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Register a new userOptical Intra-day Variability in Blazars
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A. C. Gupta
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We selected a sample of a dozen blazars which are the prime candidates for simultaneous multi-wavelength observing campaigns in their outburst phase. We searched for optical outbursts, intra-day variability and short term variability in these blazars. We carried out optical photometric monitoring of nine of these blazars in 13 observing nights during our observing run October 27, 2006 - March 20, 2007 by using the 1.02 meter optical telescope. From our observations, our data favor the hypothesis that three blazars were in the outburst state; one blazar was in the post outburst state; three blazars were in the pre/post outburst state; one blazar was in the low-state; and the state of one blazar was not known because there is not much optical data available for the blazar to compare with our observations. Out of three nights of observations of AO 0235+164, intra-day variability was detected in two nights. Out of five nights of observations of S5 0716+714, intra-day variability was detected in two nights. In one night of observations of PKS 0735+178, intra-day variability was detected. Out of six nights of observations of 3C 454.3, intra-day variability was detected in three nights. No intra-day variability was detected in S2 0109+224, OJ 287, ON 231, 3C 279 and 1ES 2344+514 in their 1, 4, 1, 2 and 1 nights of observations respectively. AO 0235+164, S5 0716+714, OJ 287, 3C 279 and 3C 454.3 were observed in more than one night and short term variations in all these blazars were also noticed. From our observations and the available data, we found that the predicted optical outburst with the time interval of ~ 8 years in AO 0235+164 and ~ 3 years in S5 0716+714 have possibly occurred.
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We carried out a pilot campaign of radio and optical band intra-day variability (IDV) observations of five blazars (3C66A, S5 0716+714, OJ287, B0925+504, and BL Lacertae) on December 18--21, 2015 by using the radio telescope in Effelsberg (Germany) and several optical telescopes in Asia, Europe, and America. After calibration, the light curves from both 5 GHz radio band and the optical R band were obtained, although the data were not smoothly sampled over the sampling period of about four days. We tentatively analyse the amplitudes and time scales of the variabilities, and any possible periodicity. The blazars vary significantly in the radio (except 3C66A and BL Lacertae with only marginal variations) and optical bands on intra- and inter-day time scales, and the source B0925+504 exhibits a strong quasi-periodic radio variability. No significant correlation between the radio- and optical-band variability appears in the five sources, which we attribute to the radio IDV being dominated by interstellar scintillation whereas the optical variability comes from the source itself. However, the radio- and optical-band variations appear to be weakly correlated in some sources and should be investigated based on well-sampled data from future observations.
We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from few minutes to up to less than a days is commonly known as intra-day variability. These fast variations are extremely useful to constrain the size of emitting region, black hole mass estimation, etc. It is noticed that in general blazars show intra-day variability in the complete electromagnetic spectrum. But some class of blazars either do not show or show very little intra-day variability in a specific band of electromagnetic spectrum. Blazars show rarely quasi-periodic oscillations in time series data in optical and X-ray bands. Other properties and emission mechanism of blazars are also briefly discussed.
We present our observations of the optical intra-day variability (IDV) in $gamma$-ray BL Lac object Mrk 501. The observations were run with the 1.02 m and 2.4 m optical telescopes at Yunnan Observatories from 2005 April to 2012 May. The light curve at the $R$ band on 2010 May 15 passes both variability tests (the $F$ test and the ANOVA test). A flare within the light curve on 2010 May 15 has a magnitude change $Delta m = 0.03 pm 0.005_{rm{stat}} pm 0.007_{rm{sys}}$ mag, textbf{a darkening timescale of $tau_{rm{d}}=$ 26.7 minutes}, and an amplitude of IDV $Amp=2.9% pm0.7%$. A decline textbf{described by 11 consecutive flux measurements} within the flare can be fitted linearly with a Pearsons correlation coefficient $r = 0.945$ at the confidence level of $> 99.99%$. Under the assumptions that the IDV is tightly connected to the mass of the black hole, textbf{and that the flare duration, being two times $tau_{rm{d}}$, is representative of the minimum characteristic timescale, we can derive upper bounds to the mass of the black hole}. In the case of the Kerr black hole, the timescale of $Delta t_{rm{min}}^{rm{ob}}=$ 0.89 hours gives $M_{bullet}la 10^{9.20} M_{odot}$, which is consistent with measurements reported in the literature. This agreement indicates that the hypothesis about $M_{bullet}$ and $Delta t_{rm{min}}^{rm{ob}}$ is consistent with the measurements/data.
We present the photometric observations of blazars S5 0716+714 and 3C 273 with high temporal resolution (30--60s) in the $I$ or $R$ bands. The observations were performed with a 1.02 m optical telescope from 2007 March 07 to 2012 May 16. The $F$-test, one-way analysis of variance (ANOVA) test, and $z$-transformed discrete correlation function (ZDCF) cross-correlation zero lag test are used to search for intra-day variability (IDV). Four and five reliable IDVs survive three tests for S5 0716+714 and 3C 273, respectively. IDVs are found for S5 0716+714 and 3C 273. A flare on 2008 May 08 has $Delta I approx$ 0.06$pm$0.01 mag in a duration of 0.54 hr for S5 0716+714. A flare on 2011 May 10 shows $Delta R approx$ 0.05$pm$0.01 mag in a duration of 0.40 hr for 3C 273. Sharp dips appear on 2011 May 9 for 3C 273, and show $Delta R approx$ 0.05$pm$0.01 mag. Under the assumptions that the IDV is tightly connected to black hole mass, $M_{bullet}$, and that the flare durations are representative of the minimum characteristic timescales, we can estimate upper bounds to $M_{bullet}$. In the case of the Kerr black holes, $M_{bullet} la 10^{8.91} M_{odot}$ and $M_{bullet} la 10^{9.02} M_{odot}$ are given for S5 0716+714 and 3C 273, respectively. These mass measurements are consistent with those measurements reported in the literatures. Also, we discuss the origins of optical variations found in this work.
Two dozens of radio loud active galactic nuclei (AGNs) have been observed with Urumqi 25 m radio telescope in order to search for intra-day variability (IDV). The target sources are blazars (namely flat spectrum radio quasars and BL Lac objects) which are mostly selected from the observing list of RadioAstron AGN monitoring campaigns. The observations were carried out at 4.8 GHz in two sessions of 8-12 February 2014 and 7-9 March respectively. We report the data reduction and the first results of observations. The results show that the majority of the blazars exhibit IDV in 99.9% confidence level, some of them show quite strong IDV. We find the strong IDV of blazar 1357 + 769 for the first time. The IDV at centimeter-wavelength is believed to be predominately caused by the scintillation of blazar emission through the local interstellar medium in a few hundreds parsecs away from Sun. No significant correlation between the IDV strength and either redshift or Galactic latitude is found in our sample. The IDV timescale along with source structure and brightness temperature analysis will be presented in a forthcoming paper.
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