arXiv subjects

A. Bazzano

Publications and source records attributed to A. Bazzano.

At least 163 records · Page 9Linked to original sources

arXiv2006

SWIFT/XRT follow-up observations of INTEGRAL AGNs

In five years of operation, data from INTEGRAL has been used to discover a large number of gamma-ray sources, a substantial fraction of which have turned out to be active galactic nuclei (AGN). Recently Bassani et al. (2006) have presented a sample of around 60 AGNs of which some still lack optical identification or information in the X-ray band. In this work we present X-ray data for 8 objects acquired with the XRT telescope on-board Swift satellite. The XRT positioning has allowed us to identify the optical counterparts and to classify their types through follow-up measurements. Analysis of these data has also provided information on their spectra below 10 keV.

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arXiv2006

Spectral analysis of the Low-Mass X-ray Binary 4U~1705--44

\integral and \sax observations of the neutron-star LMXB 4U~1705--44 have been analysed to deeply investigate the spectral state transitions nature. Its energy spectrum can be described as the sum of one or two blackbody, a 6.4-keV Fe line and a component due to thermal Comptonization. For the first time in this source, we find a strong signature of Compton reflection, presumably due to illumination of the optically-thick accretion disk by the Comptonized spectrum. Detection of two blackbody component in the soft states could originate in the disk and the neutron-star surface, and the Comptonized component arises from a hot inner flow with the seed photons coming from the disk and/or the neutron-star surface. The spectral transitions are shown to be associated with variations in the accretion rate, which changes in turn the temperature of the Comptonizing electrons and the strength of Compton reflection.

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arXiv2006

Hard tail detection in the LMXB 4U1636-53 from INTEGRAL

Recent INTEGRAL observations and archival BeppoSAX data have been analyzed to deeply investigate the hard X-ray behavior of the neutron star, atoll type, low mass X-ray Binary 4U1636-53. Our investigation in three different periods outline three corresponding different sates. Infact, the source was detected in both the canonical high and low state and moreover in one occasion \integral spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. This spectrum is fitted as the sum of a Comptonized component similar to soft state and a power-law component ($Γ=2.76$), indicating the presence of a non thermal electron distribution of velocities.

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arXiv2006

Average hard X-ray emission from NS LMXBs: Observational evidence of different spectral states in NS LMXBs

We studied and compared the long-term average hard X-ray (>20keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright ("GX") bulge sources while two are weaker ones in the 2-10keV range (H1750-440 and H1608-55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H1750-440 up to 150keV. The Z and bright "GX" Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5-3keV. Six sources show a hard tail in their average spectrum: CygX-2 (Z), GX340+0 (Z), GX17+2 (Z), GX5-1 (Z), ScoX-1 (Z) and GX13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX13+1. Using radio data from the literature we find, in all Z sources and bright "GX" Atolls, a systematic positive correlation between the X-ray hard tail (40-100keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.

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arXiv2006

The ultra-compact binary 4U1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster

The X-ray burster 4U1850-087, located in the Galactic globular cluster NGC6712, is an ultracompact binary (orbital period~21 min),likely harbouring a degenerate companion.The source has been observed at soft gamma-rays several times with the INTEGRAL satellite,during the monitoring of the Galactic plane, with an unprecedented exposure time.We analysed all available INTEGRAL observations, with the main aim of studying the long-term behaviour of this Galactic bulge X-ray burster.The spectral results are based on the systematic analysis of all INTEGRAL observations covering the source position performed between March 2003 and November 2005.The source X-ray emission is hard and is observed, for the first time, up to 100keV.A broad-band spectrum obtained combining the INTEGRAL spectrum together with a quasi-simultaneous XMM-Newton observation performed in September 2003 is well modeled with a disk-blackbody emission (with kTin=0.8keV) together with a power-law (photon index=2.1).The 2-100keV luminosity is 1.5E36 erg/s (assuming a distance of 6.8kpc).

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arXiv2006

On the multiwavelength spectrum of the microquasar 1E 1740.7-2942

Context. The microquasar 1E 1740.7-2942 is a source located in the direction of the Galactic Center. It has been detected at X-rays, soft gamma-rays, and in the radio band, showing an extended radio component in the form of a double-sided jet. Although no optical counterpart has been found so far for 1E 1740.7-2942, its X-ray activity strongly points to a galactic nature. Aims. We aim to improve our understanding of the hard X-ray and gamma-ray production in the system, exploring whether the jet can emit significantly at high energies under the light of the present knowledge. Methods. We have modeled the source emission, from radio to gamma-rays, with a cold-matter dominated jet model. INTEGRAL data combined with radio and RXTE data, as well as EGRET and HESS upper-limits, are used to compare the computed and the observed spectra. Results. From our modeling, we find out that jet emission cannot explain the high fluxes observed at hard X-rays without violating at the same time the constraints from the radio data, favoring the corona origin of the hard X-rays. Also, 1E 1740.7-2942 might be detected by GLAST or AGILE at GeV energies, and by HESS and HESS-II beyond 100 GeV, with the spectral shape likely affected by photon-photon absorption in the disk and corona photon fields.

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. V. Identification and properties of 21 southern hard X-ray sources

Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in the study of the hard X-ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of the X-ray sources detected with INTEGRAL.

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arXiv2006

A Hard X-ray View on Scorpius X-1 with INTEGRAL: non-Thermal Emission ?

We present here simultaneous INTEGRAL/RXTE observations of Sco X-1, and in particular a study of the hard X-ray emission of the source and its correlation with the position in the Z-track of the X-ray color-color diagram. We find that the hard X-ray (above about 30 keV) emission of Sco X-1 is dominated by a power-law component with a photon index of ~3. The flux in the power-law component slightly decreases when the source moves in the color-color diagram in the sense of increasing inferred mass accretion rate from the horizontal branch to the normal branch/flaring branch vertex. It becomes not significantly detectable in the flaring branch, where its flux has decreased by about an order of magnitude. These results present close analogies to the behavior of GX 17+2, one of so-called Sco-like Z sources. Finally, the hard power law in the spectrum of Sco X-1 does not show any evidence of a high energy cutoff up to 100 - 200 keV, strongly suggesting a non-thermal origin of this component.

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arXiv2006

Integral/Ibis Census of the Sky Beyond 100 kev

We report on the first census of INTEGRAL/IBIS detections ($\gtrsim 4σ$ significance) above 100 keV based on the Core Program and public Open time observations up to April 2005. There are 49 sources detected in the 100-150 keV band of which 14 are also seen in the 150-300 keV range. The low energy sample is dominated by X-ray binary systems of both low and high mass, but also includes 10 active galaxies. Of the binary systems that are detected above 150 keV, more than 50% are associated with black hole candidates, often reported as microquasars. The present survey results are then used to construct LogN-LogS curves for galactic and extragalactic objects in the 100-150 keV band: above a 1 mCrab sensitivity limit we expect that around 200 galactic sources and almost 350 active galaxies populate the sky above 100 keV. While the contribution of individual point sources to the total Galactic emission has been estimated to be around 70-80% between 100-300 keV, we find that active galaxies detected above 1 mCrab account for only about 3% of the cosmic hard X-ray background in the 100-150 keV band.

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arXiv2006

First broad band study of the mysterious source 1E 1743.1-2843

In the last years, the persistent source 1E 1743.1-2843 has been observed in the X-rays, but never above 20 keV. In previous works, it was stressed that a possible high energy emission could give further indications on the accreting object nature which remains still unknown. We present here more than two years of 1E 1743.1-2843 monitoring with INTEGRAL/IBIS as well as public XMM-Newton and Chandra X-ray observations. The temporal study in the 20-40 keV band shows a rather constant flux on few months time scale. Based on this result we have performed the broad-band spectral analysis using EPIC/IBIS non simultaneous data and ACIS-I/IBIS data collected during 2004. In ~2 Ms, we report a detection of 6 sigma in the energy range 35-70 keV. The first broad-band study (2-70 keV) shows a steep slope (~3) and a black body temperature of 1.7 keV. Combining spectral parameters and discussion about the luminosity evaluations for different possible distances, our conclusions are in favour of a LMXB system with a neutron star at distance higher than the Galactic Centre, even though a firm conclusion can not be stated.

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arXiv2006

Swift/XRT follow-up observations of TeV sources of the HESS Inner Galaxy survey

In order to provide a firm identification of the newly discovered Galactic TeV sources, a search for counterparts in a broad band from soft X-ray to soft gamma-rays is crucial as data in these wavebands allow us to distinguish between different types of suggested models (for example leptonic versus hadronic) and, in turn, to disentangle their nature. In this paper, we report the results of a set of follow-up observations performed by the Swift/X-Ray Telescope (XRT) on seven sources recently discovered by HESS, in the range from few hundred GeV to about 10 TeV, during the inner Galaxy survey (Aharonian et al. 2006). In all, but one case, we detect X-ray sources inside or close-by the extended TeV emitting region. All these putative X-ray counterparts have accurate arc-second location and are consistent with being point sources. The main result of our search is the discovery that three of them are located at the center of the diffuse radio emission of the supernova remnants, which have been spatially associated to these TeV objects. HESS J1640-465, HESS J1834-087 and HESS J1813-178 show this evidence, suggestive of a possible Pulsar Wind Nebula association.

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arXiv2006

The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL

We present for the first time a study of the 3-200 keV broad band spectra of the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity diagram. The analysis was done using the INTEGRAL public and Galactic Center deep exposure data ranging from February 2003 to October 2004. The spectra are well described by a thermal Comptonization model with an electron temperature from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab geometry. The source undergoes a transition from an intermediate/hard to a soft state where the source luminosity increases from 2 to 12% of Eddington. We have also detected 36 type I X-ray bursts two of which show photospheric radius expansion. The energetic bursts with photospheric radius expansion occurred at an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s, while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For 4U1728-34 the bursts' total fluence, and the bursts' peak flux are anti-correlated with the mass accretion rate. The type I X-ray bursts involve pure helium burning either during the hard state, or during the soft state of the source.

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. IV. A study of six new hard X-ray sources

We present further results from our ongoing optical spectrophotometric campaign at the Astronomical Observatory of Bologna in Loiano (Italy) on unidentified hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of the INTEGRAL sources IGR J00234+6141, IGR J01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR J20286+2544. We find that the first two are Galactic objects, namely a Cataclysmic Variable at a distance of about 300 pc and a Be/X transient High-Mass X-ray Binary (HMXB) located at about 6.4 kpc, respectively, whereas the last one is identified with MCG +04-48-002, a Starburst/HII galaxy at redshift z = 0.013 hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 as the (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we classify as an X-ray Bright, Optically Normal Galaxy; this is the first example of this type of object to be detected by INTEGRAL, and one of the closest of this class. We moreover confirm the possible Be/X nature of IGR J06074+2205, and we estimate it to be at a distance of about 1 kpc. We also reexamine the spectrum of the z = 0.087 elliptical radio galaxy PKS 0352-686, the possible counterpart of the INTEGRAL source IGR J03532-6829, and we find that it is a BL Lac. Physical parameters for these sources are also evaluated by discussing our findings in the context of the available multiwavelength information. These identifications further stress the importance of INTEGRAL in the study of the hard X-ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables.

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arXiv2006

Unveiling supergiant fast X-ray transient sources with INTEGRAL

Supergiant high mass X-ray binaries (SGXBs) are believed to be rare objects, as stars in the supergiant phase have a very short lifetime and to date only about a dozen of them have been discovered. They are known to be persistent and bright X-ray sources. INTEGRAL is changing this classical picture as its observations are revealing the presence of a new subclass of SGXBs which have been labelled as supergiant fast X-ray transients (SFXTs) as they are strongly characterized by fast X-ray outbursts lasting less than a day, typically a few hours. We report on IBIS detections of newly discovered fast X-ray outbursts from 10 sources, four of which have been recently optically identified as supergiant high mass X-ray binaries. In particular for one of them, IGR J11215-5952, we observe fast X-ray transient behaviour for the first time. The remaining six sources (IGR J16479-4514, IGR J16418-4532, XTE J1743-363, IGR J16195-4945=AX J161929-4945, AX J1749.1-2733, IGR J17407-2808) are still unclassified, however they can be considered as candidate SFXTs because of their similarity to the known SFXTs.

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. III. Observations of seven southern sources

Context. The detection, thanks to the INTEGRAL satellite, of a number of new hard X-ray sources calls for the identification of their nature by means of observations at longer wavelengths. Aims. In our continuing campaign to identify longer-wavelength counterparts of these newly-discovered hard X-ray sources detected by INTEGRAL, we observed the putative optical counterparts of seven southern sources at the South African Astronomical Observatory and at the European Southern Observatory. Methods. Optical spectroscopy was performed on all candidates of these X-ray sources. For two of these objects, optical photometry was also acquired. Results. These observations firmly established the nature of four of these sources: we found that IGR J10404-4625 (=LEDA 93974), 4U 1344-60 and IGR J16482-3036 are Active Galactic Nuclei (AGNs) at redshifts z = 0.0237, 0.013 and 0.0313, respectively, and that 2RXP J130159.6-635806 is a Galactic High-Mass X-ray Binary (HMXB). We also give possible optical identifications for three further objects, namely IGR J11215-5952, IGR J11305-6256 and IGR J16207-5129, which are consistent with being Galactic HMXBs. Physical parameters for these objects are also evaluated by collecting and discussing the available multiwavelength information. Conclusions. The detection of four definite or likely HMXBs out of seven objects in our sample further stresses INTEGRAL's crucial contribution in hunting this class of object. Also, the determination of the extragalactic nature of a substantial fraction of the INTEGRAL survey sources underlines the importance of hard X-ray observations for the study of background AGNs located beyond the 'Zone of Avoidance' of the Galactic Plane.

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arXiv2006

Spectral states of the X-ray binary IGR J17091--3624 observed by INTEGRAL and RXTE

IGR J17091-3624 was discovered in 2003 April by INTEGRAL/IBIS during its Galactic Centre Deep Exposure programme. The source was initially detectable only in the 40--100 keV range, but after two days was also detected in the 15-40 keV range. Its flux had by then increased to 40 mCrab and 25 mCrab in the 15-40 keV and 40-100 keV bands respectively. RXTE observed the source simultaneously on 2003 April 20, with an effective exposure of 2 ksec. We report here the spectral and temporal evolution of the source, which shows a transition between the hard and soft states. We analyse in detail the RXTE/INTEGRAL Comptonised spectrum of the hard state as well as the JEM-X detection of a blackbody component during the source softening. Even though the source spectral behaviour and time variability show a similarity with the outburst of the black-hole candidate IGR J17464-3213 (= H1743-322), observed by INTEGRAL in 2003, the nature of its compact object (BH vs. NS) remains controversial.

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arXiv2005

INTEGRAL/IBIS Extragalactic survey: 20-100 keV selected AGN

Analysis of INTEGRAL Core Program and public Open Time observations performed up to April 2005 provides a sample of 62 active galactic nuclei in the 20-100 keV band above a flux limit of ~1.5x10^-11 erg/cm2/s. Most(42) of the sources in the sample are Seyfert galaxies, almost equally divided between type 1 and 2 objects, 6 are blazars and 14 are still unclassified. Excluding the blazars, the average redshift of our sample is 0.021 while the mean luminosity is Log(L) = 43.45. We find that absorption is present in 65% of the objects with 14% of the total sample due to Compton thick active galaxies. In agreement with both Swift/BAT team results and 2-10 keV studies, the fraction of absorbed objects decreases with the 20-100 keV luminosity. All Seyfert 2s in our sample are absorbed as are 33% of Seyfert 1s. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky and to find new and/or absorbed active galaxies.

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arXiv2005

INTEGRAL observation of the X-ray burster KS 1741-293

KS 1741-293 was firstly detected in 1989 with the X-ray wide field camera TTM (3-10 keV) on board of the Rontgen-Kvant-Mir observatory. During these observations this source exhibited two X-ray bursts allowing to identify it as a neutron star in a Low mass X-ray Binary. During the BeppoSAX/WFC monitoring of the Galactic Centre Region, KS 1741-293 was also reported at a flux level of 6 mCrab in the 2-9 keV and 25 mCrab in the 9-25 keV energy range. Thanks to the deep and regular INTEGRAL observation of the Galactic Centre region, KS 1741-293 has been observed by the X-ray monitor JEM-X and the imager IBIS in a wide energy range, giving for the first time relevant information on its high energy behaviour. Furthermore, two X-ray bursts have been detected by JEM-X. We report on IBIS and JEM-X data analysis in terms of flux monitoring, spectral proprieties and bursts detection. The data reduction has been done with the most recent release of the standard analysis software (OSA 5.0).

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