arXiv subjects

S. Paiano

Publications and source records attributed to S. Paiano.

At least 19 recordsLinked to original sources

arXiv2025

SDSS low z quasar companion galaxies

We investigate the relationship between quasars and close companion galaxies using photometry and spectroscopy from the large data set of the SDSS DR16 survey. From the SDSS-QSO catalog of quasars, we selected objects with 0.1 < z < 0.35 and absolute magnitude M(r) <-21.3. For all these targets, we searched for candidate companion galaxies that have a projected distance from the target < 700 kpc and a radial velocity difference from the QSO DeltaV < 1000 km/sec. We find that in 447 QSO at least one companion galaxy is associated. A total of 691 associated galaxies are found in these QSO fields. In the majority of them there is just one associated galaxy but in few cases, many companions are also discovered and in two cases the QSO is found in rich galaxy environments. The possible contamination due to a chance projection of the companion galaxies is found less than 5%. Based on the available data we expect on average to find \sim2 associated companion galaxies for each QSO. A small fraction (13%) of the companion galaxies exhibits [OII] emission lines as signature of recent star formation. However a similar fraction (16%) of unassociated galaxies in the same QSO fields show [OII] emission. This study suggests that there is no significant link between the presence of these close companion galaxies or the signature of recent star formation and the QSO nuclear activity.

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arXiv2022

Low-redshift quasars in the SDSS Stripe 82 -- III MOS observations

We present multi object optical spectroscopy of the galaxies in the environment of 12 low-redshift (z < 0.5) quasars and of 11 inactive massive galaxies chosen to match the properties of the quasar host galaxies to probe physical association and possible events of recent star formation. The quasars are selected from a sample of QSOs in the SDSS Stripe82 region for which both the host galaxy and the large scale environments were previously investigated. The new observations complement those reported in our previous works on close companion galaxies of nearby quasars. For the whole dataset we find that for about half (19 out of 44 ) of the observed QSOs there is at least one associated companion galaxy. In addition to the new spectroscopic observations, we add data from the SDSS database for the full sample of objects. We find that the incidence of companion galaxies in the fields of QSO (17%) is not significantly different from that of inactive galaxies (19%) similar to quasar hosts in redshift and mass. Nevertheless, the companions of quasars exhibit more frequently emission lines than those of inactive galaxies, suggesting a moderate link between the nuclear activity and recent star formation in their environments.

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arXiv2022

The peculiar short-duration GRB 200826A and its supernova

Gamma-ray bursts (GRBs) are classified as long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was a SGRB, with a rest-frame duration of ~ 0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multi-wavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution is in agreement with several LGRB-SNe. Analysis of the prompt GRB shows that this event follows the $E_{\rm p,i}-E_{\rm iso}$ relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical for LGRBs, but with one of the highest star-formation rates (SFR), especially with respect to its mass ($\log M_\ast/M_\odot = 8.6$, SFR $\sim 4.0 \,M_\odot$/yr). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB.

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arXiv2021

The Spectra of IceCube Neutrino (SIN) candidate sources -- II. Source Characterisation

Eight years after the first detection of high-energy astrophysical neutrinos by IceCube we are still almost clueless as regards to their origin, although the case for blazars being neutrino sources is getting stronger. After the first significant association at the $3 - 3.5\,σ$ level in time and space with IceCube neutrinos, i.e. the blazar TXS 0506+056 at $z=0.3365$, some of us have in fact selected a unique sample of 47 blazars, out of which $\sim 16$ could be associated with individual neutrino track events detected by IceCube. Building upon our recent spectroscopy work on these objects, here we characterise them to determine their real nature and check if they are different from the rest of the blazar population. For the first time we also present a systematic study of the frequency of masquerading BL Lacs, i.e. flat-spectrum radio quasars with their broad lines swamped by non-thermal jet emission, in a $γ$-ray- and IceCube-selected sample, finding a fraction $>$ 24 per cent and possibly as high as 80 per cent. In terms of their broad-band properties, our sources appear to be indistinguishable from the rest of the blazar population. We also discuss two theoretical scenarios for neutrino emission, one in which neutrinos are produced in interactions of protons with jet photons and one in which the target photons are from the broad line region. Both scenarios can equally account for the neutrino-blazar correlation observed by some of us. Future observations with neutrino telescopes and X-ray satellites will test them out.

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arXiv2020

ZBLLAC -- A spectroscopic database of BL Lacertae objects

This paper describes the database of optical spectra of BL Lacertae (BLL) objects ZBLLAC available at the URL https://web.oapd.inaf.it/zbllac/. At the present date, it contains calibrated spectra for 295 BLL. For about 35% of them, we report a firm measure of redshift z while for 35 sources we set a lower limit on z based on the detection of intervening absorption systems, mainly ascribed to Mg II (2800 Ang). We report here on the architecture of the database and on its web front-end that permits to filter, query and interactively explore the data. We discuss some properties of the objects in the present dataset by giving the distribution of the redshifts and reporting on the detected emission lines, which turn out to be mainly forbidden and ascribed to [O II] (3737 Ang) and [O III] (5007 Ang). Finally, we discuss on intervening absorption systems detected in 35 BLLs that allow us to set lower limits to their distance.

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arXiv2020

3HSP J095507.9+355101: a flaring extreme blazar coincident in space and time with IceCube-200107A

The uncertainty region of the highly energetic neutrino IceCube200107A includes 3HSP J095507.9+355101 ($z$~=~0.557), an extreme blazar, which was detected in a high, very hard, and variable X-ray state shortly after the neutrino arrival. Following a detailed multi-wavelength investigation, we confirm that the source is a genuine BL Lac, contrary to TXS 0506+056, the first source so far associated with IceCube neutrinos, which is a "masquerading" BL Lac. As in the case of TXS0506+056, 3HSP J095507.9+355101 is also way off the so-called "blazar sequence". We consider 3HSP J095507.9+355101 a possible counterpart to the IceCube neutrino. Finally, we discuss some theoretical implications in terms of neutrino production.

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arXiv2019

Extreme high-energy peaked BL Lac objects and their TeV gamma-ray emission: are they a homogeneous population?

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. In blazars, the spectral energy distribution (SED) is dominated by the non-thermal emission of the relativistic jet, and consists of two main broad humps. For the EHBLs, these two components peak in the X-ray and GeV-TeV bands, respectively. Although the number of TeV detected extreme blazars is very limited, recent observations by Imaging Atmospheric Cherenkov Telescopes (IACTs) have revealed that in some of them the energy of the second peak exceeds several TeV (e.g. 1ES 0229+200). Their exceptional hard TeV spectra represent a challenge for the standard leptonic modeling, and a possible hadronic contribution may make these objects high-energy neutrinos producers. Moreover, they are important for the implications on the indirect measurements of the extragalactic background light and of the intergalactic magnetic field. In this contribution, we perform a comparative study of the multi-wavelength spectral energy distributions of a sample of hard X-ray selected EHBL objects. The analysis suggests that the EHBL class is not homogeneous, and a possible sub-classification may be unveiled with TeV gamma-ray observations of the candidates. With the purpose of increasing their number and settle their statistics, we discuss the potential detectability of the currently undetected TeV-emitting EHBLs in our sample by current and next generation of IACTs.

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arXiv2019

A new hard X-ray selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the spectral energy distribution (SED) plot with the synchrotron emission in X-rays and with the gamma-ray emission in the TeV range or above. These high photon energies may represent a challenge for the standard modeling of these sources. They are important for the implications on the indirect measurements of the extragalactic background light, the intergalactic magnetic field estimate, and the possible origin of extragalactic high-energy neutrinos. In this paper, we perform a comparative study of the multi-wavelength spectra of 32 EHBL objects detected by the Swift-BAT telescope in the hard X-ray band and by the Fermi-LAT telescope in the high-energy gamma-ray band. The source sample presents uniform spectral properties in the broad-band SEDs, except for the TeV gamma-ray band where an interesting bimodality seems to emerge. This suggests that the EHBL class is not homogeneous, and a possible sub-classification of the EHBLs may be unveiled. Furthermore, in order to increase the number of EHBLs and settle their statistics, we discuss the potential detectability of the 14 currently TeV gamma-ray undetected sources in our sample by the Cherenkov telescopes.

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arXiv2018

Teraelectronvolt pulsed emission from the Crab pulsar detected by MAGIC

Aims: To investigate the extension of the very-high-energy spectral tail of the Crab pulsar at energies above 400 GeV. Methods: We analyzed $\sim$320 hours of good quality data of Crab with the MAGIC telescope, obtained from February 2007 until April 2014. Results: We report the most energetic pulsed emission ever detected from the Crab pulsar reaching up to 1.5 TeV. The pulse profile shows two narrow peaks synchronized with the ones measured in the GeV energy range. The spectra of the two peaks follow two different power-law functions from 70 GeV up to 1.5 TeV and connect smoothly with the spectra measured above 10 GeV by the Large Area Telescope (LAT) on board of the Fermi satellite. When making a joint fit of the LAT and MAGIC data, above 10 GeV, the photon indices of the spectra differ by 0.5$\pm$0.1. Conclusions: We measured with the MAGIC telescopes the most energetic pulsed photons from a pulsar to date. Such TeV pulsed photons require a parent population of electrons with a Lorentz factor of at least $5\times 10^6$. These results strongly suggest IC scattering off low energy photons as the emission mechanism and a gamma-ray production region in the vicinity of the light cylinder.

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arXiv2018

The extreme HBL behaviour of Markarian 501 during 2012

A multiwavelength campaign was organized to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of $\sim$0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was $\sim$3 CU, and the peak of the high-energy spectral component was found to be at $\sim$2 TeV. This study reports very hard X-ray spectra, and the hardest VHE spectra measured to date for Mrk 501. The fractional variability was found to increase with energy, with the highest variability occurring at VHE, and a significant correlation between the X-ray and VHE bands. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency- peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The one-zone synchrotron self-Compton (SSC) scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behaviour seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays.

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arXiv2018

The two Ultraluminous X-ray sources in the galaxy NGC 925

NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ($(2$$-$$4)\times10^{40}$ erg s$^{-1}$) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc' ultraluminous X-ray source. Based on the physical interpretation of this spectral state, we suggest that ULX-1 is seen at small inclination angles, possibly through the evacuated cone of a powerful wind originating in the accretion disc. The spectral classification of ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington rates as none of its spectral/temporal properties can be associated to either the soft or hard state of Galactic accreting black hole binaries.

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arXiv2018

High redshift BL Lac objects: spectroscopy of candidates

We report on 16 BL Lacertae objects that were proposed to be at z > 1. We present spectroscopic observations secured at the 10.4m GTC that allowed us to assess the redshift of these sources. In particular, for five objects we disprove the previous value of the redshift reported in literature and found that they lie at z < 1. Moreover, two of them exhibit broad emission lines that are not characteristic of BL Lacertae object. On the other hand, for eight targets we improve the tentative value of z, previously based on only one feature, by detecting a number emission lines. Finally, in three cases we detect onset of Ly-α forest at z > 2.50. Based on the new high quality spectra we found that only half of the observed objects can be classified as bona-fine BL Lacs.

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arXiv2017

Search for very-high-energy gamma-ray emission from the microquasar Cygnus X-1 with the MAGIC telescopes

The microquasar Cygnus X-1 displays the two typical soft and hard X-ray states of a black-hole transient. During the latter, Cygnus X-1 shows a one-sided relativistic radio-jet. Recent detection of the system in the high energy (HE; $E\gtrsim60$ MeV) gamma-ray range with \textit{Fermi}-LAT associates this emission with the outflow. Former MAGIC observations revealed a hint of flaring activity in the very high-energy (VHE; $E\gtrsim100$ GeV) regime during this X-ray state. We analyze $\sim97$ hr of Cygnus X-1 data taken with the MAGIC telescopes between July 2007 and October 2014. To shed light on the correlation between hard X-ray and VHE gamma rays as previously suggested, we study each main X-ray state separately. We perform an orbital phase-folded analysis to look for variability in the VHE band. Additionally, to place this variability behavior in a multiwavelength context, we compare our results with \textit{Fermi}-LAT, \textit{AGILE}, \textit{Swift}-BAT, \textit{MAXI}, \textit{RXTE}-ASM, AMI and RATAN-600 data. We do not detect Cygnus X-1 in the VHE regime. We establish upper limits for each X-ray state, assuming a power-law distribution with photon index $Γ=3.2$. For steady emission in the hard and soft X-ray states, we set integral upper limits at 95\% confidence level for energies above 200 GeV at $2.6\times10^{-12}$~photons cm$^{-2}$s$^{-1}$ and $1.0\times10^{-11}$~photons cm$^{-2}$s$^{-1}$, respectively. We rule out steady VHE gamma-ray emission above this energy range, at the level of the MAGIC sensitivity, originating in the interaction between the relativistic jet and the surrounding medium, while the emission above this flux level produced inside the binary still remains a valid possibility.

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arXiv2017

MAGIC observations of the microquasar V404 Cygni during the 2015 outburst

The microquasar V404 Cygni underwent a series of outbursts in 2015, June 15-31, during which its flux in hard X-rays (20-40 keV) reached about 40 times the Crab Nebula flux. Because of the exceptional interest of the flaring activity from this source, observations at several wavelengths were conducted. The MAGIC telescopes, triggered by the INTEGRAL alerts, followed-up the flaring source for several nights during the period June 18-27, for more than 10 hours. One hour of observation was conducted simultaneously to a giant 22 GHz radio flare and a hint of signal at GeV energies seen by Fermi-LAT. The MAGIC observations did not show significant emission in any of the analysed time intervals. The derived flux upper limit, in the energy range 200--1250 GeV, is 4.8$\times 10^{-12}$ ph cm$^{-2}$ s$^{-1}$. We estimate the gamma-ray opacity during the flaring period, which along with our non-detection, points to an inefficient acceleration in the V404\,Cyg jets if VHE emitter is located further than $1\times 10^{10}$ cm from the compact object.

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arXiv2017

On the lensed blazar B0218+357

We present an optical spectrum (4000-10500 Å ) of the lensed blazar B0218+357 secured at the 10m GTC and aimed to investigate and clarify the properties of this intriguing system. We found that the emission line spectrum of the blazar is characterised by only one broad emission line that interpreted as Mg II 2800 Å yields z=0.95. In addition we detect narrow absorption lines of Mg II 2800 Å and Ca II (H,K) and Na I 5892 Å at z=0.68437 +/- 0.00005 due to intervening interstellar gas. No stellar absorption features attributable to the lens galaxy are revealed. Thus the assumed redshift of the lens is dubious. The continuum spectrum of the object exhibits a remarkable drop towards the short wavelengths likely due to a significant extinction. This extinction cannot be produced in the lens galaxy at z =0.684 with any value of R$_V$ under the assumption that the intrinsic shape of the blazar is dominated by a power law emission. However, the observed continuum is consistent with a power law emission assuming a standard (R$_V$ = 3.1) extinction at the source redshift (z=0.95) as supported also by the presence of Mg II absorptions at the same redshift. HST images of B0218+357 exhibit the double image of the source together with extended image of a face on spiral galaxy. We argue that this galaxy is possibly not the lensing galaxy but the host galaxy of the blazar. This has substantial consequences on the models of the system and on the derived values of the Hubble constant.

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arXiv2017

Multiwavelength observations of a VHE gamma-ray flare from PKS 1510-089 in 2015

Context. PKS 1510-089 is one of only a few flat spectrum radio quasars detected in the VHE (very-high-energy, > 100 GeV) gamma-ray band. Aims. We study the broadband spectral and temporal properties of the PKS 1510-089 emission during a high gamma-ray state. Methods. We performed VHE gamma-ray observations of PKS 1510-089 with the MAGIC telescopes during a long high gamma-ray state in May 2015. In order to perform broadband modelling of the source, we have also gathered contemporaneous multiwavelength data in radio, IR, optical photometry and polarization, UV, X-ray and GeV gamma-ray ranges. We construct a broadband spectral energy distribution (SED) in two periods, selected according to VHE gamma-ray state. Results. PKS 1510-089 has been detected by MAGIC during a few day-long observations performed in the middle of a long, high optical and gamma-ray state, showing for the first time a significant VHE gamma-ray variability. Similarly to the optical and gamma-ray high state of the source detected in 2012, it was accompanied by a rotation of the optical polarization angle and the emission of a new jet component observed in radio. However, due to large uncertainty on the knot separation time, the association with the VHE gamma-ray emission cannot be firmly established. The spectral shape in the VHE band during the flare is similar to the ones obtained during previous measurements of the source. The observed flux variability sets for the first time constraints on the size of the region from which VHE gamma rays are emitted. We model the broadband SED in the framework of the external Compton scenario and discuss the possible emission site in view of multiwavelength data and alternative emission models.

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arXiv2016

MAD Adaptive Optics Imaging of High Luminosity Quasars: A Pilot Project

We present near-IR images of five luminous quasars at z~2 and one at z~4 obtained with an experimental adaptive optics instrument at the ESO Very Large Telescope. The observations are part of a program aimed at demonstrating the capabilities of multi-conjugated adaptive optics imaging combined with the use of natural guide stars for high spatial resolution studies on large telescopes. The observations were mostly obtained under poor seeing conditions but in two cases. In spite of these non optimal conditions, the resulting images of point sources have cores of FWHM ~0.2 arcsec. We are able to characterize the host galaxy properties for 2 sources and set stringent upper limits to the galaxy luminosity for the others. We also report on the expected capabilities for investigating the host galaxies of distant quasars with adaptive optics systems coupled with future Extremely Large Telescopes. Detailed simulations show that it will be possible to characterize compact (2-3 kpc) quasar host galaxies for QSOs at z = 2 with nucleus K-magnitude spanning from 15 to 20 (corresponding to absolute magnitude -31 to -26) and host galaxies that are 4 mag fainter than their nuclei.

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arXiv2015

Probing the very-high-energy gamma-ray spectral curvature in the blazar PG 1553+113 with the MAGIC telescopes

PG 1553+113 is a very-high-energy (VHE, $E>100\,\mathrm{GeV}$) $γ$-ray emitter classified as a BL Lac object. Its redshift is constrained by intergalactic absorption lines in the range $0.4<z<0.58$. The MAGIC telescopes have monitored the source's activity since 2005. In early 2012, PG 1553+113 was found in a high-state, and later, in April of the same year, the source reached its highest VHE flux state detected so far. Simultaneous observations carried out in X-rays during 2012 April show similar flaring behaviour. In contrast, the $γ$-ray flux at $E<100\,\mathrm{GeV}$ observed by Fermi-LAT is compatible with steady emission. In this paper, a detailed study of the flaring state is presented. The VHE spectrum shows clear curvature, being well fitted either by a power law with an exponential cut-off or by a log-parabola. A simple power-law fit hypothesis for the observed shape of the PG 1553+113 VHE $γ$-ray spectrum is rejected with a high significance (fit probability P=2.6 $\times 10^{-6}$). The observed curvature is compatible with the extragalactic background light (EBL) imprint predicted by current generation EBL models assuming a redshift $z\sim0.4$. New constraints on the redshift are derived from the VHE spectrum. These constraints are compatible with previous limits and suggest that the source is most likely located around the optical lower limit, $z=0.4$, based on the detection of Ly$α$ absorption. Finally, we find that the synchrotron self-Compton (SSC) model gives a satisfactory description of the observed multi-wavelength spectral energy distribution during the flare.

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