arXiv subjects

G. Fossati

Publications and source records attributed to G. Fossati.

At least 37 records · Page 2Linked to original sources

arXiv2000

Looking for High Energy Peaked Blazars

Blazars can be classified on the basis of their overall Spectral Energy Distribution (SED). BL Lac objects are usually divided in LBL or HBL (Low or High energy peaked BL Lacs), according to the peak frequency of the synchrotron emission, if in the optical or UV-soft-X band respectively. FSRQs instead are characterized by synchrotron peaks mainly at IR-optical frequencies, similarly to LBLs. Here we report on recent BeppoSAX observations which are unveiling the high energy branch of the range of synchrotron peak frequencies. Four new "extreme" HBLs have been discovered, one of which (1ES 1426+428) peaks near or above 100 keV, in a quiescent state.

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arXiv2000

X-ray Emission of Mkn 421: New Clues From Its Spectral Evolution. I. Temporal Analysis

Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. This is the first of two papers where we present the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. Here we focus on the time analysis, while the spectral analysis and physical interpretation are presented in the companion paper. The detailed study of the flare in different energy bands reveals very important new results: i) hard photons lag the soft ones by 2-3 ks -a behavior opposite to what is normally found in high energy peak BL Lacs X-ray spectra; ii) the flare light curve is symmetric in the softest X-ray band, while it becomes increasingly asymmetric at higher energies, with the decay being progressively slower than the rise; iii) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. The temporal and spectral information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process.

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arXiv2000

X-ray Emission of Mkn 421: New Clues From Its Spectral Evolution. II. Spectral Analysis and Physical Constraints

Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. The source showed a very rich phenomenology, with remarkable spectral variability. This is the second of two papers presenting the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. The spectral analysis and correlations are presented in this paper, while the data reduction and timing analysis are the content of the companion paper. The spectral evolution during the flare has been followed over few ks intervals, allowing us to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase, and then receding. This spectral analysis nicely confirms the delay of the flare at the higher energies, which in Paper I we quantified as a hard lag of a few ks. Furthermore, at the highest energies, evidence is found of variations of the inverse Compton component. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process. A scenario accounting for all the observational constraints is discussed, where electrons are injected at progressively higher energies during the development of the flare, and the achromatic decay is ascribed to the source light crossing time exceeding the particle cooling timescales.

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arXiv2000

Gamma-loud quasars: a view with BeppoSAX

We present $Beppo$SAX observations of the $γ$-ray emitting quasars 0836+710, 1510-089 and 2230+114. All the objects have been detected in the PDS up to 100 keV and have extremely flat power-law spectra above 2 keV ($α_x$=0.3--0.5). 0836+710 shows absorption higher than the galactic value and marginal evidence for the presence of the redshifted 6.4 keV Iron line. 1510-089 shows a spectral break around 1 keV, with the low energy spectrum steeper ($α_l$=1.6) than the high energy power-law ($α_h$=0.3). The data are discussed in the light of current Inverse Compton models for the high energy emission.

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arXiv2000

New Extreme Synchrotron BL Lac Objects

We report on the BeppoSAX observations of four "extreme" BL Lacs, selected to have high synchrotron peak frequencies. All have been detected also in the PDS band. For 1ES 0120+340, PKS 0548-322 and H 2356-309 the spectrum is well fitted by a convex broken power-law, thus locating the synchrotron peak around 1 - 4 keV. 1ES 1426+428 presents a flat energy spectral index (alpha_x= 0.92) up to 100 keV, thus constraining the synchrotron peak to lie near or above that value. For their extreme properties, all sources could be strong TeV emitters.

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arXiv1999

The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lac objects

ON 231 was observed with BeppoSAX in May and June 1998, following an exceptional optical outburst which occured in April--May. We measured the X-ray spectrum from 0.1 up to 100 keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simultaneously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between the first and the second observation, and smaller variability amplitude between 4 and 10 keV. During the May observation we also detected a fast variability event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models.

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arXiv1999

BeppoSAX Observations of Mkn 421: clues on the particle acceleration ?

Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present highlights of the results of the thorough temporal and spectral analysis discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the flare of April 1998, which was simultaneously observed also at TeV energies. The detailed study of the flare in different energy bands reveals a few very important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior opposite to what is normally found in High energy peak BL Lacs X-ray spectra*; (b) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. Moreover the spectral evolution during the flare has been followed by extracting X-ray spectra on few ks intervals, allowing to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase, and then receding. The spectral analysis confirms the delay in the flare at the higher energies, as above a few keV the spectrum changes only after the peak of the outburst has occurred. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process. A theoretical picture accounting for all the observational constraints is discussed, where electrons are injected at low energies and then progressively accelerated during the development of the flare.

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arXiv1999

X-ray Rapid Variability of MKN 421

We present a detailed Cross Correlation analysis of the light curves of the X-ray bright BL Lac object Mkn 421 observed with $Beppo$SAX period 1997-1998. Monte Carlo simulations are performed in order to estimate the uncertainties. We found that in one occasion the lag is negative.

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arXiv1999

Hour-Scale Multiwavelength Variability in Markarian 421

Markarian 421 was observed for about four days with BeppoSAX and the Whipple Observatory gamma-ray telescope in April 1998. A pronounced, well-defined, flare with hour-scale variability was observed simultaneously in X-rays and very high energy gamma-rays. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours. While the rise of the flare occurred on a similar time-scale in the two wavebands, the decay of the flare was much more rapid in gamma rays, providing the first clear indication that the X-ray and gamma-ray emission may not be completely correlated in Markarian 421.

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arXiv1999

The blazar PKS 0528+134: new results from BeppoSAX observations

BeppoSAX observed 8 times the gamma-ray bright blazar PKS 0528+134 in Feb. and Mar. 1997, during a multiwavelength campaign involving EGRET, onboard CGRO, and ground based telescopes. The source was in its faintest X-ray state observed so far, with an unabsorbed [2-10] keV flux of 2.5E-12 erg/cm2/s. The spectrum can be fitted with a power law, with energy index alpha_X=0.49+-0.07 between 0.1 and 10 keV. The high energy instrument PDS detected a flux in the 15-100 keV band which appears disconnected from the lower X-ray energies, most likely due to a contaminating source. We consider our findings in the context of the overall spectral energy distribution and discuss their implications for synchrotron and inverse Compton models.

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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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arXiv1999

Progress in understanding Blazars from BeppoSAX observations

Results obtained with BeppoSAX observations of blazars within various collaborative programs are presented. The spectral similarity "paradigm", whereby the spectral energy distributions of blazars follow a sequence, leading to a unified view of the whole population, is briefly illustrated. We concentrate on recent observations of flares and associated spectral variability for three objects at the "blue" end of the spectral sequence, namely PKS 2155-304, Mkn 421 and Mkn 501. The results are discussed in terms of a general analytic synchrotron self-Compton interpretation of the overall spectrum. The physical parameters of the quasi-stationary emission region can be derived with some confidence, while the variability mechanism(s) must be complex.

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arXiv1998

Unifying All Blazars

We propose a new physical model for the unification of all Blazars. The blazar phenomenology can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. This regulates the equilibrium electron distribution and hence the shape of the spectral energy distribution (SED), and in turn the classification into a blazar "flavour". The processes inside and beside the emitting jet are the same throughout the blazar class. Objects along this sequence would be observationally classified as High-frequency-peaked BL Lac objects (HBL), Low-frequency BL Lac objects (LBL), Highly Polarized Quasars (HPQ) and Lowly Polarized Quasars (LPQ).

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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

Scientific Report on Low Energy Absorption Features Detected by BeppoSAX LECS in Bright Blazar Targets

The apparency of low energy absorption features in Blazar LECS spectra has been reported during the early phase of the BeppoSAX mission (e.g. by Grandi et al. 1997 for 3C~273 SVP data; Giommi et al. 1997 for PKS 2155--304 SVP data), with data reduction and analysis tools released prior to July 1997. We set up a robust method to investigate the reliability of absorption features detections in LECS data of bright blazars. The analysis shows that similar features are common to all datasets (with possibly the exception of 3C~273 SVP data). This suggests that most of the features observed from 0.2 to 1 keV are possibly due to miscalibration of the instrument response. The reported problems have been overcome with the latest release of calibrations (September 1997).

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arXiv1998

BeppoSAX Observations of the TeV Blazar Mkn 421

The blazar Mkn 421 has been observed, as part of the AO1 Core Program, five times from 2 to 7 May 1997. In the LECS+MECS energy band the spectrum shows convex curvature, well represented by a broken power--law. Flux variability (more than a factor 2) has been detected over the entire 0.1--10 keV range, accompanying which the spectrum steepens with the decrease in intensity. Mkn 421 has also been detected with the PDS instrument. Our preliminary analysis indicates that the PDS spectrum lies significantly above the extrapolation from the MECS, suggesting a contribution from a flatter high energy component.

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