arXiv subjects

M. Villata

Publications and source records attributed to M. Villata.

At least 91 records · Page 5Linked to original sources

arXiv2007

Swift follow-up of the gigantic TeV outburst of PKS 2155-304 in 2006

At the end of July 2006, the blazar PKS 2155-304 (z=0.116) underwent a strong outburst observed at TeV energies by HESS (up to 17 Crab flux level at E > 200 GeV). The Swift satellite followed the evolution of the source for about one month. The data analysis - reported in another paper - has shown that, despite the violent activity at TeV energies, the synchrotron energy distribution increased in normalization, but only with a small shift in frequency. In the present work, we fit the broad-band spectrum with a log-parabolic model, to search for indications of intrinsic curvature, which in turn is usually interpreted as a signature of energy-dependent acceleration mechanisms of electrons.

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arXiv2007

X-ray/UV/Optical follow-up of the blazar PKS 2155-304 after the giant TeV flares of July 2006

We present all the publicly available data, from optical/UV wavelengths (UVOT) to X-rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155-304, performed in response to the rapid alert sent out after the strong TeV activity (up to 17 Crab flux level at E > 200 GeV) at the end of July 2006. The X-ray flux increased by a factor of 5 in the 0.3-10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly one day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data, but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker.

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arXiv2007

Ten-year optical monitoring of PKS 0735+178: historical comparison, multiband behaviour and variability timescales

New data and results on the optical behaviour of the blazar PKS 0735+178 are presented. In addition the whole historical light curve, and a new photometric calibration of comparison stars are reported. Optical spectral indexes are calculated and studied on years scales, while several methods for time-series analysis are applied to the whole historical series and to each observing season of our data set. This allowed to search and identify optical variability modes, characteristic timescales and the signal power spectrum over 3 decades in time. In the last 10 years the optical flux of PKS 0735+178 exhibited a rather achromatic long-term behaviour and a variability mode resembling the shot-noise. The brightness level was in an intermediate/low state with a mild flaring activity and a superimposition/succession of rapid and slower flares, with no extraordinary/isolated outbursts but, at any rate, characterized by 1 major active phase in 2001. Several mid-term scales (days, weeks) were found, the more common falling into values of about 27-28 days, 50-56 days and 76-79 days. The rapid variability in the historical curve appear to be modulated by a general, slower and rather oscillating trend, where typical timescales of about 4.5, 8.5 and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive variability signatures at days/weeks scales, that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behaviour and the twisted radio structure observed by many years in this blazar.

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arXiv2007

Spectroscopic monitoring of the BL Lac object AO 0235+164

We performed optical spectroscopic monitoring of the BL Lac object AO 0235+164 (z=0.94) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004, during an extended WEBT campaign. The flux of this source is both contaminated and absorbed by a foreground galactic system at z=0.524, the stars of which can act as gravitational micro-lenses. In this period the object was in an optically faint, though variable state, and a broad Mg II emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the Mg II line flux of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3. Most likely, the photoionising radiation can be identified with the emission component that was earlier recognised to be present as a UV-soft-X-ray bump in the source SED and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the BLR size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the Mg II line predicted by the microlensing scenario. Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad Mg II emission line, but the time scale of variations due to these (rare) events would be of several years. In contrast, the continuum flux, coming from much smaller emission regions in the jet, could be affected by microlensing from the more plausible MACHO deflectors, with variability time scales of some months.

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arXiv2007

The radio delay of the exceptional 3C 454.3 outburst. Follow-up WEBT observations in 2005-2006

In spring 2005 the blazar 3C 454.3 was observed in an unprecedented bright state from the near-IR to the hard X-ray frequencies. A mm outburst peaked in June-July 2005, and it was followed by a flux increase at high radio frequencies. In this paper we report on multifrequency monitoring by the WEBT aimed at following the further evolution of the outburst in detail. In particular, we investigate the expected correlation and time delays between the optical and radio emissions in order to derive information on the variability mechanisms and jet structure. A comparison among the light curves at different frequencies is performed by means of visual inspection and discrete correlation function, and the results are interpreted with a simple model taking into account Doppler factor variations of geometric origin. The high-frequency radio light curves show a huge outburst starting during the dimming phase of the optical one and lasting more than 1 year. The first phase is characterized by a slow flux increase, while in early 2006 a major flare is observed. The lower-frequency radio light curves show a progressively delayed and fainter event, which disappears below 8 GHz. We suggest that the radio major peak is not physically connected with the spring 2005 optical one, but it is actually correlated with a minor optical flare observed in October-November 2005. This interpretation involves both an intrinsic and a geometric mechanism. The former is represented by disturbances travelling down the emitting jet, the latter being due to the curved-jet motion, with the consequent differential changes of viewing angles of the different emitting regions.

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arXiv2006

Multi-frequency monitoring of gamma-ray loud blazars: I. Light curves and spectral energy distributions

Context: Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets, and, ltimately, how the energy is extracted from the central black hole in radio-loud active galactic nuclei. Aims: Crucial information is provided by broad-band spectral energy distributions (SEDs), their trends with luminosity and correlated multi-frequency variability. With this study we plan to obtain a database of contemporaneous radio-to-optical spectra of a sample of blazars, which are and will be observed by current and future high-energy satellites. Methods: Since December 2004 we are performing a monthly multi-frequency radio monitoring of a sample of 35 blazars at the antennas in Medicina and Noto. Contemporaneous near-IR and optical observations for all our observing epochs are organised. Results: Until June 2006 about 4000 radio measurements and 5500 near-IR and optical measurements were obtained. Most of the sources show significant variability in all observing bands. Here we present the multi-frequency data acquired during the first eighteen months of the project, and construct the SEDs for the best-sampled sources.

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arXiv2006

Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis

A huge multiwavelength campaign targeting the blazar AO 0235+164 was organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the variability properties of the source. Monitoring observations were carried out at cm and mm wavelengths, and in the near-IR and optical bands, while three pointings by the XMM-Newton satellite provided information on the X-ray and UV emission. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR and optical activity with respect to the previous season. Increased variability is also found at the higher radio frequencies, down to 15 GHz, but not at the lower ones. The radio (and optical) outburst predicted to peak around February-March 2004 on the basis of the previously observed 5-6 yr quasi-periodicity did not occur. The analysis of the optical light curves reveals now a longer characteristic time scale of 8 yr, which is also present in the radio data. The spectral energy distributions corresponding to the XMM-Newton observations performed during the WEBT campaign are compared with those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray spectra can be described in terms of an extra component, which appears also when the source is faint through a hard UV spectrum and a curvature of the X-ray spectrum. Finally, there might be a correlation between the X-ray and optical bright states with a long time delay of about 5 yr, which would require a geometrical interpretation.

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arXiv2006

Synchrotron flaring in the jet of 3C 279

We study the synchrotron flaring behaviour of the blazar 3C279 based on an extensive dataset covering 10 years of monitoring at 19 different frequencies in the radio-to-optical range. The properties of a typical outburst are derived from the observations by decomposing the 19 lightcurves into a series of self-similar events. This analysis is achieved by fitting all data simultaneously to a succession of outbursts defined according to the shock-in-jet model of Marscher & Gear (1985). We compare the derived properties of the synchrotron outbursts in 3C279 to those obtained with a similar method for the quasar 3C273. It is argued that differences in the flaring behaviour of these two sources are intrinsic to the sources themselves rather than being due to orientation effects. We also compare the start times and flux densities of our modelled outbursts with those measured from radio components identified in Very Long Baseline Interferometry (VLBI) images. We find VLBI counterparts for most of our model outbursts, although some high-frequency peaking events are not seen in the radio maps. Finally, we study the link between the appearance of a new synchrotron component and the EGRET gamma-ray state of the source at 10 different epochs. We find that an early-stage shock component is always present during high gamma-ray states, while in low gamma-ray states the time since the onset of the last synchrotron outburst is significantly longer. This statistically significant correlation supports the idea that gamma-ray flares are associated with the early stages of shock components propagating in the jet. We note, however, that the shock wave is already beyond the broad line region during the gamma-ray flaring.

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arXiv2006

Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995--2004)

BL Lacertae has been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the Königl model. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi-wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model.

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arXiv2006

XMM-Newton observations of a sample of gamma-ray loud active galactic nuclei

(abridged) We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 gamma-ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies, that are associated with detections at energies >100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406-706. Then, the spectral characteristics of the sources in the present sample have been compared with those in previous catalogs of blazars and other AGN, in order to search for any difference or long term change. All the selected sources appear to follow the classic "blazar sequence" and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of sources. Some sources display interesting features worth noting: four of them, namely AO 0235+164, PKS 1127-145, S5 0836+710 and PKS 1830-211 show the presence of an intervening absorption system along the line of sight, but only the latter is known to be surely gravitationally lensed. AO 0235+164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in the state with respect to the previous BeppoSAX observations, that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same years. PKS 1406-706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Moreover, although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.

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arXiv2006

X-ray emission from the blazar AO 0235+16: the XMM-Newton and Chandra point of view

In this paper we analyse five observations of the BL Lac object AO 0235+16 performed with the Chandra and XMM-Newton satellites during the years 2000-2005. In the February 2002 observation the source is found in a bright state and presents a steep X-ray spectrum, while in all the other epochs it is faint and the spectrum is hard. The soft X-ray spectrum appears to be strongly absorbed, likely by the intervening system at z=0.524, which also absorbs the optical-UV radiation. We find that models that consider spectral curvature are superior to single power law ones in fitting the X-ray spectrum. In particular, we favour a double power law model, which agrees with the assumption of a superposition of two different components in the X-ray domain. Both in the Chandra and in one of the XMM-Newton observations, a tentative detection of the redshifted Fe Kalpha emission line may suggest its origin from the inner part of an accretion disc. Thermal emission from this accretion disc might explain the UV-soft-X-ray bump that appears in the spectral energy distributions, when the X-ray spectra are complemented with the optical-UV data from the Optical Monitor onboard XMM-Newton. More likely, the bump can be interpreted in terms of an additional synchrotron component emitted from an inner region of the jet with respect to that where the lower-energy emission comes from. An inspection of the X-ray light curves reveals that intraday variability occurs only when the source is in a bright state.

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arXiv2006

The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005

The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (M_B ~ -31.4). In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005. The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the

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arXiv2006

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. I. Simultaneous broadband observations during November 2003

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 framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involving a pointing by the soft-gamma-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, and Very Long Baseline Array (VLBA) monitoring. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the gamma-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature Tb>2.1x10^{14} K was inferred from the radio variability, but no corresponding signatures of IC avalanches were recorded at higher energies. The absence of IC-catastrophe signatures provides either a lower limit delta>8 to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton catastrophes in S5 0716+71.

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arXiv2005

The XMM-Newton view of gamma-ray loud active nuclei

Notwithstanding the big efforts devoted to the investigation of the mechanisms responsible for the high-energy (E>100 MeV) gamma-ray emission in active galactic nuclei (AGN), the definite answer is still missing. The X-ray energy band (0.4-10 keV) is crucial for this type of study, since both synchrotron and inverse Compton emission can contribute to the formation of the continuum. Within an ongoing project aimed at the investigation of the gamma-ray emission mechanism acting in the AGN detected by the EGRET telescope onboard CGRO, we firstly focused on the sources for which X-ray and optical/UV data are available in the XMM-Newton public archive. The preliminary results are outlined here.

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arXiv2005

The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season

A multiwavelength campaign on the BL Lac object AO 0235+16 has been organized by the WEBT collaboration during the observing seasons 2003-2004 and 2004-2005. Here we report on the results of the first season, which saw the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected. In the optical and near-IR the source has been very active in the last 3 years, though rather faint most of the time, with noticeable variations of more than a mag in a few days. In contrast, in the radio bands it appears "quiescent" since early 2000. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z=0.524. No significant variation of the X-ray flux occurred during the pointing. In contrast, contemporaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ~2-3% flux decrease in 6-7 hours, which, if intrinsic, would imply a brightness temperature well above the Compton limit. We construct the broad-band spectral energy distribution of January 18-19, 2004 with simultaneous radio data from Effelsberg, optical data from the Nordic Optical Telescope, optical-UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care was taken to correct data for extinction due to both the Milky Way and the z=0.524 absorber. The resulting SED suggests the existence of a bump in the UV spectral region.

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arXiv2004

The WEBT BL Lacertae Campaign 2001 and its extension

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present $UBVRI$ light curves obtained by the WEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the $\sim 15600$ observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a few-day time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a $\sim 0.1$ bluer-when-brighter slope in the $B-R$ versus $R$ plane; a steeper slope of $\sim 0.4$ would distinguish the shorter-term variations. This means that, when considering the long-term trend, the $B$-band flux level is related to the $R$-band one according to a power law of index $\sim 1.1$. Doppler factor variations on a "convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.

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arXiv2004

Helical jets in blazars. Interpretation of the multifrequency long-term variability of AO 0235+16

The long-term variability of the multiwavelength blazar emission can be interpreted in terms of orientation variations of a helical, inhomogeneous, non-thermally emitting jet, possibly caused by the orbital motion of the parent black hole in a binary system (Villata & Raiteri 1999). The helical-jet model is here applied to explain the quasi-periodic radio-optical light curves and the broad-band spectral energy distributions (SEDs) of the BL Lac object AO 0235+16. Through a suitable choice of the model parameters, the helix rotation can well account for the periodicity of the main radio and optical outbursts and for the corresponding SED variability, while the interspersed minor radio events could be interpreted as due either to some local distortions of the helical structure or to other phenomena contributing to the source emission. In particular, the probable existence of flow instabilities provides a viable interpretation for the non-periodic features.

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arXiv2003

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.

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