arXiv subjects

Alok C. Gupta

Publications and source records attributed to Alok C. Gupta.

At least 127 records · Page 7Linked to original sources

arXiv2015

Multi-band optical variability studies of BL Lacertae

We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during October and November 2014 including quasi-simultaneous observations in V and R bands using two optical telescopes in India. We have studied multi-band optical flux variations, colour variation and spectral changes in this blazar. Source was found to be active during the whole monitoring period and showed significant intraday variability on 3 nights in V and R filters while displayed hints of variability on 6 other dates in R passband and on 2 nights in V filter. From the colour-magnitude analysis of the source we found that the spectra of the target gets flatter as it becomes brighter on intra-night timescale. Using discrete correlation technique, we found that intraday light curves in both V and R filters are almost consistent and well correlated with each other. We also generated spectral energy distribution (SED) of the target using the B, V, R, and I data sets for all 13 nights which could help us investigate the physical process responsible for the observed variations in BL Lacertae objects. We also discuss possible physical causes of the observed spectral variability.

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arXiv2014

Spectral energy distributions of the BL Lac PKS 2155-304 from XMM-Newton

We have used all 20 archival XMM-Newton observations of PKS 2155-304 with simultaneous X-ray and UV/optical data to study its long term flux and spectral variability. We find significant variations, in all bands, on time-scales of years with an rms amplitude of ~ 35-45 per cent, through the optical/UV variations are not correlated with those in the X-ray. We constructed SEDs that span more than three orders of magnitude in frequency and we first fitted them with a log-parabolic model; such models have been applied many times in the past for this, and other, blazars. These fits were poor, so we then examined combined power-law and log-parabolic fits that are improvements. These models indicate that the optical/UV and X-ray flux variations are mainly driven by model normalization variations, but the X-ray band flux is also affected by spectral variations, as parametrized with the model 'curvature' parameter, b. Overall, the energy at which the emitted power is maximum correlates positively with the total flux. As the spectrum shifts to higher frequencies, the spectral 'curvature' increase, in contrast to what is expected if a single log-parabolic model were an acceptable representation of the broad band SEDs. Our results suggest that the optical/UV and X-ray emissions in this source may arise from different lepton populations.

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arXiv2013

Anti-correlated Optical Flux and Polarization Variability in BL Lac

We present the results of photometric (V band) and polarimetric observations of the blazar BL Lac during 2008--2010 using TRISPEC attached to the KANATA 1.5-m telescope in Japan. The data reveal a great deal of variability ranging from days to months with detection of strong variations in fractional polarization. The V band flux strongly anti-correlates with the degree of polarization during the first of two observing seasons but not during the second. The direction of the electric vector, however, remained roughly constant during all our observations. These results are consistent with a model with at least two emission regions being present, with the more variable component having a polarization direction nearly perpendicular to that of the relatively quiescent region so that a rising flux can produce a decline in degree of polarization. We also computed models involving helical jet structures and single transverse shocks in jets and show that they might also be able to agree with the anti-correlations between flux and fractional polarization.

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arXiv2012

The nature of the intra-night optical variability in blazars

In this paper we present results of a short-term optical monitoring of 13 blazars. The objects were monitored mostly in the R-band for a total of ~ 160 hours between 2006 and 2011. We study the nature of the short-term variations and show that most of them could be described as slow, smooth, and (almost) linear changes of up to ~ 0.1 mag/hour, but many objects show no short-term variations at all. In fact, we found only ~ 2 per cent chance to observe variability of more than 0.1 mag/hour for the sample we observed. Hints for quasi-periodic oscillations at very low amplitude levels are also found for some objects. We briefly discuss some of the possible mechanisms to generate the intra-night variability and the quasi-periodic oscillations.

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arXiv2012

Multiwavelength Intraday Variability of the BL Lac S5 0716+714

We report results from a 1 week multi-wavelength campaign to monitor the BL Lac object S5 0716+714 (on December 9-16, 2009). In the radio bands the source shows rapid (~ (0.5-1.5) day) intra-day variability with peak amplitudes of up to ~ 10 %. The variability at 2.8 cm leads by about 1 day the variability at 6 cm and 11 cm. This time lag and more rapid variations suggests an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 cm and 11 cm interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ~ 0.8 mag were detected in the V, R and I-bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 day time scale, favoring reprocessed inverse-Compton over synchrotron radiation in this band. The characteristic variability time scales in radio and optical bands are similar. A quasi-periodic variation (QPO) of 0.9 - 1.1 days in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical), and is consistent with a high jet opacity and a compact synchrotron component peaking at ~= 100 GHz in an ongoing very prominent flux density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution (SED), including gamma-ray data from the FERMI satellite. We obtain lower limits for the relativistic Doppler-boosting of delta >= 12-26, which for a BL\,Lac type object, is remarkably high.

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arXiv2012

Optical Flux and Spectral Variability of Blazars

We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacs and 1 Flat Spectrum Radio Quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intra-day and short-term timescales of months and have limited data for 2 more blazars. These photometric observations were made during 2009 to 2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term timescales we found significant flux variations in 9 of the sources and colour variations in 3 of them. Intra-day variability was detected on 6 nights for 2 sources out of the 18 nights and 4 sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for 3 more blazars) were used to analyze their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes 6 high-synchrotron-frequency-peaked BL Lacs (HSPs), 3 intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and 6 low-synchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the Synchrotron Self-Compton (SSC) loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.

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arXiv2011

Multiwavelength Variability of the Blazars Mrk 421 and 3C 454.3 in the High-State

We report the results of photometric observations of the blazars Mrk 421 and 3C 454.3 designed to search for intraday variability (IDV) and short-term variability (STV). Optical photometric observations were spread over eighteen nights for Mrk 421 and seven nights for 3C 454.3 during our observing run in 2009-2010 at the 1.04 m telescope at ARIES, India. Genuine IDV is found for the source 3C 454.3 but not for Mrk 421. Genuine STV is found for both sources. Mrk 421 was revealed by the MAXI X-ray detector on the International Space Station to be in an exceptionally high flux state in 2010 January - February. We performed a correlation between the X-ray and optical bands to search for time delays and found a weak correlation with higher frequencies leading the lower frequencies by about ten days. The blazar 3C 454.3 was found to be in high flux state in November-December 2009. We performed correlations in optical observations made at three telescopes, along with X-ray data from the MAXI satellite and public release gamma-ray data from the Fermi space telescope. We found strong correlations between the gamma-ray and optical bands at a time lag of about four days but the X-ray flux is not correlated with either. We briefly discuss the possible reasons for the time delays between these bands within the framework of existing models for X-ray and gamma-ray emission mechanisms.

astro-ph.CO↗

arXiv2011

Quasi-simultaneous two band optical variability of the blazars 1ES 1959+650 and 1ES 2344+514

We report the results of quasi-simultaneous two filter optical monitoring of two high-energy peaked blazars, 1ES 1959+650 and 1ES 2344+514, to search for microvariability and short-term variability (STV). We carried out optical photometric monitoring of these sources in an alternating sequence of B and R pass-bands, and have 24 and 19 nights of new data for these two sources, respectively. No genuine microvariability (intra-night variability) was detected in either of these sources. This non-detection of intra-night variations is in agreement with the conclusions of previous studies that high-energy peaked BL Lacs are intrinsically less variable than low-energy peaked BL Lacs in the optical bands. We also report the results of STV studies for these two sources between July 2009 and August 2010. Genuine STV is found for the source 1ES 1959+650 but not for 1ES 2344+514. We briefly discuss possible reasons for the difference between the intra-night variability behaviour of high- and low-energy peaked blazars.

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arXiv2011

Probing spectral properties of radio-quiet quasars searched for optical microvariability-II

In the context of AGN unification scheme rapid variability properties play an important role in understanding any intrinsic differences between sources in different classes. In this respect any clue based on spectral properties will be very useful toward understanding the mechanisms responsible for the origin of rapid small scale optical variations, or microvariability. Here we have added spectra of 46 radio-quiet quasars (RQQSOs) and Seyfert 1 galaxies to those of our previous sample of 37 such objects, all of which had been previously searched for microvariability. We took new optical spectra of 33 objects and obtained 13 others from the literature. Their \hbeta and \mgii emission lines were carefully fit to determine line widths (FWHM) as well as equivalent widths (EW) due to the broad emission line components. The line widths were used to estimate black hole masses and Eddington ratios, $\ell$. Both EW and FWHM are anticorrelated with $\ell$. Nearly all trends were in agreement with our previous work, although the tendency for sources exhibiting microvariability to be of lower luminosity was not confirmed. Most importantly, this whole sample of EW distributions provides no evidence for the hypothesis that a weak jet component in the radio quiet AGNs is responsible for their microvariability.

astro-ph.CO↗

arXiv2011

Spectral Energy Distribution variation in BL Lacs and FSRQs

We present the results of our study of spectral energy distributions (SEDs) of a sample of ten low- to intermediate-synchrotron-peaked blazars. We investigate some of the physical parameters most likely responsible for the observed short-term variations in blazars. To do so, we focus on the study of changes in the SEDs of blazars corresponding to changes in their respective optical fluxes. We model the observed spectra of blazars from radio to optical frequencies using a synchrotron model that entails a log-parabolic distribution of electron energies. A significant correlation among the two fitted spectral parameters ($a$, $b$) of log-parabolic curves and a negative trend among the peak frequency and spectral curvature parameter, $b$, emphasize that the SEDs of blazars are fitted well by log-parabolic curves. On considering each model parameter that could be responsible for changes in the observed SEDs of these blazars, we find that changes in the jet Doppler factors are most important.

astro-ph.CO↗

arXiv2011

Optical Intraday Variability Studies of Ten Low Energy Peaked Blazars

We have carried out optical (R band) intraday variability (IDV) monitoring of a sample of ten bright low energy peaked blazars (LBLs). Forty photometric observations, of an average of ~ 4 hours each, were made between 2008 September and 2009 June using two telescopes in India. Measurements with good signal to noise ratios were typically obtained within 1-3 minutes, allowing the detection of weak, fast variations using N-star differential photometry. We employed both structure function and discrete correlation function analysis methods to estimate any dominant timescales of variability and found that in most of the cases any such timescales were longer than the duration of the observation. The calculated duty cycle of IDV in LBLs during our observing run is ~ 52%, which is low compared to many earlier studies; however, the relatively short periods for which each source was observed can probably explain this difference. We briefly discuss possible emission mechanisms for the observed variability.

astro-ph.CO↗

arXiv2010

Variability of the Spectral Energy Distribution of the Blazar S5 0716+714

The emission from blazars is known to be variable at all wavelengths. The flux variability is often accompanied by spectral changes. Spectral energy distribution (SED) changes must be associated with changes in the spectra of emitting electrons and/or the physical parameters of the jet. Meaningful modeling of blazar broadband spectra is required to understand the extreme conditions within the emission region. Not only is the broadband SED crucial, but also information about its variability is needed to understand how the highest states of emission occur and how they differ from the low states. This may help in discriminating between models. Here we present the results of our SED modeling of the blazar S5 0716+714 during various phases of its activity. The SEDs are classified into different bins depending on the optical brightness state of the source.

astro-ph.CO↗

arXiv2010

Optical microvariability properties of BALQSOs

We present optical light curves of 19 radio quiet (RQ) broad absorption line (BAL) QSOs and study their rapid variability characteristics. Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to clearly detect any such microvariability exceeding 0.01--0.02 mag. Our observations cover a total of 13 nights (~72 hours) with each quasar monitored for about 4 hours on a given night. Our sample size is a factor of three larger than the number of radio-quiet BALQSOs previously searched for microvariability. We introduce a scaled F-test statistic for evaluating the presence of optical microvariability and demonstrate why it is generally preferable to the statistics usually employed for this purpose. Considering only unambiguous detections of microvariability we find that ~11 per cent of radio-quiet BALQSOs (two out of 19 sources) show microvariability for an individual observation length of about 4 hr. This new duty cycle of 11 per cent is similar to the usual low microvariability fraction of normal RQQSOs with observation lengths similar to those of ours. This result provides support for models where radio-quiet BALQSO do not appear to be a special case of the RQQSOs in terms of their microvariability properties.

astro-ph.CO↗

arXiv2010

Quasi-Periodic Oscillations of ~ 15 minutes in the Optical Light Curve of the BL Lac S5 0716+714

Over the course of three hours on 27 December 2008 we obtained optical (R-band) observations of the blazar S5 0716+714 at a very fast cadence of 10 s. Using several different techniques we find fluctuations with an approximately 15-minute quasi-period to be present in the first portion of that data at a > 3 sigma confidence level. This is the fastest QPO that has been claimed to be observed in any blazar at any wavelength. While this data is insufficient to strongly constrain models for such fluctuations, the presence of such a short timescale when the source is not in a very low state seems to favor the action of turbulence behind a shock in the blazar's relativistic jet.

astro-ph.CO↗

arXiv2010

Detection of Intra-day Variability Timescales of Four High Energy Peaked Blazars with XMM-Newton

We selected a sample of 24 XMM-Newton light curves (LCs) of four high energy peaked blazars, PKS 0548-322, ON 231, 1ES 1426+428 and PKS 2155-304. These data comprise continuous light curves of 7.67h to 18.97h in length. We searched for possible quasi-periodic oscillations (QPO) and intra-day variability (IDV) timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS 2155-304 which was reported elsewhere (Lachowicz et al. 2009). In the remaining 23 LCs we found hints of possible weak QPOs in one LC of each of ON 231 and PKS 2155-304, but neither is statistically significant. We found IDV timescales that ranged from 15.7 ks to 46.8 ks in 8 LCs. In 13 LCs any variability timescales were longer than the length of the data. Assuming the possible weak QPO periods in the blazars PKS 2155-304 and ON 231 are real and are associated with the innermost portions of their accretion disk, we can estimate that their central black hole masses exceed 1.2 $\times$ 10$^{7}$ M$_{\odot}$. Emission models for radio-loud active galactic nuclei (AGN) that could explain our results are briefly discussed.

astro-ph.CO↗

arXiv2010

Probing spectral properties of radio-quiet quasars searched for optical microvariability

We obtained SDSS spectra for a set of 37 radio-quiet quasars (RQQSOs) that had been previously examined for rapid small scale optical variations, or microvariability. Their H-beta and Mg II emission lines were carefully fit to determine line widths (FWHM) as well as equivalent widths (EW) due to the broad emission line components. The line widths were used to estimate black hole masses and Eddington ratios, ell. Both EW and FWHM are anticorrelated with ell. The EW distributions provide no evidence for the hypothesis that a weak jet component in the RQQSOs is responsible for their microvariability.

astro-ph.CO↗

arXiv2010

Short Term Flux and Colour Variations in Low-Energy Peaked Blazars

We have measured multi-band optical flux and colour variations for a sample of 12 low energy peaked blazars on short, day-to-month, timescales. Our sample contains six BL Lacertae objects and six flat spectrum radio quasars. These photometric observations, made during September 2008 to June 2009, used five optical telescopes, one in India and four in Bulgaria. We detected short term flux variations in eleven of these blazars and colour variability in eight of them. Our data indicate that six blazars (3C 66A, AO 0235+164, S5 0716+714, PKS 0735+178, OJ 287 and 3C 454.3) were observed in pre- or post-outburst states, that five (PKS 0420-014, 4C 29.45, 3C 279, PKS 1510-089 and BL Lac) were in a low state, while one (3C 273) was in an essentially steady state. The duty cycles for flux and colour variations on short timescales in these low energy peaked blazars are ~ 92 percent and ~ 33 percent, respectively. The colour vs magnitude correlations seen here support the hypothesis that BL Lac objects tend to become bluer with increase in brightness; however, flat spectrum radio quasars may show the opposite trend, and there are exceptions to these trends in both categories of blazar. We briefly discuss emission models for active galactic nuclei that might explain our results.

astro-ph.CO↗

arXiv2009

A Quasi-Periodic Oscillation of ~ 30 Minutes in the X-ray Light Curve of the BL Lac S5 0716+714

We withdraw our claim that a component in an XMM-Newton satellite light curve of the BL Lacertae object S5 0716 + 714 shows quasi-periodic oscillations (QPOs) of $\sim$30 minutes. Although both our original periodogram and wavelet analyses gave consistent results, the data do not lead to a statistically significant result once red-noise at low frequencies is properly taken into account for periodogram analyses.

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