arXiv subjects

A. Celotti

Publications and source records attributed to A. Celotti.

At least 145 records · Page 8Linked to original sources

arXiv1999

Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase

We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy, and the presence of a soft lag with a timescale of the order 10^3 s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broad band spectral energy distribution (SED) as due to synchrotron self-Compton (SSC) emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag as due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the break energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection.

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arXiv1999

Correlated variability of Mkn 421 at X-ray and TeV wavelengths on timescales of hours

Mkn 421 was observed for about two days with BeppoSAX, prior to and partly overlapping the start of a 1 week continuous exposure with ASCA in April 1998, as part of a world-wide multiwavelength campaign. A pronounced, well defined, flare observed in X-rays was also observed simultaneously at TeV energies by the Whipple Observatory's 10 m gamma-ray telescope. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours.

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arXiv1998

Quasi-thermal Comptonization and gamma-ray bursts

Quasi-thermal Comptonization in internal shocks formed between relativistic shells can account for the high energy emission of gamma-ray bursts. This is in fact the dominant cooling mechanism if the typical energy of the emitting particles is achieved either through the balance between heating and cooling or as a result of electron-positron pair production. Both processes yield sub or mildly relativistic energies. In this case the synchrotron spectrum is self-absorbed, providing the seed soft photons for the Comptonization process, whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff or a Wien peak, depending on the scattering optical depth of the emitting particles. Self-consistent particle energy and optical depth are estimated and found in agreement with the observed spectra.

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arXiv1998

Fast X-ray Variability of PKS 2155-304: A Cross Correlation Analysis

PKS 2155-304 is one of the prototypes of BL Lac objects. Its spectral energy distribution is known from the radio to the TeV band and has two peaks, one in the soft X-rays and the other in the TeV region. These may be interpreted within the synchrotron self-Compton model, which attributes the first peak to synchrotron radiation of electrons, which also produce the second peak via Compton scattering of the same synchrotron photons. The emission is relativistically boosted in the observer direction. A way of testing the model and characterizing its geometrical and kinematical properties is through the study of correlated variability in different bands. Here we report on the search and accurate study of lags in the X-ray light curves obtained with the BeppoSAX satellite during the performance verification phase in November 1996 and compare the results with the findings of a similar analysis applied to the November 1997 BeppoSAX and 1994 ASCA data.

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arXiv1998

BeppoSAX Observations of PKS 2155-304 during an active Gamma-ray State

PKS 2155-304 was observed with BeppoSAX in November 1997 for 64 ksec (total elapsed time 33.5 hours) and, for the first time, simultaneously in gamma-rays with EGRET on board the Compton Gamma Ray Observatory and with the ground based TeV telescope CANGAROO, during a phase of high brightness in the X-ray band. The LECS and MECS light curves show a pronounced flare (with an excursion of a factor 3.5 between min and max), with evidence of spectral hardening at maximum intensity. The source is weakly detected by the PDS in the 12-100 keV band with no significant evidence of variability. The broad band X-ray data from Beppo SAX are compared with the gamma-ray results and discussed in the framework of homogenous synchrotron self Compton models.

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arXiv1998

A theoretical unifying scheme for gamma-ray bright blazars

The phenomenology of gamma-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: 1) a decrease in the frequencies of the synchrotron and inverse Compton peaks; 2) an increase in the ratio of the powers of the high and low energy spectral components. Objects along this sequence would be observationally classified respectively as high frequency BL Lac objects, low frequency BL Lac objects, highly polarized quasars and lowly polarized quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 gamma ray loud blazars two of the most accepted scenarios for the broad band emission of blazars, namely the synchrotron self--Compton and external Compton models, and explains the observational trends presented by Fossati et al. (1998) in a companion paper, dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class.

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arXiv1998

Reprocessing of radiation by multi-phase gas in Low Luminosity Accretion Flows

We discuss the role that magnetic fields in low luminosity accretion flows can play in creating and maintaining a multi-phase medium, and show that small magnetically-confined clouds or filaments of dense cold gas can dramatically reprocess the `primary' radiation from tori. In particular, radio emission would be suppressed by free-free absorption, and an extra (weak) component would appear at optical wavelengths. This is expected to be a common process in various environments in the central regions of Active Galaxies, such as broad line regions, accretion disk coronae and jets.

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arXiv1998

Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4

We present a model for the different X-ray spectral states displayed by Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral implications for a magnetically structured corona in which magnetic flares result from reconnection of flux tubes rising from the accretion disk by the magnetic buoyancy instability. Using observations of one of the best studied examples, GX339-4, we identify the geometry and the physical conditions characterizing each of these states. We find that, in the Soft state, flaring occurs at small scale heights above the accretion disk. The soft thermal-like spectrum is the result of heating and consequent re-radiation of the hard X-rays produced by such flares. The hard tail is produced by Comptonization of the soft field radiation. Conversely, the hard state is the result of flares triggered high above the underlying accretion disk which produce X-rays via Comptonization of either internal synchrotron radiation or soft disk photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disk the system is photon-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disk greatly enhances the soft-photon field with the result that the spectrum softens. We interpret the two states as being related to two different phases of magnetic energy dissipation. In the Soft state, Parker instability in the disk favours the emergence of large numbers of relatively low magnetic field flux tubes. In the hard state, only intense magnetic fields become buoyant. The model can also qualitatively account for the observed short timescale variability and the characteristics of the X-ray reflected component of the hard state.

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arXiv1998

A Unifying View of the Spectral Energy Distributions of Blazars

We collect data from the radio to the gamma-ray range for three complete samples of blazars: the Slew Survey and the 1Jy samples of BL Lacs and the 2Jy sample of Flat Spectrum Radio-Loud Quasars (FSRQs). The fraction of objects detected in gamma-rays (E > 100 MeV) is 17%, 26% and 40% in the three samples respectively. Except for the Slew Survey sample, gamma-ray detected sources do not differ either from other sources in each sample, nor from all the gamma-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities and of the broad band spectral indices (radio to optical and radio to X-ray). We compute average Spectral Energy Distributions (SEDs) from radio to gamma rays for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that: i) the first peak occurs in different frequency ranges for different luminosity classes, with most luminous sources peaking at lower frequencies; ii) the peak frequency of the gamma-ray component correlates with the peak frequency of the lower energy one; iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of blazars and the systematic trends of the SEDs as a function of luminosity favor a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQ. The general implications of this unified scheme are discussed.

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arXiv1998

The ASCA spectrum of the z=4.72 blazar, GB 1428+4217

The X-ray luminous quasar GB 1428+4217 at redshift 4.72 has been observed with ASCA. The observed 0.5-10 keV flux is 3.2E-12 erg/s/cm2. We report here on the intrinsic 4-57 keV X-ray spectrum, which is very flat (photon index of 1.29). We find no evidence for flux variability within the ASCA dataset or between it and ROSAT data. We show that the overall spectral energy distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars and present models which fit the available data. The Doppler beaming factor is likely to be at least 8. We speculate on the number density of such high redshift blazars, which must contain rapidly-formed massive black holes.

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arXiv1997

BeppoSAX observations of PKS 0528+134

We report on the BeppoSAX observations of the gamma-ray blazar PKS 0528+134 performed in Feb and Mar 1997, during a multiwavelength campaign involving EGRET and ground based telescopes. The source was in a faint and hard state, with energy spectral index alpha=0.48+-0.04 between 0.1 and 10 keV, and [2-10] keV flux of 2.7E-12 erg/cm2/s. No significant variability was observed. The source was detected in the 20-120 keV band by the PDS, with a flux lying slightly above the extrapolation from lower X-ray energies. Comparing this low state with previous higher states of the source, there is an indication that the X-ray spectrum hardens and the gamma-ray spectrum steepens when the source is fainter.

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arXiv1997

A multi-wavelength study of the Seyfert 1 galaxy MCG-6-30-15

We present a multiwaveband spectroscopic study of the nearby Seyfert 1 galaxy MCG-6-30-15. New optical spectra from the Anglo-Australian Telescope are presented which clearly show the effects of dust extinction/reddening on both the emission line spectrum and the non-stellar AGN continuum. The reddening is constrained to be in the range E(B-V)=0.61-1.09. Spectroscopy in the X-ray band, with both ROSAT and ASCA, reveal absorption by the warm absorber but little or no neutral absorption expected to accompany the dust responsible for the optical reddening. The dusty warm absorber solution to this discrepancy is discussed and photoionization models of such warm absorbers are constructed. The optical spectrum also displays the relatively strong `coronal' lines of [FeX]6375, [FeXI]7892 and [FeXIV]5303. We show that these lines may plausibly originate from the outer regions of the warm absorber, although better calculations of the collision strengths for these transitions are required in order to conclusively address this issue. We also present new ultraviolet data from the International Ultraviolet Explorer and suggest that much of the observed UV flux is scattered into our line of sight (with a scattering fraction of 1-5 per cent). We conclude with a discussion of the global energetics of this system.

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arXiv1997

Thermal material in jets

The properties of thermal material coexisting with non-thermal emitting plasma and strong magnetic field in the powerful jets of Active Galactic Nuclei (AGN) are examined. Theoretical and observational constraints on the physical properties of this `cold' component are determined. While the presence of a thermal component occupying a fraction ~1.E-8 of the jet volume is possible, it seems unlikely that such a component is capable of contributing significantly to the total jet energy budget, since the thermal reprocessing signatures that should appear in the spectra have not, as yet, been detected.

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arXiv1997

Limits from rapid TeV variability of Mrk 421

The extreme variability event in the TeV emission of Mrk 421, recently reported by the Whipple team, imposes the tightest limits on the typical size of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the consequences that this imposes on the bulk Lorentz factor of the emitting plasma and on the radiation fields present in the central region of this Active Nucleus. No strong evidence is found for extreme Lorentz factors. However, energetics arguments suggest that any accretion in Mrk 421 has to take place at small rates, compatible with an advection-dominated regime.

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arXiv1997

Cyclo-synchrotron emission from magnetically dominated active regions above accretion discs

We discuss the role of thermal cyclo-synchrotron emission in a magnetically dominated corona above an accretion disk in an active galaxy or a galactic black hole candidate. The dissipation occurs in localized active regions around the central black hole. Cyclo-synchrotron radiation is found to be an important process. In the case of galactic black hole candidates, its emission can dominate the inverse Compton scattering of the soft photon field produced by the disc by thermal electrons. We discuss observational predictions and the detectability of cyclo-synchrotron radiation both for these sources and radio-quiet active galactic nuclei.

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arXiv1997

Average Spectral Energy Distributions of Blazars from Radio to Gamma-Rays

The average Spectral Energy Distributions (SED) of different classes of blazars from the radio to the gamma-ray band are studied adopting a "complete-sample" approach. The SED are globally similar, despite systematic differences in specific frequency ranges, showing two broad peaks. However, i) the first and second peak occur in different frequency ranges for different objects, with a tendency for the most luminous sources to peak at lower frequencies, ii) the ratio between the two peak frequencies *seems to be constant*, iii) the luminosity ratio between the high and low frequency component increases with the bolometric luminosity.

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arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. III. Probing the Inner Jet through Multiwavelength Correlations

In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground- based radio, IR, and optical telescopes. The light curves show a well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week there was at least one X-ray flare of comparable amplitude or perhaps ongoing stochastic X-ray variations. An extremely rapid change in UV flux was seen at the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous relativistic jet. We can rule out a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t<<hours) injection of high energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions (e.g., behind a shock front and/or in an inhomogeneous jet). The present light curves are in sharp contrast to November 1991, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hours. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes. (abbreviated abstract)

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arXiv1997

Unifying models for X-ray selected and Radio selected BL Lac Objects

We discuss alternative interpretations of the differences in the Spectral Energy Distributions (SEDs) of BL Lacs found in complete Radio or X-ray surveys. A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency. In order to explain the different properties of radio and X-ray selected BL Lacs Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high frequency spectral cut-offs are assumed to be a minority. Nevertheless they dominate the X-ray selected population due to the larger X-ray-to-radio-flux ratio. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is "primary" and that at each X-ray luminosity a minority of objects has larger radio-to-X-ray flux ratio. The predictions of the two scenarios, computed via a Montecarlo technique, are compared with the observed properties of BL Lacs in the two samples extracted respectively from the 1 Jy radio survey and the Einstein Slew Survey. We show that both models can explain a number but not all the observed features. We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs, is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity we show that the observational properties of the two surveys can be reproduced at least with the same accuracy as the two previous models.

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