arXiv subjects

Loredana Bassani

Publications and source records attributed to Loredana Bassani.

18 recordsLinked to original sources

arXiv2026

Hunting for extreme high-energy-peaked BL Lacs: Rare to find and difficult to classify

We explore the possible existence of a new population of BL Lacs, called ultra extreme high-energy-peaked BL Lacs (UEHBLs), whose synchrotron emission component peaks in the MeV band. In particular, we analysed Swift/XRT follow-up observations of a set of 10 hard X-ray sources from the Swift/BAT catalogues which were suggested to represent the first observational hints of this new blazar population on the basis of their spectral properties. We find that 6 of these candidate UEHBLs can be classified as line-emitting active galactic nuclei (AGNs), 4 of which are of type 2 and X-ray absorbed objects, while 2 are type 1 AGN and X-ray unabsorbed sources. In one more case, we find that the hard X-ray emission is probably the contribution of two line-emitting AGN, i.e. a Seyfert of type 1.9 and another of type 2. All these 7 classifications exclude the possibility that these hard X-ray sources are extreme BL Lacs. Of the remaining 3 UEHBL candidates, only 2 objects are found to have a spectral energy distribution (SED) compatible with those expected by MeV-peaked BL Lacs: however, in one of these 2 cases, an alternative Galactic association is also possible. The third source is instead likely associated with a more classical quasi-stellar object. Overall, we find that UEHBLs are extremely rare to find and probably very difficult to classify. Nevertheless, they represent a viable alternative classification for hard X-ray sources displaying no obvious or very weak counterparts.

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arXiv2026

3C403: a candidate neutrino-emitting radio galaxy

3C403 is a well-known FRII radio galaxy with jets extending up to kiloparsec scales. We report its identification as the second most significant candidate among more than 150 sources examined using the 15-year neutrino dataset from the ANTARES Collaboration, making it one of the most promising radio-galaxy candidates for high-energy neutrino emission. Motivated by previous associations between blazars and neutrino events, we investigated the jet properties of 3C403 and their possible role in neutrino production. Multi-scale radio observations, from parsec to kiloparsec scales, reveal a stable, two-sided jet lying close to the plane of the sky, with no evidence of strong Doppler boosting, while X-ray data indicate a dominant, heavily absorbed accretion-related component. We also examined the recently proposed correlation between neutrino and hard X-ray fluxes - originally identified in blazars and Seyfert galaxies - and find that 3C403 occupies an intermediate location in the $L_ν$--$L_{\rm hX}$ plane between jet-dominated and corona-dominated systems. However, the current upper limit on its neutrino flux prevents a firm assessment of whether it follows the proposed relation. With radiatively efficient accretion ($λ_{\rm Edd}\sim10^{-2}$), strong hard X-ray emission, and a powerful but misaligned jet, 3C403 provides a physically motivated laboratory for exploring the interplay between coronal activity and jet environments in multimessenger scenarios of neutrino production in active galaxies.

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arXiv2024

The challenge of identifying INTEGRAL sources on the Galactic plane

The International Gamma-ray Astrophysics Laboratory (INTEGRAL) has been surveying the sky above 20 keV since its launch in 2002 providing new insights into the nature of the sources that populate our Universe at soft gamma-ray energies. The latest IBIS/ISGRI survey lists 929 hard X-ray sources, of which 113 are reported as unidentified, i.e. lacking a lower energy counterpart or simply not studied in other wavebands. To overcome this lack of information, we either browsed the X-ray archives, or, if no data in the X-ray band were available, we requested Target of Opportunity (ToO) observations with the X-ray Telescope (XRT) on-board the Neil Gehrels Swift Observatory. Following this approach, we selected a sample of 10 objects for which X-ray data were key to investigate their nature. We found a single X-ray association for all of the sources, except for IGR J16267-3303, for which two X-ray detections were spotted within the IBIS positional uncertainty. We then browsed multi-waveband archives to search for counterparts to these X-ray detections at other wavelengths and analysed X-ray spectral properties to determine their nature and association with the high-energy emitter. As a result of our analysis, we identified the most likely counterpart for 7 sources, although in some cases its nature/class could not be definitely assessed on the basis of the information collected. Interestingly, SWIFT J2221.6+5952, first reported in the 105-month Swift/Burst Alert Telescope (BAT) survey, is the only source of the sample for which we did not find any counterpart at radio/optical/IR wavebands. Finally, we found that two IBIS source, IGR J17449-3037 and IGR J17596-2315 are positionally associated with a Fermi Large Area Telescope (LAT) object.

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arXiv2023

Dealing with broad-band X-ray spectra of faint AGN: a case study

In this work we analyse 3 average-luminosity hard X-ray selected AGN: ESO 506-G27, IGR J19039+3344 and NGC 7465. They have simultaneous Swift/XRT and NuSTAR data never published before and have been poorly studied at X-ray energies. These sources make for interesting targets both from a methodological and scientific point of view. Scientifically, they are of interest since they are possibly heavily absorbed objects, belong to a peculiar class and are variable both in flux and in spectral shape. Methodologically, because it is an interesting exercise to understand how existing spectral models can be applied to faint sources and how the use of NuSTAR data alone and then simultaneous and/or average data impacts on the spectral parameters determination. In this work we demonstrate that simultaneous data are not sufficient if their statistical quality is poor. Moreover, we show that also the use of time-averaged data when dealing with faint AGN does not always provide confident results as for brighter AGN. Regardless of the poor data quality employed in our analysis, we are able to provide insights into the spectral characteristics of each source. We analyse in detail for the first time the iron line complex of ESO 506-G27, finding not only the presence of the iron K$α$ and K$β$ lines, but also of the iron K edge around 7 keV in the NuSTAR data. We also highlight changes in the absorption properties of IGR J19039+3344 and confirm NGC 7465 to be an unabsorbed type 1 LINER.

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arXiv2022

Daily variability at milli-arcsecond scales in the radio quiet NLSy1 Mrk 110

The origin of radio emission in the majority of Active Galactic Nuclei (AGN) is still poorly understood. Various competing mechanisms are likely involved in the production of radio emission and precise diagnostic tools are needed to disentangle them, of which variability is among the most powerful. For the first time, we show evidence for significant radio variability at 5 GHz at milli-arcsecond scales on days to weeks time scales in the highly accreting and extremely radio-quiet (RQ) Narrow Line Seyfert 1 (NLSy1) Mrk110. The simultaneous Swift/XRT light curve indicates stronger soft than hard X-ray variability. The short-term radio variability suggests that the GHz emitting region has a size smaller than ~180 Schwarzschild radii. The high brightness temperature and the radio and X-ray variability rule out a star-formation and a disc wind origin. Synchrotron emission from a low-power jet and/or an outflowing corona is then favoured.

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arXiv2021

Investigating the true nature of three hard X-ray sources

Many of the new high energy sources discovered both by INTEGRAL/IBIS and Swift/BAT have been characterised thanks to extensive, multi-band follow-up campaigns, but there are still objects whose nature remains to be asserted. In this paper we investigate the true nature of three high energy sources, IGR J12134-6015, IGR J16058-7253 and Swift J2037.2+4151, employing multiwavelength data from the NIR to the X-rays. Through Gaia and ESO-VLT measurements and through Swift/XRT X-ray spectral analysis, we re-evaluate the classification for IGR J12134-6015, arguing that the source is a Galactic object and in particular a Cataclysmic Variable. We were able to confirm, thanks to NuSTAR observations, that the hard X-ray emission detected by INTEGRAL/IBIS and Swift/BAT from IGR J16058-7253 is coming from two Seyfert 2 galaxies which are both counterparts for this source. Through optical and X-ray spectral analysis of Swift J2037.2+4151 we find that this source is likely part of the rare and peculiar class of Symbiotic X-ray binaries and displays flux and spectral variability as well as interesting spectral features, such as a blending of several emission lines around the iron line complex.

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arXiv2021

INTEGRAL View of TeV Sources: A Legacy for the CTA Project

Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions of the Universe where relativistic acceleration processes are in place. Simultaneous and/or combined observations with gamma-ray satellites and ground based high-energy telescopes, have clarified the scenario of the mechanisms responsible for high energy photon emission by leptonic and hadronic accelerated particles in the presence of magnetic fields. Specifically, the European Space Agency INTEGRAL soft gamma-ray observatory has detected more than 1000 sources in the soft gamma-ray band, providing accurate positions, light curves and time resolved spectral data for them. Space observations with Fermi-LAT and observations that were carried out from the ground with H.E.S.S., MAGIC, VERITAS, and other telescopes sensitive in the GeV-TeV domain have, at the same time, provided evidence that a substantial fraction of the cosmic sources detected are emitting in the keV to TeV band via Synchrotron-Inverse Compton processes, in particular from stellar galactic BH systems as well as from distant black holes. In this work, employing a spatial cross correlation technique, we compare the INTEGRAL/IBIS and TeV all-sky data in search of secure or likely associations. Although this analysis is based on a subset of the INTEGRAL all-sky observations, we find that there is a significant correlation: 39 objects show emission in both soft gamma-ray and TeV wavebands. The full INTEGRAL database, now comprising almost 19 years of public data available, will represent an important legacy that will be useful for the Cherenkov Telescope Array (CTA) and other ground based large projects.

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arXiv2020

INTEGRAL view of AGN

AGN are among the most energetic phenomena in the Universe and in the last two decades INTEGRAL's contribution in their study has had a significant impact. Thanks to the INTEGRAL extragalactic sky surveys, all classes of soft X-ray detected (in the 2-10 keV band) AGN have been observed at higher energies as well. Up to now, around 450 AGN have been catalogued and a conspicuous part of them are either objects observed at high-energies for the first time or newly discovered AGN. The high-energy domain (20-200 keV) represents an important window for spectral studies of AGN and it is also the most appropriate for AGN population studies, since it is almost unbiased against obscuration and therefore free of the limitations which affect surveys at other frequencies. Over the years, INTEGRAL data have allowed to characterise AGN spectra at high energies, to investigate their absorption properties, to test the AGN unification scheme and to perform population studies. In this review the main results are reported and INTEGRAL's contribution to AGN science is highlighted for each class of AGN. Finally, new perspectives are provided, connecting INTEGRAL's science with that at other wavelengths and in particular to the GeV/TeV regime which is still poorly explored.

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arXiv2020

Water megamaser emission in hard X-ray selected AGN

Water megamaser emission at 22 GHz has proven to be a powerful tool for astrophysical studies of AGN allowing an accurate determination of the central black hole mass and of the accretion disc geometry and dynamics. However, after searches among thousands of galaxies, only ~ 200 of them have shown such spectroscopic features, most of them of uncertain classification. In addition, the physical and geometrical conditions under which maser activates are still unknown. In this work we aim at characterizing the occurrence of water maser emission in an unbiased sample of AGN, investigating the relation with the X-ray properties and the possible favorable geometry needed to detect water maser. We have searched for 22 GHz maser emission in a hard X-ray selected sample of AGN, taken from the INTEGRAL/IBIS survey above 20 keV. Of the 380 sources in the sample, only half have water maser data. We have also considered a sub-sample of 87 sources, volume limited, for which we obtained new Green Bank Telescope and Effelsberg observations (for 35 sources), detecting one new maser and increasing its radio coverage to 75%. The detection rate of water maser emission in the total sample is 15+/-3%, this fraction raises up to 19+/-5% for the complete sub-sample, especially if considering type 2 and Compton thick AGN. These results demonstrate that the hard X-ray selection may significantly enhance the maser detection efficiency over comparably large optical/infrared surveys. A possible decline of the detection fraction with increasing luminosity might suggest that an extreme luminous nuclear environment does not favour maser emission. The large fraction of CT AGN with water maser emission could be explained in terms of geometrical effects, being the maser medium the very edge-on portion of the obscuring medium.

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arXiv2019

HST unveils a compact mildly relativistic Broad Line Region in the candidate true type 2 NGC 3147

NGC 3147 has been considered the best case of a true type 2 AGN: an unobscured AGN, based on the unabsorbed compact X-ray continuum, which lacks a broad line region (BLR). However, the very low luminosity of NGC 3147 implies a compact BLR, which produces very broad lines, hard to detect against the dominant background host galaxy. Narrow (0.1"x0.1") slit HST spectroscopy allowed us to exclude most of the host galaxy light, and revealed an H$α$ line with an extremely broad base (FWZI$\sim27\,000$ km s$^{-1}$). The line profile shows a steep cutoff blue wing and an extended red wing, which match the signature of a mildly relativistic thin accretion disk line profile. It is indeed well fit with a nearly face on thin disk, at $i\sim23^\circ$, with an inner radius at $77\pm15$ r$_g$, which matches the prediction of $62^{+18}_{-14}$ r$_g$ from the $R_{\rm BLR} \sim L^{1/2}$ relation. This result questions the very existence of true type 2 AGN. Moreover, the detection of a thin disk, which extends below 100 r$_g$ in an $L/L_{\rm Edd}\sim10^{-4}$ system, contradicts the current view of the accretion flow configuration at extremely low accretion rates.

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arXiv2019

Swift/XRT- NuSTAR spectra of type 1 AGN]{Swift/XRT- NuSTAR spectra of type 1 AGN: confirming INTEGRAL results on the high energy cut-off

We present the 0.5 - 78 keV spectral analysis of 18 broad line AGN belonging to the INTEGRAL complete sample. Using simultaneous Swift-XRT and NuSTAR observations and employing a simple phenomenological model to fit the data, we measure with a good constraint the high energy cut-off in 13 sources, while we place lower limits on 5 objects. We found a mean high-energy cut-off of 111 keV (standard deviation = 45 keV) for the whole sample, in perfect agreement with what found in our previous work using non simultaneous observations and with what recently published using NuSTAR data. This work suggests that simultaneity of the observations in the soft and hard X-ray band is important but not always essential, especially if flux and spectral variability are properly accounted for. A lesser agreement is found when we compare our cut-off measurements with the ones obtained by Ricci et al. (2017) using Swift-BAT high energy data, finding that their values are systematically higher than ours. We have investigated whether a linear correlation exists between photon index and the cut-off and found a weak one, probably to be ascribed to the non perfect modelling of the soft part of the spectra, due to the poor statistical quality of the 2-10 keV X-ray data. No correlation is also found between the Eddington ratio and the cut-off, suggesting that only using high statistical quality broad-band spectra is it possible to verify the theoretical predictions and study the physical characteristics of the hot corona and its geometry.

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arXiv2017

The NuSTAR view of the true Type 2 Seyfert NGC3147

We present the first NuSTAR observation of a 'true' Type 2 Seyfert galaxy. The 3-40 keV X-ray spectrum of NGC3147 is characterised by a simple power-law, with a standard Γ~1.7 and an iron emission line, with no need for any further component up to ~40 keV. These spectral properties, together with significant variability on time-scales as short as weeks (as shown in a 2014 Swift monitoring campaign), strongly support an unobscured line-of-sight for this source. An alternative scenario in terms of a Compton-thick source is strongly disfavoured, requiring an exceptional geometrical configuration, whereas a large fraction of the solid angle to the source is filled by a highly ionised gas, whose reprocessed emission would dominate the observed luminosity. Moreover, in this scenario the implied intrinsic X-ray luminosity of the source would be much larger than the value predicted by other luminosity proxies, like the [OIII]λ5007 emission line extinction-corrected luminosity. Therefore, we confirm with high confidence that NGC3147 is a true Type 2 Seyfert galaxy, intrinsically characterised by the absence of a BLR.

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arXiv2015

IGR J14488-4008: an X-ray peculiar giant radio galaxy discovered by INTEGRAL

In this paper we report the discovery and detailed radio/X-ray analysis of a peculiar giant radio galaxy (GRG) detected by INTEGRAL, IGR J14488-4008. The source has been recently classified as a Seyfert 1.2 galaxy at redshift 0.123; the radio data denote the source to be a type II Fanaroff-Riley radio galaxy, with a linear projected size exceeding 1.5 Mpc, clearly assigning IGR J14488-4008 to the class of GRG. In the X-rays, the source shows a remarkable spectrum, characterised by absorption by ionised elements, a characteristic so far found in only other four broad line radio galaxies.

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arXiv2015

Double-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of galaxies

The scenario of galaxy formation is believed to follow a structure that builds up from the bottom, with large galaxies being formed by several merging episodes of smaller ones. In this scenario a number of galaxies can be expected to be seen in the merging phase, with their external regions already mixed, while their nuclei, with stronger self-gravitation, are still recognizable as such. During a photometric monitoring of AGNs in the field of a long-exposure INTEGRAL pointing, we serendipitously found an elliptical galaxy in the center of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface photometry on our images and those of the SDSS archive and obtained slit spectra of both nuclei. Aperture photometry of the two stellar-like nuclei showed very similar colors in the SDSS image and in our Johnson BVRI images, which is typical of an elliptical galaxy nucleus. The spectra of the nuclei showed the typical absorption lines of an elliptical galaxy without appreciable emission lines. The redshifts derived from each nucleus were equal and fully consistent with the literature value (0.0397). We can therefore exclude the possibility that one of the nuclei is a foreground star or a background AGN and consider this elliptical galaxy as a bona fide example of a galaxy merger.

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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.

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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.

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arXiv2012

Flat-spectrum radio sources as likely counterparts of unidentified INTEGRAL sources (Research Note)

Many sources in the fourth INTEGRAL/IBIS catalogue are still unidentified, since they lack an optical counterpart. An important tool that can help in identifying/classifying these sources is the cross-correlation with radio catalogues, which are very sensitive and positionally accurate. Moreover, the radio properties of a source, such as the spectrum or morphology, could provide further insight into its nature. Flat-spectrum radio sources at high Galactic latitudes are likely to be AGN, possibly associated to a blazar or to the compact core of a radio galaxy. Here we present a small sample of 6 sources extracted from the fourth INTEGRAL/IBIS catalogue that are still unidentified/unclassified, but which are very likely associated with a bright, flat-spectrum radio object. To confirm the association and to study the source X-ray spectral parameters, we performed X-ray follow-up observations with Swift/XRT. We report the results obtained from this search and discuss the nature of each source. 5 of the 6 radio associations are also detected in X-rays; in 3 cases they are the only counterpart found. IGR J06073--0024 is a flat-spectrum radio quasar at z=1.08, IGR J14488--4008 is a newly discovered radio galaxy, while IGR J18129--0649 is an AGN of a still unknown type. The nature of IGR J07225--3810 and IGR J19386--4653 is less well defined, since in both cases we find another X-ray source in the INTEGRAL error circle; nevertheless, the flat-spectrum radio source, likely to be a radio loud AGN, remains a viable and more convincing association in both cases. Only for IGR J11544--7618 could we not find any convincing counterpart since the radio association is not an X-ray emitter.

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arXiv2002

Unabsorbed Seyfert 2 galaxies

We present a sample of 17 type 2 Seyfert galaxies which have an X-ray column density lower than 10^{22} cm^{-2}. The Compton thin nature of these sources is strongly suggested by isotropic indicators. We estimate the fraction of these sources to be in the range of 10% - 30% of the population of type 2 Seyfert galaxies. Furthermore, this fraction appears to increase progressively at lower luminosities. The simple formulation of the Unified Model for Seyfert galaxies is not applicable in such sources since the pc-scale molecular torus is not likely to be responsible for the low column density observed, instead the absorption observed is likely to originate at larger scales. According to this hypothesis, in these objects the broad line regions are covered by some dusty obscuring material. In particular, this could occur in objects with dust lanes, patches or HII regions. However, we cannot rule out that in the lowest luminosity sources the BLR is weak, absent or has faded away. This last scenario is consistent with the predictions of some recent theoretical models for low luminosity AGNs.

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