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E. Massaro

Publications and source records attributed to E. Massaro.

At least 37 records · Page 2Linked to original sources
arXiv2012

A new flaring high energy gamma-ray source

We report the detection of a new gamma-ray source in the Fermi-LAT sky using a source detection tool based on the Minimal Spanning Tree algorithm. The source, not reported in previous LAT catalogues but very recently observed in the X-rays and optical bands, is characterized by an increasing gamma-ray activity in 2012 June-September, reaching a weekly peak flux of (3.3+-0.6)*10^-7 photons cm^-2 s^-1. A search for a possible counterpart provides indication that it can be associated with the radio source NVSS J141828+354250 whose optical SDSS colours are typical of a blazar.

astro-ph.HE↗
arXiv2011

Information dynamics algorithm for detecting communities in networks

The problem of community detection is relevant in many scientific disciplines, from social science to statistical physics. Given the impact of community detection in many areas, such as psychology and social sciences, we have addressed the issue of modifying existing well performing algorithms by incorporating elements of the domain application fields, i.e. domain-inspired. We have focused on a psychology and social network - inspired approach which may be useful for further strengthening the link between social network studies and mathematics of community detection. Here we introduce a community-detection algorithm derived from the van Dongen's Markov Cluster algorithm (MCL) method by considering networks' nodes as agents capable to take decisions. In this framework we have introduced a memory factor to mimic a typical human behavior such as the oblivion effect. The method is based on information diffusion and it includes a non-linear processing phase. We test our method on two classical community benchmark and on computer generated networks with known community structure. Our approach has three important features: the capacity of detecting overlapping communities, the capability of identifying communities from an individual point of view and the fine tuning the community detectability with respect to prior knowledge of the data. Finally we discuss how to use a Shannon entropy measure for parameter estimation in complex networks.

physics.soc-ph↗
arXiv2011

The complex behaviour of the microquasar GRS 1915+105 in the rho class observed with BeppoSAX. II: Time-resolved spectral analysis

BeppoSAX observed GRS 1915+105 on October 2000 with a long pointing lasting about ten days. During this observation, the source was mainly in the rho class characterized by bursts with a recurrence time of between 40 and 100 s. We identify five segments in the burst structure and accumulate the average spectra of these segments during each satellite orbit. We present a detailed spectral analysis aimed at determining variations that occur during the burst and understanding the physical process that produces them. We compare MECS, HPGSPC, and PDS spectra with several models. Under the assumption that a single model is able to fit all spectra, we find that the combination of a multi-temperature black-body disk and a hybrid corona is able to give a consistent physical explanation of the source behaviour. Our measured variations in KT_el, tau, KT_in, and R_in appear to be either correlated or anti-correlated with the count rate in the energy range 1.6-10 keV. The strongest variations are detected along the burst segments: almost all parameters exhibit significant variations in the segments that have the highest fluxes (pulse) with the exception of R_in, which varies continuously and reaches a maximum just before the peak. The flux of the multi-temperature disk strongly increases in the pulse and simultaneously the corona contribution is significantly reduced. The disk luminosity increases in the pulse and the R_in-T_in correlation can be most successfully interpreted in term of the slim disk model. In addition, the reduction in the corona luminosity at the bursts might represent the condensation of the corona onto the disk.

astro-ph.SR↗
arXiv2011

Stochastic Acceleration and the Evolution of Spectral Distributions in SSC Sources: A Self Consistent Modeling of Blazars' Flares

The broad-band spectral distributions of non-thermal sources, such as those of several known blazars, are well described by a log-parabolic fit. The second degree term in these fits measures the curvature in the spectrum. In this paper we investigate whether the curvature parameter observed in the spectra of the synchrotron emission can be used as a fingerprint of stochastic acceleration. As a first approach we use the multiplicative Central Limit theorem to show how fluctuations in the energy gain result in the broadening of the spectral shape, introducing a curvature into the energy distribution. Then, by means of a Monte-Carlo description, we investigate how the curvature produced in the electron distribution is linked to the diffusion in momentum space. To get a more generic description we turn to the diffusion equation in momentum space. We first study some "standard" scenarios, in order to understand the conditions that make the curvature in the spectra significant, and the relevance of the cooling over the acceleration process. We try to quantify the correlation between the curvature and the diffusive process in the pre-equilibrium stage, and investigate how the transition between the Klein-Nishina and the Thompson regime, in Inverse Compton cooling, can determine the curvature in the distribution at the equilibrium. We apply these results to some observed trends, such as the anticorrelation between the peak energy and the curvature term observed in the spectra of Mrk 421, and a sample of BL Lac objects whose synchrotron emission peaks at X-ray energies.

astro-ph.HE↗
arXiv2011

The populations of hard X- and γ-ray sources: a correlation study and new possible identifications

We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of γ-ray sources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold σ = 3. We obtain a sample of common sources emitting in both the hard X- and the γ-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.

astro-ph.HE↗
arXiv2010

The complex behaviour of the microquasar GRS 1915+105 in the rho class observed with BeppoSAX. I: Timing analysis

GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For about 80% of the time, the source was in the variability class $ρ$, characterised by a series of recurrent bursts. We describe the results of the timing analysis performed on the MECS (1.6--10 keV) and PDS (15--100 keV) data. The X-ray count rate from \grss showed an increasing trend with different characteristics in the various energy bands. Fourier and wavelet analyses detect a variation in the recurrence time of the bursts, from 45--50 s to about 75 s, which appear well correlated with the count rate. From the power distribution of peaks in Fourier periodograms and wavelet spectra, we distinguished between the {\it regular} and {\it irregular} variability modes of the $ρ$ class, which are related to variations in the count rate in the 3--10 keV range. We identified two components in the burst structure: the slow leading trail, and the pulse, superimposed on a rather stable level. We found that the change in the recurrence time of the regular mode is caused by the slow leading trails, while the duration of the pulse phase remains far more stable. The evolution in the mean count rates shows that the time behaviour of both the leading trail and the baseline level are very similar to those observed in the 1.6--3 and 15--100 keV ranges, while that of the pulse follows the peak number. These differences in the time behaviour and count rates at different energies indicate that the process responsible for the pulses must produce the strongest emission between 3 and 10 keV, while that associated with both the leading trail and the baseline dominates at lower and higher energies

astro-ph.HE↗
arXiv2009

Search for gamma-ray sources in the LAT sky with the Minimal Spanning Tree algorithm

We present the topometric MST method to search for clusters of photons in the LAT sky, which was used to obtain the seed list for the compilation of the First LAT catalog. This method works well in non-dense field and can be profitably used at energies higher than a few GeV. We describe the particular techniques developed by us to improve the cluster selection criteria and the estimate of the astronomical coordinates of the possibly associated gamma-ray sources. A simulation technique to evaluate the confidence level of the source detectionis presented.

astro-ph.HE↗
arXiv2009

The multicomponent model of the Crab Pulsar at energies above 25 GeV

We review a proposed multicomponent model to explain the features of the pulsed emission and spectrum of the Crab Pulsar, on the light of the recent detection of pulsed emission above 25 GeV from the MAGIC atmospheric Cherenkov telescope. This model explains the evolution of the pulse shape and of the phase-resolved spectra, ranging from the optical/UV to the GeV energy band, on the assumption that the observed emission is due to several components, which have spectra modelled as log-parabolic laws. We show that the new MAGIC data are well consistent with the prevision of our model.

astro-ph.HE↗
arXiv2008

Roma-BZCAT: A multifrequency catalogue of Blazars

We present a new catalogue of blazars based on multi-frequency surveys and on an extensive review of the literature. Blazars are classified as BL Lacertae objects, as flat spectrum radio quasars or as blazars of uncertain/transitional type. Each object is identified by a root name, coded as BZB, BZQ and BZU for these three subclasses respectively, and by its coordinates. This catalogue is being built as a tool useful for the identification of the extragalactic sources that will be detected by present and future experiments for X and gamma-ray astronomy, like Swift, AGILE, Fermi-GLAST and Simbol-X. An electronic version is available from the ASI Science Data Center web site at http://www.asdc.asi.it/bzcat

astro-ph↗
arXiv2008

X-ray observations of the Large Magellanic Cloud pulsar PSR B0540-69 and its PWN

PSR B0540-69 is a young pulsar in the Large Magellanic Cloud that has similar properties with respect to the Crab Pulsar, and is embedded in a Pulsar Wind Nebula. We have analyzed the complete archival RXTE dataset of observations of this source, together with new Swift-XRT and INTEGRAL-IBIS data. Accurate lightcurves are produced in various energy bands between 2 and 60 keV, showing no significant energy variations of the pulse shape. The spectral analysis shows that the pulsed spectrum is curved, and is best fitted up to 100 keV by a log-parabolic model: this strengthens the similarities with the Crab pulsar, and is discussed in the light of a phenomenologic multicomponent model. The total emission from this source is studied, the relative contributions of the pulsar and the PWN emission are derived, and discussed in the context of other INTEGRAL detected pulsar/PWN systems.

astro-ph↗
arXiv2007

GAW - An Imaging Atmospheric Cherenkov Telescope with Large Field of View

GAW, acronym for Gamma Air Watch, is a Research and Development experiment in the TeV range, whose main goal is to explore the feasibility of large field of view Imaging Atmospheric Cherenkov Telescopes. GAW is an array of three relatively small telescopes (2.13 m diameter) which differs from the existing and presently planned projects in two main features: the adoption of a refractive optics system as light collector and the use of single photoelectron counting as detector working mode. The optics system allows to achieve a large field of view (24x24 squared degrees) suitable for surveys of large sky regions. The single photoelectron counting mode in comparison with the charge integration mode improves the sensitivity by permitting also the reconstruction of events with a small number of collected Cherenkov photons. GAW, which is a collaboration effort of Research Institutes in Italy, Portugal and Spain, will be erected in the Calar Alto Observatory (Sierra de Los Filabres - Andalucia, Spain), at 2150 m a.s.l.). The first telescope will be settled within Autumn 2007. This paper shows the main characteristics of the experiment and its expected performance.

astro-ph↗
arXiv2007

The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX

We present the complete on-line catalogue of gamma-ray bursts observed by the two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to provide the community with the largest published data set of GRB's prompt emission X-ray light curves and other useful data. This catalogue (BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light curves of 56 GRB discovered or noticed shortly after the event and of other additional bursts detected in subsequent searches. Light curves are given in the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be accessed from the home web page of the ASI Science Data Center-ASDC (http://www.asdc.asi.it)

astro-ph↗
arXiv2007

Blazar surveys with WMAP and Swift

We present the preliminary results from two new surveys of blazars that have direct implications on the GLAST detection of extragalactic sources from two different perspectives: microwave selection and a combined deep X-ray/radio selection. The first one is a 41 GHz flux-limited sample extracted from the WMAP 3-yr catalog of microwave point sources. This is a statistically well defined sample of about 200 blazars and radio galaxies, most of which are expected to be detected by GLAST. The second one is a new deep survey of Blazars selected among the radio sources that are spatially coincident with serendipitous sources detected in deep X-ray images (0.3-10 keV) centered on the Gamma Ray Bursts (GRB) discovered by the Swift satellite. This sample is particularly interesting from a statistical viewpoint since a) it is unbiased as GRBs explode at random positions in the sky, b) it is very deep in the X-ray band (\fx \simgt $10^{-15}$ \ergs) with a position accuracy of a few arc-seconds, c) it will cover a fairly large (20-30 square deg.) area of sky, d) it includes all blazars with radio flux (1.4 GHz) larger than 10 mJy, making it approximately two orders of magnitude deeper than the WMAP sample and about one order of magnitude deeper than the deepest existing complete samples of radio selected blazars, and e) it can be used to estimate the amount of unresolved GLAST high latitude gamma-ray background and its anisotropy spectrum.

astro-ph↗
arXiv2007

ROXA J081009.9+384757.0: a $10^{47}$ erg/s blazar with hard X-ray synchrotron peak or a new type of radio loud AGN?

We report the discovery of ROXA J081009.9+384757.0 = SDSS J081009.9+384757.0, a z=3.95 blazar with a highly unusual Spectral Energy Distribution (SED). This object was first noticed as a probable high $f_x/f_r$, high-luminosity blazar within the error region of a $\approx 10^{-12}$ erg/s cm$^2$ ROSAT source which, however, also included a much brighter late-type star. We describe the results of a recent Swift observation that establishes beyond doubt that the correct counterpart of the X-ray source is the flat spectrum radio quasar. With a luminosity well in excess of $10^{47}$ erg/s, ROXA J081009.9+384757.0 is therefore one of the most luminous blazars known. We consider various possibilities for the nature of the electromagnetic emission from this source. In particular, we show that the SED is consistent with that of a blazar with synchrotron power peaking in the hard X-ray band. If this is indeed the case, the combination of high-luminosity and synchrotron peak in the hard-X-ray band contradicts the claimed anti-correlation between luminosity and position of the synchrotron peak usually referred to as the "blazar sequence". An alternative possibility is that the X-rays are not due to synchrotron emission, in this case the very peculiar SED of ROXA J081009.9+384757.0 would make it the first example of a new class of radio loud AGN.

astro-ph↗
arXiv2007

A multicomponent model for the optical to gamma-ray emission from the Crabpulsar

We present a multicomponent model to explain the features of the pulsed emission and spectrum of the Crab Pulsar, on the basis of X and gamma-ray observations performed with BeppoSAX, INTEGRAL and CGRO. This model explains the evolution of the pulse shape and of the phase-resolved spectra, ranging from the optical/UV to the GeV energy band, on the assumption that the observed emission is due to several components. The first component, C_O, is assumed to have the pulsed double-peaked profile observed at the optical frequencies, while the second component, C_X, isdominant in the interpeak and second peak phase regions. The spectra of these components are modelled with log-parabolic laws. Moreover, to explain the properties of the pulsed emission in the MeV-GeV band, we introduce two more components, C_O_gamma and C_X_gamma, with phase distributions similar to those of C_O and C_X and log-parabolic spectra with the same curvature but different peak energies. This multicomponent model is able to reproduce both the broadband phase-resolved spectral behaviour and the changes of the pulse shape with energy. We also propose some possible physical interpretations in which C_O and C_X are emitted by secondary pairs via a synchrotron mechanism while C_O_gamma and C_X_gamma can originate either from Compton scattered or primary curvature photons.

astro-ph↗
arXiv2006

SWIFT observations of TeV BL Lac objects

Context: We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. Aims: We are mainly interested in measuring the spectral parameters, and particularly the intrinsic curvature in the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac TeV objects using either a log-parabolic or a simple power-law model . Results: We found that many of the objects in our sample do show significant spectral curvature, whereas those having the peak of the spectral energies distribution at energies lower than ~0.1 keV show power law spectra. In these cases, however, the statistics are generally low thus preventing a good estimate of the curvature. Simultaneous UVOT observations are important to verify how X-ray spectra can be extrapolated at lower frequencies and to search for multiple emission components. Conclusions: The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.

astro-ph↗
arXiv2006

Swift XRT and UVOT deep observations of the high energy peaked BL Lac object PKS 0548-322 close to its brightest state

We present the results of a spectral analysis of 5 Swift XRT and UVOT observations of the BL Lac object PKS 0548-322 carried out over the period April-June 2005. The X-ray flux of this high energy peaked BL Lac (HBL) source was found to be approximately constant at a level of F(2-10 keV) ~ 4x10^-11 erg cm^-2 s^-1, a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power law spectral distribution. A log-parabolic model describes well the X-ray data and gives a best fit curvature parameter of 0.18 and peak energy in the Spectral Energy Distribution of about 2 keV. The UV spectral data from Swift UVOT join well with a power law extrapolation of the soft X-ray data points suggesting that the same component is responsible for the observed emission in the two bands. The combination of synchrotron peak in the X-ray band and high intensity state confirms PKS 0548-322 as a prime target for TeV observations. X-ray monitoring and coordinated TeV campaigns are highly advisable.

astro-ph↗