arXiv subjects

R. Landi

Publications and source records attributed to R. Landi.

At least 19 recordsLinked to original sources

arXiv2023

Update of the INTEGRAL/IBIS AGN catalogue: deeper on the Galactic plane and wider beyond

In this work we have updated the list of AGN detected by INTEGRAL taking into account the new objects listed in the last published INTEGRAL/IBIS survey. We have collected 83 new AGN increasing the number of INTEGRAL detected active galaxies (436) by 19%. Half of these new additions are located behind the Galactic plane; for most of them we have full X-ray coverage obtained through archival data from Swift/XRT, XMM-Newton and NuSTAR. This allowed us to associate each high-energy emitter with a single or multiple X-ray counterpart/s and characterise the spectral shape of these new AGN by estimating the photon index, the intrinsic absorption and the 2-10 keV flux. A few cases where two soft X-ray counterparts fall within the INTEGRAL error circle and at least one is classified as an AGN have been found and discussed in detail. Thirty-four sources originally listed as AGN candidates or unidentified objects have been recognised as AGN by employing three diagnostic tests: WISE colours, radio emission and morphology. For 12 sources, among the 34 AGN candidates, we reduced optical spectra and confirmed their AGN nature, providing also their optical class and redshift. This paper is part of an on-going effort to keep the INTEGRAL AGN catalogue updated in order to provide the scientific community with a hard X-ray selected sample of active galaxies well classified and spectrally characterised.

astro-ph.HE↗

arXiv2017

Investigating the X-ray counterparts to unidentified sources in the 1000-orbit INTEGRAL/IBIS catalogue

The latest INTEGRAL/IBIS all-sky survey lists 219 hard X-ray sources whose nature is still unknown. We report on our ongoing campaign aimed at identifying these high-energy emitters by exploiting the focusing capabilities of the X-ray Telescope (XRT, 0.2-10 keV) on board Swift, which allow an enhancement of the source localisation to arcsec level, thus facilitating the identification of the likely counterpart. By cross-correlating the list of the unidentified IBIS sources included in the latest IBIS catalogue with Swift/XRT archival data, we found a set of 14 objects, not yet reported in the literature, for which XRT data were available. We found no detection in only one case, a single X-ray association in 9 sources, and 2/3 associations in the remaining objects. We then made use of multi-waveband archives to search for counterparts at other wavelengths of these XRT detections and exploited X-ray spectral information in an attempt to determine their nature and association with the IBIS object. As a result of our analysis, we identified a single counterpart for 13 sources, although in some cases its nature/class could not be assessed on the basis of the information collected. More specifically, we found that SWIFT J0924.2-3141 and SWIFT J1839.1-5717 are absorbed AGN, while SWIFT J0800.7-4309 and 1SWXRT J230642.8+550817 are Cataclysmic Variable binary systems. Finally, we found that IGR J14059-6116 is likely associated with the Fermi source 3FGL J1405.4-6119. In the case of XMMSL1 J030715.5-545536 no XRT counterpart was detected. In all the other cases, optical/infrared spectroscopy is necessary to classify properly each X-ray counterpart and confirm their association with the INTEGRAL/IBIS detection.

astro-ph.HE↗

arXiv2017

The nature of fifty Palermo Swift-BAT hard X-ray objects through optical spectroscopy

We present the nature of 50 hard X-ray emitting objects unveiled through an optical spectroscopy campaign performed at seven telescopes in the northern and southern hemispheres. These objects were detected with Swift-BAT and listed as of unidentified nature in the 54-month Palermo BAT catalogue. In detail, 45 sources in our sample are identified as active galactic nuclei of which, 27 are classified as type 1 (with broad and narrow emission lines) and 18 are classified as type 2 (with only narrow emission lines). Among the broad-line emission objects, one is a type 1 high-redshift quasi-stellar object, and among the narrow-line emission objects, one is a starburst galaxy, one is a X-ray bright optically normal galaxy, and one is a low ionization nuclear emission line region. We report 30 new redshift measurements, 13 confirmations and 2 more accurate redshift values. The remaining five objects are galactic sources: three are Cataclismic Variables, one is a X-ray Binary probably with a low mass secondary star, and one is an active star.

astro-ph.HE↗

arXiv2016

Looking for blazars in a sample of unidentified high-energy emitting Fermi sources

Context. Based on their overwhelming dominance among associated Fermi gamma ray catalogue sources, it is expected that a large fraction of the unidentified Fermi objects are blazars. Through crossmatching between the positions of unidentified gamma ray sources from the First Fermi Catalog of gamma ray sources emitting above 10 GeV (1FHL) and the ROSAT and Swift XRT catalogues of X ray objects and between pointed XRT observations, a sample of 36 potential associations was found in previous works with less than 15 arcsec of positional offset. One third of them have recently been classified; the remainder, though believed to belong to the blazar class, still lack spectroscopic classifications. Aims. We study the optical spectrum of the putative counterparts of these unidentified gamma ray sources in order to find their redshifts and to determine their nature and main spectral characteristics. Methods. An observational campaign was carried out on the putative counterparts of 13 1FHL sources using medium resolution optical spectroscopy from the Osservatorio Astronomico di Bologna in Loiano, Italy; the Telescopio Nazionale Galileo and the Nordic Optical Telescope, both in the Canary Islands, Spain; and the Observatorio Astronomico Nacional San Pedro Martir in Baja California, Mexico. Results. We were able to classify 14 new objects based on their continuum shapes and spectral features. Conclusions. Twelve new blazars were found, along with one new quasar and one new narrow line Seyfert 1 (NLS1) to be potentially associated with the 1FHL sources of our sample. Redshifts or lower limits were obtained when possible alongside central black hole mass and luminosity estimates for the NLS1 and the quasar.

astro-ph.HE↗

arXiv2016

The column density distribution of hard X-ray radio galaxies

In order to investigate the role of absorption in AGN with jets, we have studied the column density distribution of a hard X-ray selected sample of radio galaxies, derived from the INTEGRAL/IBIS and Swift/BAT AGN catalogues (~7-10% of the total AGN population). The 64 radio galaxies have a typical FRII radio morphology and are characterized by high 20-100 keV luminosities (from 10^42 to 10^46 erg/s) and high Eddington ratios (Log L_{Bol}/L_{Edd} typically larger than ~0.01). The observed fraction of absorbed AGN (N_{H} > 10^{22} cm^{-2}) is around 40% among the total sample, and ~75% among type 2 AGN. The majority of obscured AGN are narrow line objects, while unobscured AGN are broad line objects, obeying to the zeroth-order predictions of unified models. A significant anti-correlation between the radio core dominance parameter and the X-ray column density is found. The observed fraction of Compton thick AGN is ~2-3%, in comparison with the 5-7% found in radio-quiet hard X-ray selected AGN. We have estimated the absorption and Compton thick fractions in a hard X-ray sample containing both radio galaxies and non-radio galaxies and therefore affected by the same selection biases. No statistical significant difference was found in the absorption properties of radio galaxies and non radio galaxies sample. In particular, the Compton thick objects are likely missing in both samples and the fraction of obscured radio galaxies appears to decrease with luminosity as observed in hard X-ray non-radio galaxies.

astro-ph.HE↗

arXiv2016

The INTEGRAL/IBIS AGN catalogue: an update

In the most recent IBIS survey based on observations performed during the first 1000 orbits of INTEGRAL, are listed 363 high energy emitters firmly associated with AGN, 107 of which are reported here for the first time. We have used X-ray data to image the IBIS 90% error circle of all the AGN in the sample of 107, in order to obtain the correct X-ray counterparts, locate them with arcsec accuracy and therefore pinpoint the correct optical counterparts. This procedure has led to the optical and spectral characterization of the entire sample. This new set consists of 34 broad line or type 1 AGN, 47 narrow line or type 2 AGN, 18 Blazars and 8 sources of unknown class. These 8 sources have been associated with AGN from their positional coincidence with 2MASX/Radio/X-ray sources. Seven high energy emitters have been included since they are considered to be good AGN candidates. Spectral analysis has been already performed on 55 objects and the results from the most recent and/or best statistical measurements have been collected. For the remaining 52 sources, we report the spectral analysis for the first time in this work. We have been able to obtain the full X-ray coverage of the sample making use of data from Swift/XRT, XMM-Newton and NuSTAR. In addition to the spectral characterization of the entire sample, this analysis has enabled us to identify peculiar sources and by comparing different datasets, highlight flux variability in the 2-10 keV and 20-40 keV bands.

astro-ph.HE↗

arXiv2015

Swift/XRT counterparts to unassociated Fermi high-energy LAT sources

We report the results from our analysis of a large set of archival data acquired with the X-ray telescope (XRT) onboard Swift, covering the sky region surrounding objects from the first Fermi Large Area Telescope (LAT) catalogue of high-energy sources (1FHL), which still lack an association. Of the 23 regions analysed, ten did not show any evidence of X-ray emission, but 13 were characterised by the presence of one or more objects emitting in the 0.3-10 keV band. Only in a couple of cases is the X-ray counterpart located outside the Fermi positional uncertainty, while in all other cases the associations found are compatible with the high-energy error ellipses. All counterparts we found have been studied in detail by means of a multi-waveband approach to evaluate their nature or class; in most cases, we have been able to propose a likely or possible association except for one Fermi source whose nature remains doubtful at the moment. The majority of the likely associations are extragalactic in nature, most probably blazars of the BL Lac type.

astro-ph.HE↗

arXiv2015

Using X-ray catalogues to find counterparts to unassociated high-energy Fermi/LAT sources

The first Fermi Large Area Telescope (LAT) catalogue of sources (1FHL) emitting at high energies (above 10 GeV) reports the details of 514 objects detected in the first three years of the Fermi mission. Of these, 71 were reported as unidentified in the 1FHL catalogue, although six are likely to be associated with a supernova remnant (SNR), a Pulsar Wind Nebula (PWN) or a combination of both, thereby leaving a list of 65 still unassociated objects. Herein, we report a preliminary analysis on this sample of objects concentrating on nine 1FHL sources, which were found to have a clear optical extragalactic classification. They are all blazar, eight BL Lac and one flat spectrum radio quasar, typically at redshift greater than 0.1.

astro-ph.HE↗

arXiv2014

Scientific prospects in soft gamma-ray astronomy enabled by the LAUE project

This paper summarizes the development of a successful project, LAUE, supported by the Italian Space Agency (ASI) and devoted to the development of long focal length (up to 100 m) Laue lenses for hard X--/soft gamma--ray astronomy (80-600 keV). The apparatus is ready and the assembling of a prototype lens petal is ongoing. The great achievement of this project is the use of bent crystals. From measurements obtained on single crystals and from simulations, we have estimated the expected Point Spread Function and thus the sensitivity of a lens made of petals. The expected sensitivity is a few $\times10^{-8}$ photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$. We discuss a number of open astrophysical questions that can settled with such an instrument aboard a free-flying satellite.

astro-ph.IM↗

arXiv2013

The counterpart/s of IGR J20159+3713/SWIFT J2015.9+3715: dissecting a complex region with emission from keV to TeV

We report on the identification of a new soft gamma-ray source, namely IGR J20159+3713/SWIFT J2015.9+3715, first detected by INTEGRAL/IBIS and then confirmed by Swift/BAT. The source, which has an observed 20-100 keV flux in the range (0.7-1.4) x 10^(-11) erg cm^(-2) s^(-1), encloses a Fermi variable source (2FGL J2015.6+3709) and is spatially close to a TeV emitter (VER J2016+372). Thanks to X-ray follow-up observations performed with the X-ray telescope on board Swift, we have been able to identify the new IBIS/BAT detection with the combined emission of the blazar B2013+370 and the cataclysmic variable RX J2015.6+3711. Both objects show variability in X-rays, with the CV being the most variable of the two. At high energies (above 20 keV) the emission is likely dominated by B2013+370, but the contribution from RX J2015.6+3711 is not negligible. The blazar emits up to GeV frequencies where it is seen by Fermi, while the cataclysmic variable has a bremsstrahlung temperature which is too low to provide any contribution at these high energies. These findings also indicate that the INTEGRAL/Swift source is not associated with the TeV emission, which is most likely due to the supernova remnant (SNR)/pulsar wind nebula (PWN) CTB 87.

astro-ph.HE↗

arXiv2013

BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects

The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.

astro-ph.HE↗

arXiv2013

Symbiotic Stars in OGLE Data I. Large Magellanic Cloud Systems

Symbiotic stars are long-orbital-period interacting-binaries characterized by extended emission over the whole electromagnetic range and by complex photometric and spectroscopic variability. In this paper, the first of a series, we present OGLE light curves of all the confirmed symbiotic stars in the Large Magellanic Cloud, with one exception. By careful visual inspection and combined time-series analysis techniques, we investigate for the first time in a systematic way the photometric properties of these astrophysical objects, trying in particular to distinguish the nature of the cool component (e.g., Semi-Regular Variable vs. OGLE Small-Amplitude Red Giant), to provide its first-order pulsational ephemerides, and to link all this information with the physical parameters of the binary system as a whole. Among the most interesting results, there is the discovery of a 20-year-long steady fading of Sanduleak's star, a peculiar symbiotic star known to produce the largest stellar jet ever discovered. We discuss by means of direct examples the crucial need for long-term multi-band observations to get a real understanding of symbiotic and other interacting binary stars. We eventually introduce BOMBOLO, a multi-band simultaneous imager for the SOAR 4m Telescope, whose design and construction we are currently leading.

astro-ph.SR↗

arXiv2013

Unveiling the nature of INTEGRAL objects through optical spectroscopy. X. A new multi-year, multi-observatory campaign

Within the framework of our program (running since 2004) of identification of hard X-ray INTEGRAL sources through optical spectroscopy, we present the results concerning the nature of 33 high-energy objects. The data were acquired with the use of six telescopes of different sizes and from one on-line archive. The results indicate that the majority of these objects (23 out of 33) are active galactic nuclei (AGNs), whereas 10 are sources in the local Universe with eight of which in the Galaxy and two in the Small Magellanic Cloud (SMC). Among the identified AGNs, 13 are of Type 1 (i.e., with broad emission lines), eight are of Type 2 (with narrow emissions only), and two are X-ray bright, optically normal galaxies with no apparent nuclear activity in the optical. Six of these AGNs lie at high redshift (z > 0.5). Concerning local objects, we found that five of them are Galactic cataclysmic variables, three are high-mass X-ray binaries (two of which lying in the SMC), one is a low-mass X-ray binary, and one is classified as a flare star that is likely of RS CVn type. The main optical properties and inferred physical characteristics of these sources are presented and discussed.

astro-ph.HE↗

arXiv2013

Stellar population properties for a sample of hard X-ray AGNs

The aim of this paper is to study the stellar population of galaxies hosting an active galactic nucleus (AGN). We studied a sub-sample of hard X-ray emitting AGNs from the INTEGRAL and Swift catalogs which were previously identified and characterized through optical spectroscopy. Our analysis provides complementary information, namely age and metallicity, which is necessary to complete the panoramic view of such interesting objects. We selected hard X-ray emitting objects identified as AGNs by checking their optical spectra in search for absorption lines suitable for the stellar population analysis. We obtained a final sample consisting of 20 objects with redshift lower than 0.3. We used the full-spectrum fitting method and, in particular, the penalized pixel one applying the PPXF code. After masking all the regions affected by emission lines, we fitted the spectra with the MILES single stellar population templates and we derived mass-weighted ages and metallicities. Most of the objects in our sample show an old stellar population, but three of them are characterized by a bimodal distribution with a non negligible contribution from young stars. The values of the mass-weighted metallicity span a large range of metallicity with most of them slightly above the solar value. No relations between the stellar population properties and the morphological ones have been found.

astro-ph.CO↗

arXiv2013

X-ray, optical and infrared investigation of the candidate Supergiant Fast X-ray Transient IGR J18462-0223

We report on a broad-band X-ray study (0.5-60 keV) of the poorly known candidate Supergiant Fast X-ray Transient (SFXT) IGR J18462-0223, and on optical and near-infrared (NIR) followup observations of field objects. The out-of-outburst X-ray state has been investigated for the first time with archival INTEGRAL/IBIS, ASCA, Chandra and Swift/XRT observations. This allowed us to place stringent 3 sigma upper limits on the soft (0.5-10 keV) and hard (18-60 keV) X-ray emission of 2.9x10^-13 erg cm^-2 s^-1 and 8x10^-12 erg cm^-2 s^-1, respectively; the source was also detected during an intermediate soft X-ray state with flux equal to 1.6x10^-11 erg cm^-2 s^-1 (0.5-10 keV). In addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray flares (18-60 keV) having a duration in the range 1-12 hours: the flaring behavior was also investigated in soft X-rays (3-10 keV) with archival INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges (> 1,380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively) were measured for the first time. Archival UKIDSS JHK NIR data, together with our deep R-band imaging of the field, unveiled a single, very red object inside the intersection of the Swift/XRT and XMM-Newton error circles: this source has optical/NIR photometric properties compatible with a very heavily absorbed blue supergiant located at about 11 kpc, thus being a strong candidate counterpart for IGR J18462-0223. NIR spectroscopy is advised to confirm the association. Finally, a hint of a possible orbital period was found at about 2.13 days. If confirmed by further studies, this would make IGR J18462-0223 the SFXT with the shortest orbital period among the currently known systems.

astro-ph.HE↗

arXiv2013

Swift/XRT observations of newly discovered INTEGRAL sources

With respect to the recent INTEGRAL/IBIS 9-year Galactic Hard X-ray Survey (Krivonos et al. 2012), we use archival Swift/XRT observations in conjunction with multi-wavelength information to discuss the counterparts of a sample of newly discovered objects. The X-ray telescope (XRT, 0.3-10 keV) on board Swift, thanks to its few arcseconds source location accuracy, has been proven to be a powerful tool with which the X-ray counterparts to these IBIS sources can be searched for and studied. In this work, we present the outcome of this analysis by discussing four objects (SWIFT J0958.0-4208, SWIFT J1508.6-4953, IGR J17157-5449, and IGR J22534+6243) having either X-ray data of sufficient quality to perform a reliable spectral analysis or having interesting multiwaveband properties. We find that SWIFT J1508.6-4953 is most likely a Blazar, while IGR J22534+6243 is probably a HMXB. The remaining two objects may be contaminated by nearby X-ray sources and their class can be inferred only by means of optical follow-up observations of all likely counterparts.

astro-ph.HE↗

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

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↗