In this paper, we report results of our near-infrared (NIR) photometric variability studies of the BL Lacertae object S5 0716+714. NIR photometric observations spread over 7 nights during our observing run April 2-9, 2007 at 1.8 meter telescope equipped with KASINICS (Korea Astronomy and Space Science Institute Near Infrared Camera System) and J, H, and Ks filters at Bohyunsan Optical Astronomy Observatory (BOAO), South Korea. We searched for intra-day variability, short term variability and color variability in the BL Lac object. We have not detected any genuine intra-day variability in any of J, H, and Ks passbands in our observing run. Significant short term variability ~ 32.6%, 20.5% and 18.2% have been detected in J, H, Ks passbands, respectively, and ~ 11.9% in (J-H) color.
We present results of a periodicity search of 20 intra-day variable optical light curves of the blazar S5 0716+714, selected from a database of 102 light curves spanning over three years. We use a wavelet analysis technique along with a randomization test and find strong candidates for nearly periodic variations in eight light curves, with probabilities ranging from 95% to >99%. This is the first good evidence for periodic, or more-precisely, quasi-periodic, components in the optical intra-day variable light curves of any blazar. Such periodic flux changes support the idea that some active gal…
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 bla…
The recent developments in studies of TeV radiation from blazars are highlighted and the implications of these results for derivation of cosmologically important information about the cosmic infrared background radiation are discussed.
Two days of observations were conducted at the Palomar Observatory during the nights of 25 and 26 February 1997 with the Hale telescope, in order to search for rapid variability in the near-infrared (NIR) bands J, H, Ks for a selection of eight blazars. With the possible exception of PKS 1156+295, no intraday or day-to-day variability was observed during these two nights. However, for these eight blazars, we have measured the NIR spectral indices.
In this paper, the historical infrared (JHK) data compiled from the published literature are presented here for 30 blazars. Maximum near-IR variations are found and compared with the optical ones. For the color-magnitude relation, some objects show that the color index increases with magnitude while 1253-055 shows complex behaviour, which perhaps suggests that the emission mechanism consists of, at least, two parts. The spectral indexes are in the range of $α_{IR}$ = 0.77 to 2.37.
We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sampled and sensitive R-band CCD observations, carried out from November 1998 through May 2002 during a total of 47 nights with an average of 6.5 hours/night, have enabled us to detect variability amplitudes as low as $\sim 1%$ on intra-night time scales. A distinction is found for the first time between the intra-night optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud…
The rapid variability of blazars in almost all wavelengths is now well established. Two days of observations were conducted at the Palomar Observatory during the nights of 25 and 26 February 1997 with the 5-meter Hale telescope, in order to search for rapid variability in the near-infrared (NIR) bands J, H, Ks for a selection of eight blazars. With the possible exception of 1156+295 (4C 29.45), no intraday or day-to-day variability was observed during these two nights. However, for these eight blazars, we have measured the NIR luminosities and spectral indices. It has recently been reported th…
We present the results of the blazar 3C 345 monitoring in Johnson-Cousins BVRI bands for the period 1996 - 2006. We have collected 29 V and 43 R data points for this period; the BI light curves contain a few measurements only. The accuracy of our photometry is not better than 0.03 mag in the VR bands. The total amplitude of the variability obtained from our data is 2.06 mag in the V band and 2.25 mag in the R one. 3C 345 showed periods of flaring activity during 1998/99 and 2001: a maximum of the blazar brightness was detected in 2001 February - 15.345 mag in the V band and 14.944 mag in the R…
Rapid, multi-color optical monitoring carried out for a multiwavelength campaign during 22-27 August 2000 is presented for the bright and variable BL Lac object PKS 2005-489. Previous multiwavelength monitoring campaigns of PKS 2005-489 have found complex variability behavior both in flux density and in broadband spectrum, with different modes and timescales from days to hours. During this campaign, PKS 2005-489 was observed to be in an unusual state, bright in the optical but faint in the X-ray, just prior to an X-ray flare that began on 2 September 2000. PKS 2005-489 was also observed to hav…
Aims: We derive limits on the EBL density from the energy spectra of distant sources of very high energetic gamma-rays (VHE gamma-rays). Methods: VHE gamma-rays are attenuated by the photons of the EBL via pair production, which leaves an imprint in the measured spectra from distant sources. In this paper we present a new method to derive constraints on the EBL. Hereby, we use only very basic assumptions about TeV blazar physics and no pre-defined EBL model, but rather a large number of generic shapes constructed from a grid in EBL density vs. wavelength. In our study we utilize spectral data …
We report results of multiband optical monitoring of two well known blazars, S5 0716+714 and BL Lacertae, carried out in 1996 and 2000-01 with an aim to study optical variations on time scales from minutes to hours and longer.The light curves were derived relative to comparison stars present on the CCD frames. Night to night flux variations of >0.1 mag were observed in S5 0716+714 during a campaign of ~2 weeks in 1996.A good correlation between the lightcurves in different optical bands was found for both inter-night and intra-night observations. Two prominent events of intra-night optical var…
Motivated by the idea that the recently detected near-infrared (1.2-4 micron) excess over the contribution of known galaxies is due to redshifted light from the first cosmic stars (Salvaterra & Ferrara 2003), we have used the effect caused by photon-photon absorption on gamma-ray spectra of blazars to put constraints on extragalactic background light (EBL) from the optical to the far-IR bands. Our analysis is mainly based on the blazar H 1426+428, for which we assume a power-law unabsorbed spectrum. We find that an EBL model with no excess over known galaxies in the near-infrared background (N…
The study of the Extragalactic Background Light (EBL) is crucial to understand many astrophysical problems (as the formation of first stars, the evolution of galaxies and the role of dust emission). At present, one of the most powerful ways to put constraints on EBL is represented by the study of the photon-photon absorption on gamma-ray spectra of TeV blazars. Adopting this method, we found that, if the only contribution to the optical and Near Infrared (NIR) background is given by galaxies, the spectrum of the blazar H1426+428 cannot be fitted. To reproduce the observational data of H1426+42…
Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10^{12} K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, possible under non-stationary conditions, would lead to IC avalanches in the soft-gamma-ray energy band during transient periods. For the first time, broadband signatures of possible IC catastrophes were searched for in S5 0716+71. A multifrequency observing campaign targetting S5 0716+71 was carried out in November 2003 under the framewor…
We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86 GHz and 229 GHz with the IRAM 30 m telescope within a coordinated broad spectral band campaign, centred around an INTEGRAL observation during November 10 to 16, 2003. Our aim was to search for signatures of inverse-Compton "catastrophes". At 86 GHz, making use of a new calibration strategy, we reach a relative rms accuracy of the flux density measurements of 1.2%. At this frequency, S5 0716+71 showed no intra-day variability, but showed remarkable inter-day variability with a flux den…
In the present paper we report the detection of intra-night variability in some of the RQQSOs and one LDQ. To study intra-night variability, we carried out photometric monitoring of seven RQQSOs and one LDQ in Johnson V-passband using 1.2 meter optical/IR telescope at Gurushikhar, Mount Abu, India. Observations were made in nine nights during the first half of the year 2000; seven RQQSOs: 0748+291, 0945+438, 1017+280, 1029+329, 1101+319, 1225+317, 1252+020 and one LDQ: 1103-006 were observed. RQQSOs 0748+291, 1225+317 and LDQ 1103-006 have shown existence of intra-night variations. In the case…
We present the results of a series of Swift and quasi simultaneous ground-based infra-red observations of the blazar 3C 454.3 carried out in April-May 2005 when the source was 10 to 30 times brighter than previously observed. We found 3C 454.3 to be very bright and variable at all frequencies covered by our instrumentation. The broad-band Spectral Energy Distribution (SED) shows the usual two-bump shape (in nu-nu f(nu) space) with the Infra-red, optical and UV data sampling the declining part of the synchrotron emission that, even during this extremely large outburst, had its maximum in the fa…
With the arrival of powerful, ground-based gamma-ray detectors, we can now begin to seriously probe, via simultaneous X-ray/TeV observations, the origin of the gamma-ray emission in the blazars Mrk 421 and 501. If the synchrotron-Compton emission model turns out to work, then we know that the same electrons are responsible for both the X-ray and the gamma-ray emission of these objects. In this case, we show that we can use their observed X-ray spectra to robustly estimate their intrinsic gamma-ray spectra. Among blazars, Mrk 421/501 are particularly well-suited for this task because the Compto…
The last years have seen a revolution in ground-based gamma-ray detectors. We can now detect the spectra of nearby TeV blazars like Mrk 421 and 501 out to approximately 20 TeV, and during the strongest flares, we can now follow fluctuations in these spectra on timescales close to the shortest ones likely in these objects. We point out that this represents a unique opportunity. Using these and future detectors in combination with broadband X-ray satellites like SAX and RXTE, we will be able to simultaneously follow all significant X-ray/gamma-ray variations in a blazar's emission. This will pro…
The blazar, S5 2007+77 shows intraday variability in cm wavelengths. Seven epochs of VSOP observations were carried out in 1998 and 1999 at 5 GHz to look for the origin of the variability with the highest achievable angular resolution at cm wavelengths. Here the results of four epochs are analysed, which revealed ~10% variations in polarized flux density.
We determined the kinematics at the jet of 0716+714 from a reanalysis of multi-frequency VLBI data (5, 8.4, 15, 22 GHz) obtained during the last 10 years combined with data from the literature. For this intra-day variable blazar, only a lower limit of its distance is known (z >= 0.3). We find that 0716+714 is a relatively fast superluminal source (with a Lorentz factor of gamma >15), revising earlier results showing much slower motions. We discuss the new findings with emphasis on the interpretation of the observed rapid radio variability.
Blazars are known to display strong and erratic variability at almost all the wavelengths of electromagnetic spectrum. Presently, variability studies at high-energies (hard X-rays, gamma-rays) are hampered by low sensitivity of the instruments. Nevertheless, the latest generation of satellites (INTEGRAL, Swift) have given suggestions not yet fully confirmed of variability on intraday timescales. Some specific cases recently observed are presented and physical implications are discussed (e.g. NRAO 530). The contribution that SIMBOL-X could give in this research topic is also outlined.
Variability amplitudes larger than 1 magnitude over time-scales of a few tens of minutes have recently been reported in the optical light-curves of several blazars. In order to independently verify the real occurrence of such extremely violent events, we undertook an observational study of a selected sample of three blazars: PKS 0048-097, PKS 0754+100, and PKS 1510-089. Possible systematic error sources during data acquisition and reduction were carefully evaluated. We indeed found flux variability at intra-night time-scales in all three sources, although no extremely violent behaviour, as rep…
The flat-spectrum radio quasar 3C 454.3 is well known to be a highly active and variable source with outbursts occurring across the whole electromagnetic spectrum over the last decades. In spring 2005, 3C 454.3 has been reported to exhibit a strong optical outburst which subsequently triggered multi-frequency observations of the source covering the radio up to gamma-ray bands. Here, we present first results of our near-IR/optical (V, R, I, H band) photometry performed between May 11 and August 5, 2005 with the Rapid Eye Mount (REM) at La Silla in Chile and the Automatic Imaging Telescope (AIT)…
The blazar 3C 454.3 was revealed by the Fermi Gamma-ray Space Telescope to be in an exceptionally high flux state in July 2008. Accordingly, we performed a multi-wavelength monitoring campaign on this blazar using IR and optical observations from the SMARTS telescopes, optical, UV and X-ray data from the Swift satellite, and public-release gamma-ray data from Fermi. We find an excellent correlation between the IR, optical, UV and gamma-ray light curves, with a time lag of less than one day. The amplitude of the infrared variability is comparable to that in gamma-rays, and larger than at optica…
The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. The observing campaign involved simultaneous monitoring at radio to IR/optical wavelengths centered around a 500-ks INTEGRAL pointing (November 10-17, 2003). We present the combined analysis and results of the cm- to sub-mm observations including a detailed study of the inter- to intra-day variability and spectral characteristics of 0716+714.…
In this paper, analytical arguments are presented that there exists a synchrotron X-ray jet on large scales in most radio-loud AGNs, based on the knowledge of the nature and physics of blazars. In blue blazars and blue-blazar-like radio galaxies, the large scale X-ray jet may get faint along the jet, while in most red blazars and red-blazar-like radio galaxies, the X-ray jet is bright on 10 kpc scales whether the jet is highly relativistic on large scales or not. In extreme red blazars in which the jet is still highly relativistic on large scales and the synchrotron peak of the inner jet lies …
