arXiv subjects

A. Bazzano

Publications and source records attributed to A. Bazzano.

At least 109 records · Page 6Linked to original sources

arXiv2010

A new determination of the INTEGRAL/IBIS point source location accuracy

AIMS: To determine the Point Source Location Accuracy (PSLA) for the INTEGRAL/IBIS telescope based on analysis of archival in-flight data. METHODS: Over 40000 individual pointings (science windows) of INTEGRAL/IBIS data were analysed using the latest Off-line Science Analysis software, version 7.0. Reconstructed source positions were then compared against the most accurate positions available, determined from focusing X-ray telescopes. Since the PSLA is a strong function of source detection significance, the offsets from true position were histogrammed against significance, so that the 90% confidence limits could be determined. This has been done for both sources in the fully coded field of view (FCFOV) and partially coded field of view (PCFOV). RESULTS: The PSLA is found to have improved significantly since values derived from early mission data and software for both FCFOV and PCFOV. CONCLUSIONS: This result has implications for observers executing follow-up programs on IBIS sources since the sky area to be searched is reduced by >50% in some cases.

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arXiv2010

The Soft X-ray Imager on board EXIST

The Soft X-ray Imager (SXI) is one of the three instruments on board EXIST, a multi-wavelength observatory in charge of performing a global survey of the sky in hard X-rays searching for Super-massive Black Holes (Grindlay & Natalucci, these Proceedings). One of the primary objectives of EXIST is also to study with unprecedented sensitivity the most unknown high energy sources in the Universe, like high redshift GRBs, which will be pointed promptly by the Spacecraft by autonomous trigger based on hard X-ray localization on board. The presence of a soft X-ray telescope with an effective area of about 950cm2 in the energy band 0.2-3 keV and extended response up to 10 keV will allow to make broadband studies from 0.1 to 600 keV. In particular, investigations of the spectra components and states of AGNs and monitoring of variability of sources, study of the prompt and afterglow emission of GRBs since the early phases, which will help to constrain the emission models and finally, help the identification of sources in the EXIST hard X-ray survey and the characterization of the transient events detected. SXI will also perform surveys: a scanning survey with sky coverage 2pi and a limiting flux of 5x10^(-14) cgs plus other serendipitous.

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arXiv2010

Revealing the nature of new unidentified INTEGRAL sources

Since its launch on October 2002, the INTEGRAL satellite has revolutionized our knowledge of the hard X-ray sky thanks to its unprecedented imaging capabilities and source detection positional accuracy above 20 keV. Nevertheless, many of the newly-detected sources in the INTEGRAL sky surveys are of unknown nature. However, the combined use of available information at longer wavelengths (mainly soft X-rays and radio) and of optical spectroscopy on the putative counterparts of these new hard X-ray objects allows pinpointing their exact nature. Continuing our long-standing program running since 2004 (and with which we identified more than 100 INTEGRAL objects) here we report the classification, through optical spectroscopy, of 25 unidentified high-energy sources mostly belonging to the recently published 4th IBIS survey.

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arXiv2010

Searching for 511 keV annihilation line emission from galactic compact objects with IBIS

The IBIS imager on board the INTEGRAL satellite, thanks to the large field of view and good sensitivity, gave us a unique opportunity to search for possible 511 keV point sources either previously unknown or associated to known objects such as X-ray binaries or supernovae. The IBIS sensitivity at 511 keV depends on the gamma ray detector quantum efficiency at this energy and on the background. Both these quantities have been estimated. Reducing all the available IBIS data up to April 2008 we have produced a 5 years full sky 511 keV map with 10 Ms exposure in the Galactic Center. We did not find any significant signal at this energy. The lack of detection of 511 keV galactic point sources is in agreement with the idea that a significant part of the galactic positrons originates in compact objects and then propagates in the interstellar medium.

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arXiv2010

EXIST perspective for Supergiant Fast X-ray Transients

Supergiant Fast X-ray Transients (SFXTs) are one of the most intriguing (and unexpected) results of the INTEGRAL mission. They are a new class of High Mass X-ray Binaries involving about 20 sources to date, with 8 firmly identified SFXTs and many candidates. They are composed by a massive OB supergiant star as companion donor and a compact object. At least four SFXTs host a neutron star, because X-ray pulsations have been discovered, while for the others a black hole cannot be excluded. SFXTs display short X-ray outbursts (compared with Be/X-ray transients) characterized by fast flares on brief timescales of hours and large flux variability typically in the range 1,000-100,000. The INTEGRAL/IBIS sensitivity allowed to catch only the bright flares (peaking at 1E36-1E37erg/s), without persistent or quiescent emission. The investigation of their properties, in particular the rapid variability time scales of their flaring activity, will greatly benefit from observations with the Energetic X-ray Imaging Survey Telescope (EXIST), with the possibility to perform a long term and continuous as possible monitoring of the hard X-ray sky.

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arXiv2010

New results with Swift on Supergiant Fast X-ray Transients

We report here on the most recent results obtained on a new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients. Since October 2007, we have been performing a monitoring campaign with Swift of four SFXTs (IGRJ17544-2916, XTEJ1739-302, IGRJ16479-4514 and the X-ray pulsar AXJ1841.0-0536) for about 1-2 ks, 2-3 times per week, allowing us to derive the previously unknown long term properties of this new class of sources (their duty cycles, spectral properties in outbursts and out-of-outbursts, temporal behaviour). We also report here on additional Swift observations of two SFXTs which are not part of the monitoring: IGRJ18483-0311 (observed with Swift/XRT during a whole orbital cycle) and SAXJ1818.6-1703 (observed for the first time simultaneously in the energy range 0.3-100 keV during a bright flare).

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arXiv2010

Swift observations of the SFXT SAX J1818.6-1703 in outburst

We present the Swift observations of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 collected during the most recent outburst, which occurred on May 6 2009. In particular, we present broad-band spectroscopic and timing analysis as well as a Swift/XRT light curve that spans more than two weeks of observations. The broad-band spectral models and length of the outburst resemble those of the prototype of the SFXT class, XTE J1739-302, further confirming SAX J1818.6-1703 as a member of this class.

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arXiv2010

The peculiar high-mass X-ray binary 1ES 1210-646

Using data collected with the BeppoSAX, INTEGRAL and Swift satellites, we report and discuss the results of a study on the X-ray emission properties of the X-ray source 1ES 1210-646, recently classified as a high-mass X-ray binary through optical spectroscopy. This is the first in-depth analysis of the X-ray spectral characteristics of this source. We found that the flux of 1ES 1210-646 varies by a factor of about 3 on a timescale of hundreds of seconds and by a factor of at least 10 among observations acquired over a time span of several months. The X-ray spectrum of 1ES 1210-646 is described using a simple powerlaw shape or, in the case of INTEGRAL data, with a blackbody plus powerlaw model. Spectral variability is found in connection with different flux levels of the source. A strong and transient iron emission line with an energy of about 6.7 keV and an equivalent width of about 1.6 keV is detected when the source is found at an intermediate flux level. The line strength seems to be tied to the orbital motion of the accreting object, as this feature is only apparent at the periastron. Although the X-ray spectral description we find for the 1ES 1210-646 emission is quite atypical for a high-mass X-ray binary, the multiwavelegth information available for this object leads us to confirm this classification. The results presented here allow us instead to definitely rule out the possibility that 1ES 1210-646 is a (magnetic) cataclysmic variable as proposed previously and, in a broader sense, a white dwarf nature for the accretor is disfavoured. X-ray spectroscopic data actually suggest a neutron star with a low magnetic field as the accreting object in this system.

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arXiv2009

Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model

The aim of this work is to investigate in a physical and quantitative way the spectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs), with special regard to the transient hard X-ray tails. We analyzed INTEGRAL data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source, X-ray spectra from different states were fitted with the recently proposed model compTB. The spectra have been fit with a two-compTB model. In all cases the first compTB describes the dominant part of the spectrum that we interpret as thermal Comptonization of soft seed photons (< 1 keV), likely from the accretion disk, by a 3-5 keV corona. In all cases, this component does not evolve much in terms of Comptonization efficiency, with the system converging to thermal equilibrium for increasing accretion rate. The second compTB varies more dramatically spanning from bulk plus thermal Comptonization of blackbody seed photons to the blackbody emission alone. These seed photons (R < 12 km, kT_s > 1 keV), likely from the neutron star and the innermost part of the system, the Transition Layer, are Comptonized by matter in a converging flow. The presence and nature of this second compTB component (be it a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behaviour of the hard tail: high values of accretion rates correspond to an efficient Bulk Comptonization process (bulk parameter delta > 0) while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (delta=0).

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arXiv2009

Swift/XRT observations of unidentified INTEGRAL/IBIS sources

The 4th IBIS/ISGRI survey lists 723 hard X-ray sources many still unidentified. We cross-correlated the list of the sources included in the 4th IBIS catalogue with the Swift/XRT data archive, finding a sample of 20 objects for which XRT data could help in the search for the X-ray and hence optical counterpart and/or in the study of the source spectral and variability properties below 10 keV. Four objects (IGR J00465-4005, LEDA 96373, IGR J1248.2-5828 and IGR J13107-5626) are confirmed or likely absorbed active galaxies, while two (IGR J14080-3023 and 1RXS J213944.3+595016) are unabsorbed AGN. We find three peculiar extragalactic objects, NGC 4728 being a Narrow Line Seyfert galaxy, MCG+04-26-006 a type 2 LINER and PKS 1143-693 probably a QSO; furthermore, our results indicate that IGR J08262+4051 and IGR J22234-4116 are candidate AGN, which require further optical spectroscopic follow-up observations to be fully classified. In the case of 1RXS J080114.6-462324 we are confident that the source is a Galactic object. For IGR J10447-6027, IGR J12123-5802 and IGR J20569+4940 we pinpoint one X-ray counterpart, although its nature could not be assessed despite spectral and sometimes variability information being obtained. Clearly, we need to perform optical follow-up observations in order to firmly assess their nature. There are five objects for which we find no obvious X-ray counterpart (IGR J07506-1547 and IGR J17008-6425) or even no detection (IGR J17331-2406, IGR J18134-1636 and IGR J18175-1530); apart from IGR J18134-1636, all these sources are found to be variable in the IBIS energy band, therefore it is difficult to catch them even in X-rays.

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arXiv2009

XMM-Newton and INTEGRAL study of the SFXT IGR J18483-0311 in quiescence: hint of a cyclotron emission feature?

We report the results from archival XMM-Newton and INTEGRAL observations of the Supergiant Fast X-ray Transient (SFXT) IGR J18483-0311 in quiescence. The 18-60 keV hard X-ray behaviour of the source is presented here for the first time, it is characterized by a spectral shape ($Γ$ about 2.5) similar to that during outburst activity and the lowest measured luminosity level is about 10^34 erg s^-1. The 0.5-10 keV luminosity state, measured by XMM-Newton during the apastron passage, is about one order of magnitude lower and it is reasonably fitted by an absorbed black body model yielding parameters consistent with previous measurements. In addition, we find evidence (about 3.5 sigma significance) of an emission-like feature at about 3.3 keV in the quiescent 0.5-10 keV source spectrum. The absence of any known or found systematic effects, which could artificially introduce the observed feature, give us confidence about its non-instrumental nature. We show that its physical explanation in terms of atomic emission line appears unlikely and conversely we attempt to ascribe it to an electron cyclotron emission line which would imply a neutron star magnetic field of the order of about 3x10^11 G. Importantly, such direct estimation is in very good agreement with that independently inferred by us in the framework of accretion from a spherically symmetric stellar wind. If firmly confirmed by future longer X-ray observations, this would be the first detection ever of a cyclotron feature in the X-ray spectrum of a SFXT, with important implications on theoretical models.

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arXiv2009

The INTEGRAL view of Gamma-Ray Bursts

After more than six and half years in orbit, the ESA space observatory INTEGRAL has provided new, exciting results in the soft gamma-ray energy range (from a few keV to a few MeV). With the discovery of about 700 hard X-Ray sources, it has changed our previous view of a sky composed of peculiar and ``monster'' sources. The new high energy sky is in fact full of a large variety of normal, very energetic emitters, characterized by new accretion and acceleration processes (see also IBIS cat4, Bird et al. 2009). If compared to previous IBIS/ISGRI surveys it is clear that there is a continual increase in the rate of discoveries of HMXB and AGN, including a variety of distant QSOs. This is basically due to increased exposure away from the Galactic Plane, while the percentage of sources without an identification has remained constant. At the same time, about one GRB/month is detected and imaged by the two main gamma-ray instruments on board: IBIS and SPI. INTEGRAL, after six and half years of observations, has completed the Core Programme phase and is now fully open to the scientific community for Open Time and Key Programme observations, with AO7 recently announced by ESA. In this paper we review the major achievements of the INTEGRAL Observatory in the field of Gamma Ray Bursts.

astro-ph.CO↗

arXiv2009

The SXI telescope on board EXIST: scientific performances

The SXI telescope is one of the three instruments on board EXIST, a multiwavelength observatory in charge of performing a global survey of the sky in hard X-rays searching for Supermassive Black Holes. One of the primary objectives of EXIST is also to study with unprecedented sensitivity the most unknown high energy sources in the Universe, like high redshift GRBs, which will be pointed promptly by the Spacecraft by autonomous trigger based on hard X-ray localization on board. The recent addition of a soft X-ray telescope to the EXIST payload complement, with an effective area of ~950 cm2 in the energy band 0.2-3 keV and extended response up to 10 keV will allow to make broadband studies from 0.1 to 600 keV. In particular, investigations of the spectra components and states of AGNs and monitoring of variability of sources, study of the prompt and afterglow emission of GRBs since the early phases, which will help to constrain the emission models and finally, help the identification of sources in the EXIST hard X-ray survey and the characterization of the transient events detected. SXI will also perform surveys: a scanning survey with sky coverage of about 2pi and limiting flux of 5x10^{-14}cgs plus other serendipitous. We give an overview of the SXI scientific performance and also describe the status of its design emphasizing how it has been derived by the scientific requirements.

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arXiv2009

The Fermi/LAT sky as seen by INTEGRAL/IBIS

In this letter we present the result of the cross correlation between the 4th INTEGRAL/IBIS soft gamma-ray catalog, in the range 20-100 keV, and the Fermi LAT bright source list of objects emitting in the 100 MeV - 100 GeV range. The main result is that only a minuscule part of the more than 720 sources detected by INTEGRAL and the population of 205 Fermi LAT sources are detected in both spectral regimes. This is in spite of the mCrab INTEGRAL sensitivity for both galactic and extragalactic sources and the breakthrough, in terms of sensitivity, achieved by Fermi at MeV-GeV energies. The majority of the 14 Fermi LAT sources clearly detected in the 4th INTEGRAL/IBIS catalog are optically identified AGNs (10) complemented by 2 isolated pulsars (Crab and Vela) and 2 High Mass X-Ray Binaries (HMXB, LS I +61 303 and LS 5039). Two more possible associations have been found: one is 0FGL J1045.6-5937, possibly the counterpart at high energy of the massive colliding wind binary system Eta Carinae, discovered to be a soft gamma ray emitter by recent INTEGRAL observations and 0FGL J1746.0-2900 coincident with IGR J17459-2902, but still not identified with any known object at lower energy. For the remaining 189 Fermi LAT sources no INTEGRAL counterpart was found and we report the 2 sigma upper limit in the energy band 20-40 keV.

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arXiv2009

The 4th IBIS/ISGRI soft gamma-ray survey catalog

In this paper we report on the fourth soft gamma-ray source catalog obtained with the IBIS gamma-ray imager on board the INTEGRAL satellite. The scientific dataset is based on more than 70Ms of high quality observations performed during the first five and a half years of Core Program and public observations. Compared to previous IBIS surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 700 high-energy sources detected in the energy range 17--100 keV, including both transients and faint persistent objects which can only be revealed with longer exposure times. A comparison is provided with the latest Swift/BAT survey results.

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arXiv2009

Hard X-ray properties of magnetic cataclysmic variables

Hard X-ray surveys have proven remarkably efficient in detecting intermediate polars and asynchronous polars, two of the rarest type of cataclysmic variable (CV). Here we present a global study of hard X-ray selected intermediate polars and asynchronous polars, focusing particularly on the link between hard X-ray properties and spin/orbital periods. To this end, we first construct a new sample of these objects by cross-correlating candidate sources detected in INTEGRAL/IBIS observations against catalogues of known CVs. We find 23 cataclysmic variable matches, and also present an additional 9 (of which 3 are definite) likely magnetic cataclysmic variables (mCVs) identified by others through optical follow-ups of IBIS detections. We also include in our analysis hard X-ray observations from Swift/BAT and SUZAKU/HXD in order to make our study more complete. We find that most hard X-ray detected mCVs have P_{spin}/P_{orb}<0.1 above the period gap. In this respect we also point out the very low number of detected systems in any band between P_{spin}/P_{orb}=0.3 and P_{spin}/P_{orb}=1 and the apparent peak of the P_{spin}/P_{orb} distribution at about 0.1. The observational features of the P_{spin} - P_{orb} plane are discussed in the context of mCV evolution scenarios. We also present for the first time evidence for correlations between hard X-ray spectral hardness and P_{spin}, P_{orb} and P_{spin}/P_{orb}. An attempt to explain the observed correlations is made in the context of mCV evolution and accretion footprint geometries on the white dwarf surface.

astro-ph.GA↗

arXiv2009

Accurate classification of 17 AGNs detected with Swift/BAT

Through an optical campaign performed at 5 telescopes located in the northern and the southern hemispheres, plus archival data from two on line sky surveys, we have obtained optical spectroscopy for 17 counterparts of suspected or poorly studied hard X-ray emitting active galactic nuclei (AGNs) detected with Swift/BAT in order to determine or better classify their nature. We find that 7 sources of our sample are Type 1 AGNs, 9 are Type 2 AGNs, and 1 object is an X-ray bright optically normal galaxy; the redshifts of these objects lie in a range between 0.012 and 0.286. For all these sources, X-ray data analysis was also performed to estimate their absorption column and to search for possible Compton thick candidates. Among our type 2 objects, we did not find any clear Compton thick AGN, but at least 6 out of 9 of them are highly absorbed (N_H > 10^23 cm^-2), while one does not require intrinsic absorption; i.e., it appears to be a naked Seyfert 2 galaxy.

astro-ph.CO↗

arXiv2009

Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period

IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray Transients. This system is one of the two members of this class, together with IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are known. We report on the first complete monitoring of the X-ray activity along an entire orbital period of a Supergiant Fast X-ray Transient. These Swift observations, lasting 28d, cover more than one entire orbital phase consecutively. They are a unique data-set, which allows us to constrain the different mechanisms proposed to explain the nature of this new class of X-ray transients. We applied the new clumpy wind model for blue supergiants developed by Ducci et al. (2009), to the observed X-ray light curve. Assuming an eccentricity of e=0.4, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.

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