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Register a new userIntra-Day Variability Observations of Two Dozens of Blazars at 4.8 GHz
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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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We aim to search for evidence of annual modulation in the time scales of the BL Lac object S5 0716+714. The intra-day variability (IDV) observations were carried out monthly from 2005 to 2009, with the Urumqi 25m radio telescope at 4.8 GHz. The source has shown prominent IDV as well as long-term flux variations. The IDV time scale does show evidence in favor of an annual modulation, suggesting that the IDV of 0716+714 is dominated by interstellar scintillation. The source underwent a strong outburst phase between mid-2008 and mid-2009; a second intense flare was observed in late 2009, but no correlation between the total flux density and the IDV time scale is found, implying that the flaring state of the source does not have serious implications for the general characteristics of its intra-day variability. However, we find that the inner-jet position angle is changing throughout the years, which could result in an annual modulation noise in the anisotropic ISS model fit. There is also an indication that the lowest IDV amplitudes (rms flux density) correspond to the slowest time scales of IDV, which would be consistent with an ISS origin of the IDV of 0716+714.
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 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.
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 the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8,GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid ($<$3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations. $gamma$-ray loud sources were found to vary by up to a factor 4 more than $gamma$-ray quiet ones, with 4$sigma$ significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as $sim$70% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN.
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