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A. Malizia

Publications and source records attributed to A. Malizia.

At least 91 records · Page 5Linked to original sources
arXiv2008

High-redshift blazar identification for Swift J1656.3-3302

We report on the high-redshift blazar identification of a new gamma-ray source, Swift J1656.3-3302, detected with the BAT imager onboard the Swift satellite and the IBIS instrument on the INTEGRAL satellite. Follow-up optical spectroscopy has allowed us to identify the counterpart as an R-band 19 mag source that shows broad Lyman-alpha, Si IV, He II, C IV, and C III] emission lines at redshift z = 2.40+-0.01. Spectral evolution is observed in X-rays when the INTEGRAL/IBIS data are compared to the Swift/BAT results, with the spectrum steepening when the source gets fainter. The 0.7-200 keV X-ray continuum, observed with Swift/XRT and INTEGRAL/IBIS, shows the power law shape typical of radio loud (broad emission line) active galactic nuclei (with a photon index around 1.6) and a hint of spectral curvature below 2 keV, possibly due to intrinsic absorption (N_H about 7e22 cm-2) local to the source. Alternatively, a slope change (of about 1 in terms of photon index) around 2.7 keV can describe the X-ray spectrum equally well. At this redshift, the observed 20-100 keV luminosity of the source is about 1e48 erg s-1 (assuming isotropic emission), making Swift J1656.3-3302 one of the most X-ray luminous blazars. This source is yet another example of a distant gamma-ray loud quasar discovered above 20 keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy.

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arXiv2007

INTEGRAL detection of the pulsar wind nebula in PSR J1846-0258

We communicate the detection of soft (20--200 keV) gamma-rays from the pulsar and pulsar wind nebula of PSR J1846-0258 and aim to identify the component of the system which is responsible for the gamma-ray emission. To pinpoint the source of gamma-ray emission we combine spectral information from the INTEGRAL gamma-ray mission with archival data from the Chandra X-ray Observatory. Our analysis shows that the soft gamma-rays detected from PSR J1846-0258 include emission from both the pulsar and the pulsar wind nebula, but the measured spectral shape is dominated by the pulsar wind nebula. We further discuss PSR J1846-0258 in the context of rotation-powered pulsars with high magnetic field strengths and review the anomalously high fraction of spin-down luminosity converted into X- and gamma-ray emission in light of a possible overestimate of the distance to this pulsar.

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arXiv2007

IGR J22517+2218=MG3 J225155+2217: a new gamma-ray lighthouse in the distant Universe

We report on the identification of a new soft gamma ray source, namely IGR J22517+2218, detected with IBIS/INTEGRAL. The source, which has an observed 20-100 keV flux of ~4 x10^-11 erg cm-2 s-1, is spatially coincident with MG3 J225155+2217, a quasar at z=3.668. The Swift/XRT 0.5-10 keV continuum is flat (Gamma=1.5) with evidence for a spectral curvature below 1-2 keV either due to intrinsic absorption (NH=3 +/- 2 x 10^22 cm-2) or to a change in slope (Delta Gamma= 0.5). X-ray observations indicate flux variability over a 6 days period which is further supported by a flux mismatch between Swift and INTEGRAL spectra. IGR J22517+2218 is radio loud and has a flat radio spectrum; optically it is a broad line emitting quasar with the atypical property of hosting a narrow line absorption system. The Source Spectral Energy Distribution is unusual compared to blazars of similar type: either it has the synchrotron peak in the X/gamma-ray band (i.e. much higher than generally observed) or the Compton peak in the MeV range (i.e. lower than typically measured). IGR J22517+2218=MG3 J225155+2217 is the second most distant blazar detected above 20 keV and a gamma-ray lighthouse shining from the edge of our Universe.

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arXiv2007

Broadband X-ray spectrum of the newly discovered broad line radio galaxy IGR J21247+5058

In this paper we present radio and high energy observations of the INTEGRAL source IGR J21247+5058, a broad line emitting galaxy obscured by the Galactic plane. Archival VLA radio data indicate that IGR J21247+5058 can be classified as an FRII Broad Line Radio Galaxy. The spectrum between 610 MHz and 15 GHz is typical of synchrotron self-absorbed radiation with a peak at 8 GHz and a low energy turnover; the core fraction is 0.1 suggestive of a moderate Doppler boosting of the base of the jet. The high energy broad-band spectrum was obtained by combining XMM-Newton and Swift/XRT observation with INTEGRAL/IBIS data. The 0.4-100 keV spectrum is well described by a power law, with slope $Γ$=1.5, characterised by complex absorption due to two layers of material partially covering the source and a high energy cut-off around 70-80 keV. Features such as a narrow iron line and a Compton reflection component, if present, are weak, suggesting that reprocessing of the power law photons in the accretion disk plays a negligible role in the source.

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arXiv2007

The AGN nature of 11 out of 12 Swift/RXTE unidentified sources through optical and X-ray spectroscopy

The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude survey in the 14-195 keV band at a flux limit of ~10^{-11} erg cm^{-2} s^{-1}, leading to the discovery of new high energy sources, most of which have not so far been properly classified. A similar work has also been performed with the RXTE slew survey leading to the discovery of 68 sources detected above 8 keV, many of which are still unclassified. Follow-up observations with the Swift X-ray Telescope (XRT) provide, for many of these objects, source localization with a positional accuracy of few arcsec, thus allowing the search for optical counterparts to be more efficient and reliable. We present the results of optical/X-ray follow-up studies of 11 Swift BAT detections and one AGN detected in the RXTE Slew Survey, aimed at identifying their counterparts and at assessing their nature. These data allowed, for the first time, the optical classification of 8 objects and a distance determination for 3 of them. For another object, a more refined optical classification than that available in the literature is also provided. For the remaining sources, optical spectroscopy provides a characterization of the source near in time to the X-ray measurement. The sample consists of 6 Seyfert 2 galaxies, 5 Seyferts of intermediate type 1.2-1.8, and one object of Galactic nature - an Intermediate Polar (i.e., magnetic) Cataclysmic Variable. Out of the 11 AGNs, 8 (~70%) including 2 Seyferts of type 1.2 and 1.5, are absorbed with NH > 10^{22} cm^{-2}. Up to 3 objects could be Compton thick (i.e. NH > 1.5 x 10^{24} cm^{-2}), but only in one case (Swift J0609.1-8636) does all the observational evidence strongly suggests this possibility.

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arXiv2007

IGR J16194-2810: a new symbiotic X-ray binary

We here report on the multiwavelength study which led us to the identification of X-ray source IGR J16194-2810 as a new Symbiotic X-ray Binary (SyXB), that is, a rare type of Low Mass X-ray Binary (LMXB) composed of a M-type giant and a compact object. Using the accurate X-ray position allowed by Swift/XRT data, we pinpointed the optical counterpart, a M2 III star. Besides, the combined use of the spectral information afforded by XRT and INTEGRAL/IBIS shows that the 0.5-200 keV spectrum of this source can be described with an absorbed Comptonization model, usually found in LMXBs and, in particular, in SyXBs. No long-term (days to months) periodicities are detected in the IBIS data. The time coverage afforded by XRT reveals shot-noise variability typical of accreting Galactic X-ray sources, but is not good enough to explore the presence of X-ray short-term (seconds to hours) oscillations in detail. By using the above information, we infer important parameters for this source such as its distance (about 3.7 kpc) and X-ray luminosity (about 1.4e35 erg/s in the 0.5-200 keV band), and we give a description for this system (typical of SyXBs) in which a compact object (possibly a neutron star) accretes from the wind of its M-type giant companion. We also draw some comparisons between IGR J16194-2810 and other sources belonging to this subclass, finding that this object resembles SyXBs 4U 1700+24 and 4U 1954+31.

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arXiv2007

Swift-XRT observation of 34 new INTEGRAL/IBIS AGNs: discovery of Compton thick and other peculiar sources

For a significant number of the sources detected at high energies (>10 keV) by the INTEGRAL/IBIS and Swift/BAT instruments there is either a lack information about them in the 2-10 keV range or they are totally unidentified. Herein, we report on a sample of 34 IBIS AGN or AGN candidate objects for which there is X-ray data in the Swift/XRT archive. Thanks to these X-ray follow up observations, the identification of the gamma ray emitters has been possible and the spectral shape in terms of photon index and absorption has been evaluated for the first time for the majority of our sample sources. The sample, enlarged to include 4 more AGN already discussed in the literature, has been used to provide photon index and column density distribution. We obtain a mean value of 1.88 with a dispersion of 0.12, i.e. typical of an AGN sample. Sixteen objects (47%) have column densities in excess of 10^{22} cm^{-2} and, as expected, a large fraction of the absorbed sources are within the Sey 2 sample. We have provided a new diagnostic tool (NH versus F(2-10)keV/F(20-100)keV softness ratio) to isolate peculiar objects; we find at least one absorbed Sey 1 galaxy, 3 Compton thick AGN candidates; and one secure example of a "true" type 2 AGN. Within the sample of 10 still unidentified objects, 3 are almost certainly AGN of type 2; 3 to 4 have spectral slopes typical of AGN; and two are located high on the galactic plane and are strong enough radio emitters so that can be considered good AGN candidates.

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arXiv2007

INTEGRAL high energy detection of the transient IGR J11321-5311

Context: The transient hard X-ray source IGR J11321-5311 was discovered by INTEGRAL on June 2005, during observations of the Crux spiral arm. To date, this is the only detection of the source to be reported by any X/gamma-ray mission. Aims: To characterize the behaviour and hence the nature of the source through temporal and spectral IBIS analysis. Methods: Detailed spectral and temporal analysis has been performed using standard INTEGRAL software OSA v.5.1. Results: To date, IGR J11321-5311 has been detected only once. It was active for about 3.5 hours, a short and bright flare lasting about 1.5 hours is evident in the IBIS light curve. It reached a peak flux of about 80 mCrab or 2.2x10E-9 erg cmE-2 sE-1 (20--300 keV),corresponding to a peak luminosity of 1.1x10E37 erg sE-1 (assuming a distance of 6.5 kpc). During the outburst, the source was detected with a significance of 18 sigma (20--300 keV) and 8 sigma (100--300 keV). The spectrum of the total outburst activity (17--300 keV) is best fitted by the sum of a power law (Gamma=0.55+/-0.18) plus a black body (kT=1.0{+0.2}_{-0.3} keV), with no evidence for a break up to 300 keV. A spectral analysis at Science Window level revealed an evident hardening of the spectrum through the outburst. The IBIS data were searched for pulsations with no positive result. Conclusions: The X-ray spectral shape and the flaring behaviour favour the hypothesis that IGR J11321-5311 is an Anomalous X-ray Pulsar, though a different nature can not be firmly rejected at the present stage.

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arXiv2007

IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL

IGR J18483-0311 is a poorly known transient hard X-ray source discovered by INTEGRAL during observations of the Galactic Center region performed between 23--28 April 2003. Aims: To detect new outbursts from IGR J18483-0311 using INTEGRAL and archival Swift XRT observations and finally to characterize the nature of this source using the optical/near-infrared (NIR) information available through catalogue searches. Results: We report on 5 newly discovered outbursts from IGR J18483-0311 detected by INTEGRAL.For two of them it was possible to constrain a duration of the order of a few days. The strongest outburst reached a peak flux of 120 mCrab (20--100 keV): its broad band JEM--X/ISGRI spectrum (3--50 keV) is best fitted by an absorbed cutoff power law with photon index=1.4+/-0.3, cutoff energy of ~22 keV and Nh ~9x10^22 cm^-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of 18.52 days and 21.0526 seconds which are likely the orbital period of the system and the spin period of the X-ray pulsar respectively. Swift XRT observations of IGR J18483$-$0311 provided a very accurate source position which strongly indicates a highly reddened star in the USNO--B1.0 and 2MASS catalogues as its possible optical/NIR counterpart. Conclusions: The X-ray spectral shape, the periods of 18.52 days and 21.0526 seconds, the high intrinsic absorption, the location in the direction of the Scutum spiral arm and the highly reddened optical object as possible counterpart, all favour the hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact companion. The system is most likely a Be X-ray binary, but a Supergiant Fast X-ray Transient nature can not be entirely excluded.

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arXiv2007

The 3rd IBIS/ISGRI soft gamma-ray survey catalog

In this paper we report on the third soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific dataset is based on more than 40 Ms of high quality observations performed during the first three and a half years of Core Program and public IBIS/ISGRI observations. Compared to previous IBIS/ISGRI surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 400 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.

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arXiv2006

Soft X and Gamma ray emission from TeV sources observed with Swift and INTEGRAL

The soft X-ray and soft gamma observations of the new discovered TeV sources by HESS and MAGIC are crucial to discriminate between various emitting scenarios and to fully understand their nature. The INTEGRAL Observatory has regularly observed the entire galactic plane during the first 1000 day in orbit providing a survey in the 20-100 keV range resulted in a soft gamma-ray sky populated with more than 200 sources. In the case of HESS J1813-178 INTEGRAL found the hard X-ray counterpart IGR J18135-1751 and Swift/XRT Telescope performed follow-up observations on this source. Here we present the soft/hard X-ray spectral study. We reported on the INTEGRAL observation of LS 5039, the first microquasar that have been observed by HESS up to now.

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arXiv2006

INTEGRAL and Swift observations of the supergiant fast X-ray transient AXJ1845.0-0433=IGRJ18450-0435

Context: AXJ1845.0-0433 was discovered by ASCA in 1993 during fast outburst activity characterized by several flares on short timescales. Up to now, the source was not detected again by any X-ray mission. Its optical counterpart is suggested to be an O9.5I supergiant star, which is the only remarkable object found inside the ASCA error box. Aims: To detect and characterize new fast outbursts of AXJ1845.0-0433 and confirm its supergiant HMXB nature, using INTEGRAL and archival Swift XRT observations. Methods: We performed an analysis of INTEGRAL IBIS and JEM-X data with OSA 5.1 as well as an analysis of archive Swift XRT data. Results: We report on fast flaring activity from the source on timescales of a few tens of minutes, the first to be reported since its discovery in 1993. Two outbursts have been detected by INTEGRAL (Apr 2005 and Apr 2006) with similar peak fluxes and peak luminosities of 80 mCrab and 9.3X10^35 erg s^-1 (20--40 keV), respectively. Two other outbursts were detected by Swift XRT on Nov 2005 and Mar 2006. The refined Swift XRT position of AXJ1845.0-0433 confirms its association with the supergiant star previously proposed as optical counterpart. Conclusions: Our INTEGRAL and Swift results fully confirm the supergiant HMXB nature of AXJ1845.0-0433 which can therefore be classified as a supergiant fast X-ray transient. Moreover they provide for the first time evidence of its recurrent fast transient behaviour.

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arXiv2006

INTEGRAL/IBIS 20-100 keV Extragalactic survey: an update

Analysis of INTEGRAL Core Program and public Open Time observations has recently provided a sample of 60 extragalactic sources selected in the 20-100 keV band above a flux of 1.5 10^-11 erg/cm2/s. As this band probes heavily obscured regions/objects, i.e. those that could be missed in optical, UV, and even X-ray surveys, our sample offers the opportunity to study the extragalactic sky from a different point of view with respect to surveys at lower energies. We present an update of our analysis, including the first sample of AGNs detected above 100 keV. We also discuss the results of follow up observations performed at optical and X-ray frequencies with the aim of classifying our objects and studying the effects of intrinsic absorption in gamma-ray selected AGNs. The average redshift of our sample is 0.134 while the mean 20-100 keV luminosity in Log is 43.84; if blazars are excluded these numbers become 0.022 and 43.48 respectively. Defining an absorbed object as one with NH above 10^22 atoms/cm2, we find that absorption is present in 60% of the objects with at most 14% of the total sample due to Compton thick active galaxies. Almost all Seyfert 2s in our sample are absorbed as are 24% of Seyfert 1s. We also present broad-band spectral information on a sub-sample of the brightest objects: our observations indicate a mean photon index of 1.8 spanning from 30-50 keV to greater than 200 keV. Finally, we discuss the LogN/LogS distribution in the 20-100 and 100-150 keV bands derived from our sample. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky, to discover new AGNs and to find absorbed objects.

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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

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

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

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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