arXiv subjects

G. Tagliaferri

Publications and source records attributed to G. Tagliaferri.

At least 325 records · Page 18Linked to original sources

arXiv2002

The Brera Multi-scale Wavelet ROSAT HRI source catalogue (BMW-HRI)

We present the Brera Multi-scale Wavelet ROSAT HRI source catalogue (BMW-HRI) derived from all ROSAT HRI pointed observations with exposure time longer than 100 s available in the ROSAT public archives. The data were analyzed automatically using a wavelet detection algorithm suited to the detection and characterization of both point-like and extended sources. This algorithm is able to detect and disentangle sources in very crowded fields and/or in presence of extended or bright sources. Images have been also visually inspected after the analysis to ensure verification. The final catalogue, derived from 4,303 observations, consists of 29,089 sources detected with a detection probability of greater or equal 4.2 sigma. For each source, the primary catalogue entries provide name, position, count rate, flux and extension along with the relative errors. In addition, results of cross-correlations with existing catalogues at different wavelengths (FIRST, IRAS, 2MASS and GSC2) are also reported. All these information are available on the web via the DIANA Interface. As an external check, we compared our catalogue with the previously available ROSHRICAT catalogue (both in its short and long versions) and we were able to recover, for the short version, \~90% of the entries. We computed the sky coverage of the entire HRI data set by means of simulations. The complete BMW-HRI catalogue provides a sky coverage of 732 square degrees down to a limiting flux of ~1E-12 erg/s/cm2 and of 10 square degrees down to ~1E-14 erg/s/cm2. We were able to compute the cosmological log(N)-log(S) distribution down to a flux of about 1.2x1E-14 erg/s/cm2.

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arXiv2002

BL Lacertae: the multiwavelength campaign of 2000

We present two BeppoSAX observations of BL Lacertae as part of a multiwavelength radio-to-TeV campaign. During the first observation we observe a faint Compton spectrum, while during the second, we detect a synchrotron spectrum with the highest [2-10] keV flux ever measured; above 10 keV an inverse Compton component begin to dominate. The synchrotron flux is very variable with time scales of 1 hr. We describe four different SED shifting the synchrotron peak both in frequency and flux intensity and we sketch a scenario in which a blob moves along a jet and can be located in or outside the BLR. This implies different radiative mechanism at work, SSC or external Compton, producing different high energy spectra.

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arXiv2002

Polarimetric observations of GRB 011211

We present and discuss polarimetric observations performed with the VLT-UT3 (Melipal) on the afterglow of GRB 011211, ~35 hours after the burst onset. The observations yielded a 3-sigma upper limit of P<2.7%. We discuss this result in combination with the lightcurve evolution, that may show a break approximately at the time of our observation. We show that our upper limit is consistent with the currently favored beamed fireball geometry, especially if the line of sight was not too close to the edge of the cone.

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arXiv2002

Spectral Energy Distributions of Flat Spectrum Radio Quasars observed with BeppoSAX

We report the BeppoSAX observations of 6 Flat Spectrum Radio Quasars. Three of them have a clear detection up to 100 keV with the PDS instrument. For 4 objects the X-ray spectrum is satisfactorily fitted by a power-law continuum with Galactic absorption. 2251+158 show the presence of absorption higher than the galactic value, while the spectrum of the source 0208-512 shows a complex structure, with evidence of absorption at low energy. We construct the Spectral Energy Distributions adding historical data to the broad band X-ray spectra obtained with $Beppo$SAX and reproduce them with a one-zone Synchrotron-Inverse Compton model (including both SSC and External Compton). The implications are briefly discussed.

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arXiv2002

Broadband Spectra and Variability of BL Lacertae in 2000

We have organized an extensive multiwavelength campaign on BL Lacertae in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio frequencies, in the optical - carried out by the WEBT collaboration -, in X-rays - using BeppoSAX and RXTE -, and at VHE gamma-rays with the CAT and HEGRA Cherenkov telescope facilities. In this paper, we are presenting first results from this campaign. The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10 - 20 hour segments, and revealed intraday variability on time scales of ~ 1.5 hours. The X-ray observations of Nov. 1 - 2, 2000, revealed significant variability on similar time scales, and provided evidence for the synchrotron spectrum extending out to ~ 10 keV during that time. At higher energies, the onset of the hard power-law commonly observed in radio-loud quasars and radio-selected BL Lac objects is seen. From the energy-resolved X-ray variability patterns, we find evidence that electron cooling might not be dominated by one single radiation mechanism; rather, a combination of synchrotron and Compton cooling seems to be required.

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arXiv2002

A full orbit XMM-Newton observation of PKS 2155-304

XMM observed the BL Lac PKS 2155-304 for a full orbit (about 150 ksec) on 2000 November 19-21. Preliminary results on the temporal and spectral analysis of data from the EPIC PN camera and Optical Monitor are presented. The variability amplitude depends systematically on energy, however the slopes of the structure functions of the light-curves in different bands do not appear to be significantly different. No evidence of time lags is found by cross correlating the light-curves in different bands.

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arXiv2002

Four Years Monitoring of Blazar PKS 2155-304 with BeppoSAX: Probing the Dynamics of the Jet

PKS 2155--304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with BeppoSax during three long campaigns of about 2 days each in November of 1996, 1997 and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The rich phenomenology is interpreted in the context of a model where relativistic electrons are accelerated through internal shocks taking place in the jets. The most important parameter turns out to be the initial time interval between the two shells ejected from the central engine to produce the flare, which may determine the structure of the shock and in turn the physical quantities of the emitting region to reproduce the observed trends of the X-ray variability (abridged).

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arXiv2002

BL Lacertae: complex spectral variability and rapid synchrotron flare detected with BeppoSAX

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite different spectra: in June it was concave with a very hard component above 5-6 keV (alpha_1 ~ 1.6; alpha_2 ~ 0.15); in December it was well fitted by a single power law (alpha ~ 0.6). During the first BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3 - 2 keV flux doubled in ~ 20 minutes, while the flux above 4 keV was almost constant. This frequency-dependent event is one of the shortest ever recordered for BL Lac objects and places lower limits on the dimension and the magnetic field of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of ~ 20% in 20 min. We fit the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but effective SSC + external Compton scenario, highlightning the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to gamma-ray Spectral Energy Distributions.

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arXiv2002

The BMW Detection Algorithm applied to the Chandra Deep Field south: deeper and deeper

Chandra deep fields represent the deepest look at the X-ray sky. We analyzed the Chandra Deep Field South (CDFS) with the aid of a dedicated wavelet-based algorithm. Here we present a detailed description of the procedures used to analyze this field, tested and verified by means of extensive simulations. We show that we can safely reconstruct the LogN-Log S source distribution of the CDFS down to limiting fluxes of 2.4x10^-17 and 2.1x10^-16 erg s^-1 cm^-2 in the soft (0.5-2 keV) and hard (2-10 keV) bands, respectively, fainter by a factor ~ 2 than current estimates. At these levels we can account for ~ 90% of the 1-2 keV and 2-10 keV X-ray background.

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arXiv2001

BeppoSAX Spectral Survey of BL Lacs - new spectra and results

We present BeppoSAX LECS, MECS, and PDS spectra of eleven X-ray selected BL Lacertae objects. Combining these sources with the ones presented elsewhere we have a sample of 21 BL Lacs from the Einstein Medium Sensitivity and Einstein Slew Survey. The sample shows strong correlations of several physical parameters with the peak frequency of the synchrotron branch of the spectral energy distribution. In particular the peak frequency is correlated to the X-ray spectral shape: objects with the peak near to the X-ray band show harder and straighter X-ray spectra than those of the low energy peaked sources. This work shows that the recently proposed unification scenario for different types of blazars can hold also within the class of high frequency peaked BL Lac objects.

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arXiv2001

Spectral Energy Distributions of 3C 279 revisited: BeppoSAX observations and variability models

We present the BeppoSAX observations of 3C 279 performed in 1997 January 13-23 simultaneously with gamma-ray pointings with EGRET on board CGRO and optical observations at the Torino Observatory (1997 January 11-18). ISO data close to this epoch are also available. We compare the derived Spectral Energy Distribution with those obtained at all other epochs with adequate multiwavelength coverage. Simple spectral models fitted to the multiepoch SEDs suggest that changes in intensity can be ascribed mainly to the variation of the bulk Lorentz factor of the plasma in the jet.

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arXiv2001

Multiwavelenght analysis of Gl 355 (LQ Hya)

We discuss ROSAT, ASCA, {\it Beppo}SAX and optical observations of the young active star Gl 355. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. We have modeled the flare and derived a loop semi--length of the order of $\sim 1.5$ stellar radii. ROSAT, ASCA and {\it Beppo}SAX data suggest that the coronal abundance of Gl 355 is subsolar, in the range $0.1 ÷0.3 Z/Z_\odot$. A preliminary analysis of optical spectra allows us to compare the photospheric and coronal metal abundances.

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arXiv2001

The deepest X-ray look at the Universe

The origin of the X-ray background, in particular at hard (2-10 keV) energies, has been a debated issue for more than 30 years. The Chandra deep fields provide the deepest look at the X-ray sky and are the best dataset to study the X-ray background. We searched the Chandra Deep Field South for X-ray sources with the aid of a dedicated wavelet-based algorithm. We are able to reconstruct the Log N-Log S source distribution in the soft (0.5-2 keV) and hard (2-10 keV) bands down to limiting fluxes of 2x10^{-17} erg s^{-1} cm^{-2} and 2x10^{-16} erg s^{-1} cm^{-2}, respectively. These are a factor ~5 deeper than previous investigations. We find that the soft relation continues along the extrapolation from higher fluxes, almost completely accounting for the soft X-ray background. On the contrary, the hard distribution shows a flattening below ~2x10^{-14} erg s^{-1} cm^{-2}. Nevertheless, we can account for >68% of the hard X-ray background, with the main uncertainty being the sky flux itself.

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arXiv2001

Hard X-ray properties of blazars

We have considered all blazars observed in the X-ray band and for which the slope of the X-ray spectrum is available. We have collected 421 spectra of 268 blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose analysisis presented here. The X-ray spectra of blazars show trends as a function of their power, confirming that the blazar overall energy distribution can be parameterized on the basis of one parameter only, i.e. the bolometric luminosity. This is confirmed by the relatively new hard (2-10 keV) X-ray data. Our results confirm the idea that in low power objects the X-ray emission mechanism is the synchrotron process, dominating both the soft and the hard X-ray emissions. Low energy peaked BL Lac objects are intermediate, often showing harder spectra in the hard X-ray band, suggesting that the synchrotron process dominates in the soft band, with the inverse Compton process dominating at high energies. The most powerful objects have X-ray spectra that are flat both in the soft and in the hard band, consistent with a dominating inverse Compton component.

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arXiv2001

BeppoSAX Observations of 1 Jy BL Lacertae Objects. I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1 Jy sample plus one additional source. The collected data cover the energy range 0.1 - 10 keV (observer's frame), reaching ~ 50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the LBL type) display a relatively flat (alpha_x ~ 0.9) X-ray spectrum, which we interpret as inverse Compton emission. Four objects (2/3 of the LBL) show some evidence for a low-energy steepening which is likely due to the synchrotron tail merging into the inverse Compton component around ~ 1 - 3 keV. If this were generally the case with LBL, it would explain why the 0.1 - 2.4 keV ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones (by ~ 0.5 in alpha_x). The broad-band spectral energy distributions fully confirm this picture and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequencies, we confirm and clarify the dependence of the X-ray spectral index on synchrotron peak frequency originally found in ROSAT data.

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arXiv2001

Extreme synchrotron BL Lac objects

We performed an observational program with the X-ray satellite BeppoSAX to study objects with extreme synchrotron peak frequencies (nu_peak > 1 keV). Of the seven sources observed, four showed peak frequencies in the range 1-5 keV, while one (1ES 1426+428) displayed a flat power law spectrum (alpha= 0.92), locating its synchrotron peak at or above 100 keV. This is the third source of this type ever found, after Mkn 501 and 1ES 2344+514. Our data confirm the large nu_peak variability of this class of sources, compared with lower peaked objects. The high synchrotron peak energies, flagging the presence of high energy electrons, make the extreme BL Lacs also good candidates for TeV emission, and therefore good probes for the IR background.

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arXiv2001

Quiescent and flare analysis for the chromospherically active star Gl355 (LQHya)

We discuss ROSAT and ASCA observations of the young active star Gl355}. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. The global coronal metallicity $Z/Z_{\odot} \sim 0.1$ derived from both ROSAT and ASCA data is much lower than solar and presumably also much lower than the photospheric abundance expected for this very young star. The temperature structure of the quiescent corona was about the same during the various observations, with a cooler component at $T_1 \sim 7$ MK and a hotter component (to which only ASCA was sensitive) at $T_2 \sim 20$ MK. During the flare, the low temperature component remained approximately constant and equal to the quiescent value, while the high-temperature component was the only one that varied. We have modeled the flare with the hydrodynamic-decay sustained-heating approach of Reale at al. (1997) and we have derived a loop semi--length of the order of $\sim 1.5$ stellar radii, i.e. much larger than the dimensions of flares on the Sun, but comparable with the typical dimensions inferred for other stellar flares. We have compared the derived loop size with that estimated with a simpler (but physically inconsistent) approach, finding that for this, as well for several other stellar flares, the two methods give comparable loop sizes. Possible causes and consequences of this result are discussed.

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arXiv2001

BMW: a ROSAT-HRI source catalogue obtained with a wavelet transform detection algorithm

In collaboration with the Observatories of Palermo and Rome and the SAX-SDC we are constructing a multi-site interactive archive system featuring specific analysis tools. In this context we developed a detection algorithm based on the Wavelet Transform (WT) and performed a systematic analysis of all ROSAT-HRI public data (~3100 observations +1000 to come). The WT is specifically suited to detect and characterize extended sources while properly detecting point sources in very crowded fields. Moreover, the good angular resolution of HRI images allows the source extension and position to be accurately determined. This effort has produced the BMW (Brera Multiscale Wavelet) catalogue, with more than 19,000 sources detected at the ~4.2sigma level. For each source detection we have information on the position, X-ray flux and extension. This allows for instance to select complete samples of extended X-ray sources such as candidate clusters of galaxies or SNR's. Details about the detection algorithm and the catalogue can be found in Lazzati et al. (1999), Campana et al. (1999) and Panzera et al. (2001). Here we shall present an overview of first results from several undergoing projects which make use of the BMW catalogue.

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