arXiv subjects

A. Bazzano

Publications and source records attributed to A. Bazzano.

At least 73 records · Page 4Linked to original sources

arXiv2013

In-flight calibration of the INTEGRAL/IBIS mask

Since the release of the INTEGRAL Offline Scientific Analysis (OSA) software version 9.0, the ghost busters module has been introduced in the INTEGRAL/IBIS imaging procedure, leading to an improvement of the sensitivity around bright sources up to a factor of 7. This module excludes in the deconvolution process the IBIS/ISGRI detector pixels corresponding to the projection of a bright source through mask elements affected by some defects. These defects are most likely associated with screws and glue fixing the IBIS mask to its support. Following these major improvements introduced in OSA 9, a second order correction is still required to further remove the residual noise, now at a level of 0.2-1% of the brightest source in the field of view. In order to improve our knowledge of the IBIS mask transparency, a calibration campaign has been carried out during 2010-2012. We present here the analysis of these data, together with archival observations of the Crab and Cyg X-1, that allowed us to build a composite image of the mask defects and to investigate the origin of the residual noise in the IBIS/ISGRI images. Thanks to this study, we were able to point out a simple modification of the ISGRI analysis software that allows to significantly improve the quality of the images in which bright sources are detected at the edge of the field of view. Moreover, a refinement of the area excluded by the ghost busters module is considered, and preliminary results show improvements to be further tested. Finally, this study indicates further directions to be investigated for improving the ISGRI sensitivity, such as taking into account the thickness of the screws in the mask model or studying the possible discrepancy between the modeled and actual mask element bridges.

astro-ph.IM↗

arXiv2013

Identification of newly-discovered sources belonging to the 4th IBIS catalog and to the 54 months Palermo Swift/BAT catalog

The most recent all-sky surveys performed with the INTEGRAL and SWIFT satellites allowed the detection of more than 1500 sources in hard X-rays above 20 keV. About one quarter of them has no obvious counterpart at other wavelengths and therefore could not be associated with any known class of high-energy emitting objects. Although cross-correlation with catalogues or surveys at other wavelengths (especially soft X-rays) is of invaluable support in pinpointing the putative optical candidates, only accurate optical spectroscopy can reveal the true nature of these sources. With the aim of identifying them, we started in 2004 an optical spectroscopy program which uses data from several telescopes worldwide and which proved extremely successful, leading to the identification of about 200 INTEGRAL objects and nearly 130 Swift sources. Here we want to present a summary of this identification work and an outlook of our preliminary results on identification of newly-discovered sources belonging to the 4th IBIS catalog and to the 54 months Palermo Swift/BAT catalog.

astro-ph.HE↗

arXiv2013

The INTEGRAL/IBIS Complete Sample of Type 1 AGN

The determination of the broad (0.1-100 keV) spectra of active galaxies is crucial for understanding and discriminating among emission models, for estimating the properties of the Comptonising/reflecting region around the central black hole and for obtaining a firm description of the contribution of AGN to the Cosmic X-ray Background. Although broad-band X-ray measurements of AGN have been made in the past, these did not generally pertain to a complete sample of sources. Since few years, we have started a systematic analysis of the 0.1-100 keV spectra of a complete sample of AGN selected in the hard X-ray band (20-40 keV) using low energy data (not always of good quality) from a set of operating X-ray telescopes. Thanks to data obtained through an XMM-Newton Large Programme, we have now high quality 0.1-10 keV data for all sources in the sample; these combined with high energy observations from INTEGRAL/IBIS and Swift/BAT will allow us to study the spectral properties of this complete sample. Here in particular, we report the progress made on type 1 AGN, focusing in particular on the continuum and its high energy cut-off, the reflection fraction, the absorption properties and the presence of soft excesses and warm absorbers in our sources. More specifically we discuss the broad-band properties of 4 sources IGR J00333+6122, Swift J0917.2-6221, GRS 1734-292 and NGC 6814, which can be considered as the most representative objects of our sample.

astro-ph.HE↗

arXiv2013

Chasing extreme blazars with INTEGRAL

Within the blazar population, hard X-ray selected objects are of particular interest as they tend to lie at each end of the blazar sequence. In particular, flat spectrum radio quasars located at high redshifts display the most powerful jets, the largest black hole masses and the most luminous accretion disks: their spectral energy distribution has a Compton peak in the sub-MeV region which favours their detection by instruments like INTEGRAL/IBIS and Swift/BAT. These sources are even more extreme than blazars selected in other wavebands, like, for example, the gamma-ray range explored by Fermi. Here we report on a sample of 12 high redshift blazars detected so far by INTEGRAL, including 3 newly identified objects. Some properties of the combined IBIS/BAT sample of high redshift blazars (z>2) are also compared to those of a similar similar sample obtained by Fermi.

astro-ph.HE↗

arXiv2013

Review on Active Galactic Nuclei at hard X-ray energies

Hard X-ray surveys are an important tool for the study of active galactic nuclei (AGN): they provide almost an unbiased view of absorption in the extragalactic population, allow the study of spectral features such as reflection and high energy cut-off which would otherwise be unexplored and favour the discovery of some blazars at high redshift. Here, we present the absorption properties of a large sample of INTEGRAL detected AGN, including an update on the fraction of Compton thick objects. For a sub-sample of 87 sources, which represent a complete set of bright AGN, we will discuss the hard X-ray (20-100 keV) spectral properties, also in conjunction with Swift/BAT 58 month data, providing information on BAT/IBIS cross-calibration constant, average spectral shape and spectral complexity. For this complete sample, we will also present broad-band data using soft X-ray observations, in order to explore the complexity of AGN spectra both at low and high energies and to highlight the variety of shapes. Future prospects for AGN studies with INTEGRAL will also be outlined.

astro-ph.HE↗

arXiv2012

The INTEGRAL source IGR J16328-4726: a High Mass X-ray Binary from the Beppo SAX era

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions.

astro-ph.HE↗

arXiv2012

The INTEGRAL/IBIS AGN catalogue I: X-ray absorption properties versus optical classification

In this work we present the most comprehensive INTEGRAL AGN sample which lists 272 objects. Here we mainly use this sample to study the absorption properties of active galaxies, to probe new AGN classes and to test the AGN unification scheme. We find that half (48%) of the sample is absorbed while the fraction of Compton thick AGN is small (~7%). In line with our previous analysis, we have however shown that when the bias towards heavily absorbed objects which are lost if weak and at large distance is removed, as it is possible in the local Universe, the above fractions increase to become 80% and 17%. We also find that absorption is a function of source luminosity, which implies some evolution in the obscuration properties of AGN. Few peculiar classes, so far poorly studied in the hard X-ray band, have been detected and studied for the first time such as 5 XBONG, 5 type 2 QSOs and 11 LINERs. In terms of optical classification, our sample contains 57% of type 1 and 43% of type 2 AGN; this subdivision is similar to that found in X-rays if unabsorbed versus absorbed objects are considered, suggesting that the match between optical and X-ray classification is overall good. Only a small percentage of sources (12%) does not fulfill the expectation of the unified theory as we find 22 type 1 AGN which are absorbed and 10 type 2 AGN which are unabsorbed. Studying in depth these outliers we found that most of the absorbed type 1 AGN have X-ray spectra characterized by either complex or warm/ionized absorption more likely due to ionized gas located in an accretion disk wind or in the biconical structure associated to the central nucleus, therefore unrelated to the toroidal structure. Among 10 type 2 AGN which resulted to be unabsorbed, at most 3-4% is still eligible to be classified as a "true" type 2 AGN.

astro-ph.HE↗

arXiv2012

Accurate classification of 29 objects detected in the 39 months Palermo Swift/BAT hard X-ray catalogue

Through an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, plus archival data from two on-line sky surveys, we have obtained optical spectroscopy for 29 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 39 months Palermo catalogue. All these objects have also observations taken with Swift/XRT or XMM-EPIC which not only allow us to pinpoint their optical counterpart, but also to study their X-ray spectral properties (column density, power law photon index and F2-10 keV flux). We find that 28 sources in our sample are AGN; 7 are classified as type 1 while 21 are of type 2; the remaining object is a galactic cataclysmic variable. Among our type 1 AGN, we find 5 objects of intermediate Seyfert type (1.2-1.9) and one Narrow Line Seyfert 1 galaxy; for 4 out of 7 sources, we have been able to estimate the central black hole mass. Three of the type 2 AGN of our sample display optical features typical of the LINER class and one is a likely Compton thick AGN. All galaxies classified in this work are relatively nearby objects since their redshifts lie in the range 0.008-0.075; the only galactic object found lies at an estimated distance of 90 pc. We have also investigated the optical versus X-ray emission ratio of the galaxies of our sample to test the AGN unified model. For them, we have also compared the X-ray absorption (due to gas) with the optical reddening (due to dust): we find that for most of our sources, specifically those of type 1.9-2.0 the former is higher than the latter confirming early results by Maiolino et al. (2001); this is possibly due to the properties of dust in the circumnuclear obscuring torus of the AGN.

astro-ph.HE↗

arXiv2012

IGR J12319-0749: evidence for another extreme blazar found with INTEGRAL

We report on the identification of a new soft gamma-ray source, IGR J12319-0749, detected with the IBIS imager on board the INTEGRAL satellite. The source, which has an observed 20-100 keV flux of ~8.3 x 10^{-12} erg cm^{-2} s^{-1}, is spatially coincident with an AGN at redshift z=3.12. The broad-band continuum, obtained by combining XRT and IBIS data, is flat (Gamma ~ 1.3) with evidence for a spectral break around 25 keV (100 keV in the source rest frame). X-ray observations indicate flux variability which is further supported by a comparison with a previous ROSAT measurement. IGR J12319-0749 is also a radio emitting object likely characterized by a flat spectrum and high radio loudness; optically it is a broad-line emitting object with a massive black hole (2.8 x 10^{9}$ solar masses) at its center. The source Spectral Energy Distribution is similar to another high redshift blazar, 225155+2217 at z=3.668: both objects are bright, with a large accretion disk luminosity and a Compton peak located in the hard X-ray/soft gamma-ray band. IGR J12319-0749 is likely the second most distant blazar detected so far by INTEGRAL.

astro-ph.HE↗

arXiv2012

X-ray Pulsations from the region of the Supergiant Fast X-ray Transient IGR J17544-2619

Phase-targeted RXTE observations have allowed us to detect a transient 71.49 \pm 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544-2619. The phase-folded light curve shows a possible double-peaked structure with a pulsed flux of ~4.8*10^-12 erg cm^-2 s^-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544-2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of HMXB in Porb - Pspin space.

astro-ph.HE↗

arXiv2012

Unveiling the nature of INTEGRAL objects through optical spectroscopy. IX. 22 more identifications, and a glance into the far hard X-ray Universe

(Abridged) Since its launch in 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. 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 us to pinpoint their exact nature. Continuing our long-standing program that has been running since 2004, and using 6 different telescopes of various sizes, we report the classification through optical spectroscopy of 22 more unidentified or poorly studied high-energy sources detected with the IBIS instrument onboard INTEGRAL. We found that 16 of them are active galactic nuclei (AGNs), while the remaining 6 objects are within our Galaxy. Among the identified extragalactic sources, 14 are Type 1 AGNs; of these, 6 lie at redshift larger than 0.5 and one has z = 3.12, which makes it the second farthest object detected in the INTEGRAL surveys up to now. The remaining AGNs are of type 2, and one of them is a pair of interacting Seyfert 2 galaxies. The Galactic objects are identified as two cataclysmic variables, one high-mass X-ray binary, one symbiotic binary and two chromospherically active stars. We thus still find that AGNs are the most abundant population among hard X-ray objects identified through optical spectroscopy. Moreover, we note that the higher sensitivity of the more recent INTEGRAL surveys is now enabling the detection of high-redshift AGNs, thus allowing the exploration of the most distant hard X-ray emitting sources and possibly of the most extreme blazars.

astro-ph.HE↗

arXiv2011

SFXTs as best candidate counterparts of unidentified transient MeV-GeV sources: the test case of IGR J17354-3255/AGL J1734-3310

In the last few years Fermi and AGILE observations have indicated the existence of a possible population of transient MeV-GeV sources located on the Galactic plane and characterized by fast flares lasting only a very few days. Notably, no blazar-like counterparts are known within their error boxes so they could represent a completely new class of Galactic transient high energy emitters. The task of identifying their counterparts at lower energies remains very challenging. Despite this difficulty, INTEGRAL observations have provided intriguing hints that reliable candidate counterparts for these unidentified MeV-GeV transients could be found among the members of the recently discovered class of Supergiant Fast X-ray Transients (SFXTs). In this context, to date the best test case is represented by the association between the two sources IGR J17354-3255 and AGL J1734-3310. We will discuss their possible physical link and implications stemming from this association.

astro-ph.HE↗

arXiv2011

Accurate classification of 28 objects detected in the 39 months Palermo Swift/BAT hard X-ray catalogue

astro-ph.CO↗

arXiv2011

INTEGRAL view of the Extragalactic Sky

Presented in this work is the most comprehensive INTEGRAL AGN sample: it includes 272 active galaxies for which optical identifications and redshifts are available. 2-10 keV fluxes and column density measurements are also collected for all the sample sources. For 33 new hard X-ray discovered AGN with no previous X-ray coverage, XRT and XMM data analysis is reported for the first time in this work. Examples of future studies which are being developed using this large X-ray selected sample are presented together with some early results.

astro-ph.HE↗

arXiv2011

Broadband X-ray properties of absorbed AGN

In this paper we report on the broadband X-ray properties of a complete sample of 33 absorbed Seyfert galaxies hard X-ray selected with integral. The high quality broadband spectra obtained with both xmm, and integral-IBIS data are well reproduced with an absorbed primary emission with a high energy cutoff and its scattered fraction below 2-3 keV, plus the Compton reflection features. A high energy cut-off is found in 30% of the sample, with an average value below 150 keV. The diagnostic plot NH vs Fobs(2-10 keV)/F(20-100 keV) allowed the isolation of the Compton thick objects, and may represent a useful tool for future hard X-ray observations of newly discovered AGN. We are unable to associate the reflection components with the absorbing gas as a torus, a more complex scenario being necessary. In the Compton thin sources, a fraction (but not all) of the Fe K line needs to be produced in a gas possibly associated with the optical Broad Line Region, responsible also for the absorption. We still need a Compton thick medium (not intercepting the line of sight) likely associated to a torus, which contributes to the Fe line intensity and produces the observed reflection continuum above 10 keV. The so-called Iwasawa-Taniguchi effect can not be confirmed with our data. Finally, the comparison with a sample of unobscured AGN shows that, type 1 and type 2 (once corrected for absorption) Seyfert are characterized by the same nuclear/accretion properties (luminosity, bolometric luminosity, Eddington ratio), supporting the "unified" view.

astro-ph.CO↗

arXiv2011

The nature of 19 X-ray sources detected with INTEGRAL

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. 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, here we report the classification, through optical spectroscopy, of 19 more unidentified or poorly studied high-energy sources detected with the IBIS instrument onboard INTEGRAL.

astro-ph.HE↗

arXiv2011

Broadband study of hard X-ray selected absorbed AGN

[Abridged]We report on the broadband X-ray properties of a complete sample of absorbed Seyfert galaxies hard X-ray selected with INTEGRAL. The sample is composed of 33 sources: 15 are newly discovered above 20 keV while 18 are already known AGN. For 17 sources we have performed a broadband analysis with XMM and INTEGRAL data. We have complemented the analysis of the 16 remaining sources with existing broadband studies. The spectra are well reproduced with an absorbed primary emission with a high energy cutoff and its scattered fraction below 2-3 keV, plus the Compton reflection features. A high energy cut-off is found in 30% of the sample, with an average value below 150 keV. The hard X-ray selection favours the detection of more obscured sources, with the log NH average value of 23.15. The diagnostic plot NH vs F(corr)(2-10keV)/F(20-100keV) allowed the isolation of the Compton thick objects and may represent a useful tool for future hard X-ray observations of newly discovered AGN. We are unable to associate the reflection components (continuum and Fe line) with the absorbing gas as a torus, a more complex scenario being necessary. In the Compton thin sources, a fraction (but not all) of the Fe line needs to be produced in a gas located closer to the BH than the thick torus, and this is possibly associated with the optical BLR, responsible also for the absorption. We still need a Compton thick medium (not intercepting the line of sight) likely associated to a torus, which contributes to the Fe line and produces the observed reflection continuum above 10 keV. The Iwasawa-Taniguchi effect can not be confirmed with our data. Finally, the comparison with a sample of unobscured AGN shows that, type 1 and type 2 (once corrected for absorption) Seyfert are characterized by the same nuclear/accretion properties (luminosity, bolometric luminosity, Eddington ratio), supporting the unified view.

astro-ph.CO↗

arXiv2011

The brightest gamma-ray flaring blazar in the sky: AGILE and multi-wavelength observations of 3C 454.3 during November 2010

Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all frequencies, and during the last four years it has exhibited more than one gamma-ray flare per year, becoming the most active gamma-ray blazar in the sky. We present for the first time the multi-wavelength AGILE, SWIFT, INTEGRAL, and GASP-WEBT data collected in order to explain the extraordinary gamma-ray flare of 3C 454.3 which occurred in November 2010. On 2010 November 20 (MJD 55520), 3C 454.3 reached a peak flux (E>100 MeV) of F_gamma(p) = (6.8+-1.0)E-5 ph/cm2/s on a time scale of about 12 hours, more than a factor of 6 higher than the flux of the brightest steady gamma-ray source, the Vela pulsar, and more than a factor of 3 brighter than its previous super-flare on 2009 December 2-3. The multi-wavelength data make a thorough study of the present event possible: the comparison with the previous outbursts indicates a close similarity to the one that occurred in 2009. By comparing the broadband emission before, during, and after the gamma-ray flare, we find that the radio, optical and X-ray emission varies within a factor 2-3, whereas the gamma-ray flux by a factor of 10. This remarkable behavior is modeled by an external Compton component driven by a substantial local enhancement of soft seed photons.

astro-ph.HE↗