155-304 with Suzaku During 2005-2014

We have examined 13 pointed observations of the TeV emitting high synchrotron peak blazar PKS 2155-304, taken by the Suzaku satellite throughout its operational period. We found that the blazar showed large-amplitude intraday variabilities in the soft (0.8 - 1.5 keV) and the hard (1.5 - 8.0 keV) bands in the light curves. Spectral variability on intraday timescales is estimated using the hardness ratio. The blazar usually becomes harder when brighter and vice versa, following the typical behavior of high synchrotron peak blazars. The power spectral density (PSD) analyses of 11 out of 13 light curves in the total energy (0.8 - 8.0 keV) are found to be red-noise dominated, with power-law spectral indices that span a large range, from -2.81 to -0.88. Discrete correlation function analyses of all the 13 light curves between the soft and the hard bands show that they are well correlated and peak at, or very close to, zero lag. This indicates that the emissions in soft and hard bands are probably cospatial and emitted from the same population of leptons. Considering fluxes versus variability timescales, we found no correlation on intraday timescales, implying that X-ray emission from PKS 2155-304 is not dominated by simple changes in the Doppler factor. We briefly discuss the most likely emission mechanisms responsible for the observed flux and spectral variabilities and place constraints on magnetic field strength and Lorentz factors of the electrons emitting the X-rays in the most likely scenario.

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arXiv2020

X-ray Spectral Evolution of High Energy Peaked Blazars

The synchrotron hump of the high energy peaked blazars generally lies in the 0.1-10 keV range and such sources show extreme flux and spectral variability in X-ray bands. Various spectral studies showed that the X-ray spectra of high energy peaked blazars are curved and better described by the log-parabolic model. The curvature is attributed to the energy dependent statistical acceleration mechanism. In this work, we review the X-ray spectral studies of high energy peaked blazars. It is found that the log-parabolic model well describes the spectra in a wide energy interval around the peak. The log-parabolic model provides the possibility of investigating the correlation between the spectral parameters derived from it. Therefore, we compiled the studies of correlations between the various parameters derived from the log-parabolic model and their implications to describe the variability mechanism of blazars.

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arXiv2020

X-ray Flux and Spectral Variability of Blazar H 2356-309

We present the results of timing and spectral analysis of the blazar H 2356-309 using XMM-Newton observations. This blazar is observed during 13 June 2005-24 December 2013 in total nine observations. Five of the observations show moderate flux variability with amplitude 1.7-2.2 percent. We search for the intra-day variability timescales in these five light curves, but did not find in any of them. The fractional variability amplitude is generally lower in the soft bands than in the hard bands, which is attributed to the energy dependent synchrotron emission. Using the hardness ratio analysis, we search for the X-ray spectral variability along with flux variability in this source. However, we did not find any significant spectral variability on intra-day timescales. We also investigate the X-ray spectral curvature of blazar H 2356-309 and found that six of our observations are well described by the log parabolic model with alpha=1.99-2.15 and beta=0.03-0.18. Three of our observations are well described by power law model. The break energy of the X-ray spectra varies between 1.97-2.31 keV. We investigate the correlation between various parameters that are derived from log parabolic model and their implications are discussed.

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arXiv2019

Properties of radio-loud quasars in Sloan Digital Sky Survey

We present a study of a sample of 223 radio-loud quasars (up to redshift $<$0.3) in order to investigate their spectral properties. Twenty-six of these radio-loud quasars are identified as Flat Spectrum Radio Quasars (FSRQs) and fifty-four are identified as Steep Spectrum Radio Quasars (SSRQs) based on their radio spectral index. We study the [O III] line properties of these quasars to investigate the origin and properties of blue wings (shift of the profile towards lower wavelengths) and blue outliers (shift of the whole spectroscopic feature). Most of the quasars show blue wings with velocities up to 420 km $s^{-1}$. We find that around 17\% of the quasars show outliers with velocities spanning 419 to -315 km $s^{-1}$. Finally, we revisit the $\it M_{\rm BH} - σ$ relation of our sample using [S II]$λ$6716, 6731 and [O III] linewidths as surrogates for stellar velocity dispersions, $σ$, to investigate their location on the $\it M_{\rm BH} - σ$ relation for quiescent galaxies. Due to the strong blending of [S II] with $\rm H_α$, we could estimate $σ_{[\rm SII]}$ of only 123 quasars. We find that the radio-loud quasars do not show a relationship between $\it M_{\rm BH}$ and $σ_{\rm [SII]/[OIII]}$ up to a redshift of 0.3, although they cluster around the local relation. We find an overall offset of 0.12$\pm$0.05 dex of our sample of radio-loud quasars from the $\it M_{\rm BH} - σ$ relation of quiescent galaxies. Quasars in our highest redshift bin (z=0.25-0.3) show a deviation of $\sim$0.33 $\pm$ 0.06 dex with respect to the local relation.

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arXiv2019

X-ray Intra-day Variability of the TeV Blazar Mrk 421 with Suzaku

We present X-ray flux and spectral analyses of the three pointed Suzaku observations of the TeV high synchrotron peak blazar Mrk 421 taken throughout its complete operational duration. The observation taken on 5 May 2008 is, at 364.6 kiloseconds (i.e., 101.3 hours), the longest and most evenly sampled continuous observation of this source, or any blazar, in the X-ray energy 0.8 - 60 keV until now. We found large amplitude intra-day variability in all soft and hard bands in all the light curves. The discrete correction function analysis of the light curves in soft and hard bands peaks on zero lag, showing that the emission in hard and soft bands are cospatial and emitted from the same population of leptons. The hardness ratio plots imply that the source is more variable in the harder bands compared to the softer bands. The source is harder-when-brighter, following the general behavior of high synchrotron peak blazars. Power spectral densities of all three light curves are red noise dominated, with a range of power spectra slopes. If one assumes that the emission originates very close to the central super massive black hole, a crude estimate for its mass, of ~ 4 * 10^{8} M_{\odot}, can be made; but if the variability is due to perturbations arising there that are advected into the jet and are thus Doppler boosted, substantially higher masses are consistent with the quickest seen variations. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux and spectral variability.

astro-ph.HE↗