Rapid optical variability of TeV blazars

Gopal-Krishna, . .; Goyal, A.; Joshi, Santosh; Karthick, Chrisphin; Sagar, Ram; Wiita, Paul J.; Anupama, G. C.; Sahu, D. K.

doi:10.1111/j.1365-2966.2011.19014.x

Cited by 36 publications

(33 citation statements)

References 110 publications

(170 reference statements)

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“…very similar to the value recently estimated for the subset of blazars detected at TeV energies (Gopal-Krishna et al 2011). In contrast, the other two classes of radio-loud AGN, namely LDQs and LPCDQs, were found to exhibit only low-level INOV and that too with a small duty cycle of only around 10-15%, which is akin to the INOV behaviour exhibited by RQQs (Stalin et al 2004a,b;also, Ramírez et al 2009).…”

Section: Referencessupporting

confidence: 47%

“…With a typical stellar velocity of < ∼ 10 3 km s −1 the expected crossing time of the inner jet by the star is > ∼ 10 2 -10 3 yr and so the postulated enhanced turbulence level in the affected jet sedgement can be a long lasting feature, consistent with the persistence of enhanced INOV in blazars underscored in the present work. However, within this basic scenario it remains to be clearly understood why, unlike the γ-ray flaring, INOV has been so rarely detected on sub-hour time scales (e.g., Gopal-Krishna et al 2011).…”

Section: Discussionmentioning

confidence: 99%

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Intra-night optical variability of core dominated radio quasars: the role of optical polarization

Goyal

Gopal-Krishna

Wiita

et al. 2012

A&A

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Context. Rapid variations in optical flux are seen in many quasars and all blazars. The amount of variability in different classes of active galactic nuclei has been studied extensively but many questions remain unanswered. Aims. We present the results of a long-term programme to investigate the intra-night optical variability (INOV) of powerful flat spectrum radio core-dominated quasars (CDQs), with a focus on probing the relationship of INOV to the degree of optical polarization. Methods. We observed a sample of 16 bright CDQs showing strong broad optical emission lines and consisting of both high and low optical polarization quasars (HPCDQs and LPCDQs). In this first systematic study of its kind, we employed the 104-cm Sampurnanand telescope, the 201-cm Himalayan Chandra telescope and the 200-cm IUCAA-Girawali Observatory telescope, to carry out R-band monitoring on a total of 47 nights. Using the CCD as an N-star photometer to densely monitor each quasar for a minimum duration of about 4 h per night, INOV exceeding ∼1-2 per cent could be reliably detected. Combining these INOV data with those taken from the literature, after ensuring conformity with the basic selection criteria we adopted for the 16 CDQs monitored by us, we were able to increase the sample size to 21 CDQs (12 LPCDQs and 9 HPCDQs) monitored on a total of 73 nights. Results. As the existence of a prominent flat-spectrum radio core signifies that strong relativistic beaming is present in all these CDQs, the definitions of the two sets differ primarily in fractional optical polarization, with the LPCDQs showing a very low median P op 0.4 per cent. Our study yields an INOV duty cycle (DC) of ∼28 per cent for the LPCDQs and ∼68 percent for HPCDQs. If only strong INOV with fractional amplitude above 3 per cent is considered, the corresponding DCs are ∼7 per cent and ∼40 per cent, respectively. Conclusions. From this strong contrast between the two classes of luminous, relativistically beamed quasars, it is apparent that relativistic beaming is normally not a sufficient condition for strong INOV and a high optical polarization is the other necessary condition. Moreover, the correlation is found to persist for many years after the polarization measurements were made. Some possible implications of this result are pointed out, particularly in the context of the recently detected rapid γ-ray variability of blazars.

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Section: Referencessupporting

confidence: 47%

Section: Discussionmentioning

confidence: 99%

Intra-night optical variability of core dominated radio quasars: the role of optical polarization

Goyal

Gopal-Krishna

Wiita

et al. 2012

A&A

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“…The emission observed from blazars is known to be variable at all wavelengths on different timescales down to hours or even minutes in the E-mail: alicja.wierzcholska@ifj.edu.pl most extreme cases (e.g. Wagner & Witzel 1995;Aharonian et al 2007c;Gopal-Krishna et al 2011;Saito et al 2013;Liao & Bai 2015). Also, flux changes in blazars are often associated with spectral variability (e.g.…”

Section: Introductionmentioning

confidence: 99%

X-ray spectral studies of TeV γ-ray emitting blazars

Wierzcholska

Wagner

2016

Mon. Not. R. Astron. Soc.

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This work is a summary of the X-ray spectral studies of 29 TeV γ-ray emitting blazars observed with Swift/XRT especially focusing on sources for which X-ray regime allows to study the low and the high energy ends of the particle distributions function. Variability studies require simultaneous coverage, ideally sampling different flux states of each source. This is achieved using X-ray observations by disentangling the high-energy end of the synchrotron emission and the low-energy end of the Compton emission, which are produced by the same electron population. We focused on a sample of 29 TeV γ-ray emitting blazars with the best signal-to-noise X-ray observations collected with Swift/XRT in the energy range of 0.3-10 keV during 10 years of Swift/XRT operations. We investigate the X-ray spectral shapes and the effects of different corrections for neutral hydrogen absorption and decompose the synchrotron and inverse Compton components. In the case of 5 sources (3C 66A, S5 0716+714, W Comae, 4C +21.35 and BL Lacertae) a superposition of both components is observed in the X-ray band, permitting simultaneous, time resolved studies of both ends of the electron distribution. The analysis of multi-epoch observations revealed that the break energy of X-ray spectrum varies only by a small factor with flux changes. Flux variability is more pronounced in the synchrotron domain (high-energy end of the electron distribution) than in the Compton domain (low energy end of the electron distribution). The spectral shape of the Compton domain is stable, while the flux of the synchrotron domain is variable. These changes cannot be described by simple variations of the cut-off energy, suggesting that the high-energy end of the electron distribution is not generally well-described by cooling only.

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“…Thus, while making comparison with the above mentioned other AGN classes, our present estimate of INOV duty cycle for RQWLQs (∼ 5 per cent) may be treated as a lower limit. This cautionary remark is underscored by the fact that both events of INOV detection reported here (Table 4) are marked by extremely large amplitudes (ψ ∼ 30% peak-to-peak, occurring on hour-like time scale), rivaling blazars in their highly active phases (e.g, Sagar et al 2004;Gopal-Krishna et al 2011;Goyal et al 2012). Clearly, it would be very interesting to check if a factor of 2 − 3 improvement in ψ lim would reveal many more events of INOV among RQWLQs, yielding a statistically robust estimate for the duty cycle of strong INOV (ψ > 3%) for RQWLQs, which is distinctly higher than the present estimate of ∼ 5%, perhaps even approaching the high values established for blazars.…”

Section: Resultsmentioning

confidence: 76%

Intranight optical variability of radio-quiet weak emission line quasars – III

Gopal-Krishna¹,

Joshi²,

Chand³

2015

Monthly Notices of the Royal Astronomical Society

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This is continuation of our programme to search for the elusive radio-quiet BL Lacs, by carrying out a systematic search for intranight optical variability (INOV) in a subset of 'weak-line quasars' which are already designated as 'high-confidence BL Lac candidate' and are also known to be radio-quiet. For 6 such radio-quiet weak-line quasars (RQWLQs), we present here new INOV observations taken in 11 sessions of duration >3 hours each. Combining these data with our previously published INOV monitoring of RQWLQs in 19 sessions yields INOV observations for a set of 15 RQWLQs monitored in 30 sessions, each lasting more than 3 hours. The 30 differential light curves, thus obtained for the 15 RQWLQs, were subjected to a statistical analysis using the F−test, and the deduced INOV characteristics of the RQWLQs then compared with those published recently for several prominent AGN classes, also applying the F−test. From our existing INOV observations, there is a hint that RQWLQs in our sample show a significantly higher INOV duty cycle than radio-quiet quasars and radio lobedominated quasars. Two sessions when we have detected strong (blazar-like) INOV for RQWLQs are pointed out, and these two RQWLQs are therefore the best known candidates for radio-quiet BL Lacs, deserving to be pursued. For a proper comparison with the INOV properties already established for (brighter) members of several prominent classes of AGN, a factor of 2 − 3 improvement in the INOV detection threshold for the RQWLQs is needed and it would be very interesting to check if that would yield a significantly higher estimate for INOV duty cycle than is found here.

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Rapid optical variability of TeV blazars

Cited by 36 publications

References 110 publications

Intra-night optical variability of core dominated radio quasars: the role of optical polarization

Intra-night optical variability of core dominated radio quasars: the role of optical polarization

X-ray spectral studies of TeV γ-ray emitting blazars

Intranight optical variability of radio-quiet weak emission line quasars – III

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