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Register a new userMultiwavelength Cross-Correlations and Flaring Activity in Bright Blazars
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Blazars are known for their energetic multiwavelength flares from radio wavelengths to high-energy $gamma$-rays. In this work, we study radio, optical, and $gamma$-ray light curves of 145 bright blazars spanning up to 8~yr, to probe the flaring activity and interband correlations. Of these, 105 show $>1sigma$ correlations between one or more wavebands, 26 of which have a $>3sigma$ correlation in at least one wavelength pair, as measured by the discrete correlation function. The most common and strongest correlations are found between the optical and $gamma$-ray bands, with fluctuations simultaneous within our $sim 30$~d resolution. The radio response is usually substantially delayed with respect to the other wavelengths with median time lags of $sim 100$--160~d. A systematic flare identification via Bayesian block analysis provides us with a first uniform sample of flares in the three bands, allowing us to characterise the relative rates of multiband and orphan flares. Multiband flares tend to have higher amplitudes than orphan flares.
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We present measurements of rotations of the optical polarization of blazars during the second year of operation of RoboPol, a monitoring programme of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events, and we analyse the large set of rotation events discovered in two years of observation. We investigate patterns of variability in the polarization parameters and total flux density during the rotation events and compare them to the behaviour in a non-rotating state. We have searched for possible correlations between average parameters of the polarization-plane rotations and average parameters of polarization, with the following results: (1) there is no statistical association of the rotations with contemporaneous optical flares; (2) the average fractional polarization during the rotations tends to be lower than that in a non-rotating state; (3) the average fractional polarization during rotations is correlated with the rotation rate of the polarization plane in the jet rest frame; (4) it is likely that distributions of amplitudes and durations of the rotations have physical upper bounds, so arbitrarily long rotations are not realised in nature.
An exhaustive analysis of 9-year optical R-band photopolarimetric data of the flat-spectrum radio quasar 3C279 from 2008 February 27 to 2017 May 25 is presented, alongside with multiwavelength observing campaigns performed during the flaring activity exhibited in 2009 February/March, 2011 June, 2014 March/April, 2015 June and 2017 February. In the R-band, this source showed the maximum brightness state of $13.68pm 0.11$ mag ($1.36pm0.20$ mJy) on 2017 March 02, and the lowest brightness state ever recorded of $18.20pm 0.87$ mag ($0.16pm0.03$ mJy) on 2010 June 17. During the entire period of observations, the polarization degree varied between $0.48pm0.17$% and $31.65pm0.77$% and the electric vector position angle exhibited large rotations between $82.98^circ pm0.92$ and $446.32^circ pm1.95$. Optical polarization data show that this source has a stable polarized component that varied from $sim$6% (before the 2009 flare) to $sim$13% after the flare. The overall behavior of our polarized variability data supports the scenario of jet precessions as responsible of the observed large rotations of the electric vector position angle. Discrete correlation function analysis show that the lags between gamma-rays and X-rays compared to the optical R-band fluxes are $Delta t sim$ 31 d and $1$ d in 2009. Lags were also found among gamma-rays compared with X-rays and radio of $Delta t sim$ 30 d and $43$ d in 2011, and among radio and optical-R band of $Delta t sim$ 10 d in 2014. A very intense flare in 2017 was observed in optical bands with a dramatic variation in the polarization degree (from $sim$ 6% to 20%) in 90 days without exhibiting flaring activity in other wavelengths.
Blazars are an extreme subclass of active galactic nuclei. Their rapid variability, luminous brightness, superluminal motion, and high and variable polarization are probably due to a beaming effect. However, this beaming factor (or Doppler factor) is very difficult to measure. Currently, a good way to estimate it is to use the time scale of their radio flares. In this $Letter$, we use multiwavelength data and Doppler factors reported in the literatures for a sample of 86 flaring blazars detected by Fermi to compute their intrinsic multiwavelength data and intrinsic spectral energy distributions, and investigate the correlations among observed and intrinsic data. Quite interestingly, intrinsic data show a positive correlation between luminosity and peak frequency, in contrast with the behavior of observed data, and a tighter correlation between $gamma$-ray luminosity and the lower energy ones. For flaring blazars detected by Fermi, we conclude that (1) Observed emissions are strongly beamed; (2) The anti-correlation between luminosity and peak frequency from the observed data is an apparent result, the correlation between intrinsic data being positive; and (3) Intrinsic $gamma$-ray luminosity is strongly correlated with other intrinsic luminosities.
We have been using the 0.76-m Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory to optically monitor a sample of 157 blazars that are bright in gamma rays, being detected with high significance ($ge 10sigma$) in one year by the Large Area Telescope (LAT) on the {it Fermi Gamma-ray Space Telescope}. We attempt to observe each source on a 3-day cadence with KAIT, subject to weather and seasonal visibility. The gamma-ray coverage is essentially continuous. KAIT observations extend over much of the 5-year {it Fermi} mission for several objects, and most have $>100$ optical measurements spanning the last three years. These blazars (flat-spectrum radio quasars and BL~Lac objects) exhibit a wide range of flaring behavior. Using the discrete correlation function (DCF), here we search for temporal relationships between optical and gamma-ray light curves in the 40 brightest sources in hopes of placing constraints on blazar acceleration and emission zones. We find strong optical--gamma-ray correlation in many of these sources at time delays of $sim 1$ to $sim 10$ days, ranging between $-40$ and +30 days. A stacked average DCF of the 40 sources verifies this correlation trend, with a peak above 99% significance indicating a characteristic time delay consistent with 0 days. These findings strongly support the widely accepted leptonic models of blazar emission. However, we also find examples of apparently uncorrelated flares (optical flares with no gamma-ray counterpart and gamma-ray flares with no optical counterpart) that challenge simple, one-zone models of blazar emission. Moreover, we find that flat-spectrum radio quasars tend to have gamma rays leading the optical, while intermediate and high synchrotron peak blazars with the most significant peaks have smaller lags/leads.
The supermassive black hole Sgr A* is located at the Milky Way center. We studied its flaring activity close to the DSO/G2 pericenter passage to constrain the physical properties and origin of the flares. Simultaneous/coordinated observations were made in 2014 Feb-Apr with XMM-Newton, HST/WFC3, VLT/SINFONI, VLA and CARMA. We detected 2 X-ray and 3 NIR flares on Mar. 10 and Apr. 2 with XMM-Newton and HST and 2 NIR flares on Apr. 3 and 4 with VLT. The Mar. 10 X-ray flare has a long rise and a rapid decay. Its NIR counterpart peaked 4320s before the X-ray peak implying a variation in the X-ray-to-NIR flux ratio. This flare may be a single flare where change in the flux ratio is explained by the adiabatic compression of a plasmon or 2 close flares with simultaneous X-ray/NIR peaks. We observed an increase in the rising radio flux density on Mar. 10 with the VLA. It could be the delayed emission from a NIR/X-ray flare preceding our observation. The Apr. 2 X-ray flare occurred for HST in the Earth occultation of Sgr A*. We thus only observed the start of its NIR counterpart. After the occultation, we observed the decay phase of a bright NIR flare with no X-ray counterpart. On Apr. 3, 2 CARMA flares were observed. The 1rst one may be the delayed emission of a VLT NIR flare. We thus observed 7 NIR flares whose 3 have an X-ray counterpart. We studied the physical parameters of the flaring region for each NIR flare but none of the possible radiative processes can be ruled out for the X-ray flares creation. Our X-ray flaring rate is consistent with those observed in the 2012 Chandra XVP campaign. No increase in the flaring activity was thus triggered close to the DSO/G2 pericenter passage. Moreover, higher X-ray flaring rates had already been observed with no increase in the quiescent level. There is thus no direct link between an X-ray flaring-rate increase and an accretion-rate change. (abridged)
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