arXiv subjects

G. Cusumano

Publications and source records attributed to G. Cusumano.

At least 19 recordsLinked to original sources

arXiv2023

Swift/UVOT discovery of Swift J221951-484240: a UV luminous ambiguous nuclear transient

We report the discovery of Swift J221951-484240 (hereafter: J221951), a luminous slow-evolving blue transient that was detected by the Neil Gehrels Swift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during the follow-up of Gravitational Wave alert S190930t, to which it is unrelated. Swift/UVOT photometry shows the UV spectral energy distribution of the transient to be well modelled by a slowly shrinking black body with an approximately constant temperature of T~2.5x10^4 K. At a redshift z=0.5205, J221951 had a peak absolute magnitude of M_u,AB = -23 mag, peak bolometric luminosity L_max=1.1x10^45 erg s^-1 and a total radiated energy of E>2.6x10^52 erg. The archival WISE IR photometry shows a slow rise prior to a peak near the discovery date. Spectroscopic UV observations display broad absorption lines in N V and O VI, pointing toward an outflow at coronal temperatures. The lack of emission in the higher H~Lyman lines, N I and other neutral lines is consistent with a viewing angle close to the plane of the accretion or debris disc. The origin of J221951 can not be determined with certainty but has properties consistent with a tidal disruption event and the turn-on of an active galactic nucleus.

astro-ph.HE↗

arXiv2022

ASTRI Mini-Array Core Science at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Project led by the Italian National Institute for Astrophysics (INAF) is developing and will deploy at the Observatorio del Teide a mini-array (ASTRI Mini-Array) composed of nine telescopes similar to the small-size dual-mirror Schwarzschild-Couder telescope (ASTRI-Horn) currently operating on the slopes of Mt. Etna in Sicily. The ASTRI Mini-Array will surpass the current Cherenkov telescope array differential sensitivity above a few tera-electronvolt (TeV), extending the energy band well above hundreds of TeV. This will allow us to explore a new window of the electromagnetic spectrum, by convolving the sensitivity performance with excellent angular and energy resolution figures. In this paper we describe the Core Science that we will address during the first four years of operation, providing examples of the breakthrough results that we will obtain when dealing with current open questions, such as the acceleration of cosmic rays, cosmology and fundamental physics and the new window, for the TeV energy band, of the time-domain astrophysics.

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arXiv2022

Extragalactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI Mini-Array is a next-generation system of nine imaging atmospheric Cherenkov telescopes that is going to be built at the Observatorio del Teide site. After a first phase, in which the instrument will be operated as an experiment prioritizing a schedule of primary science cases, an observatory phase is foreseen in which other significant targets will be pointed. We focus on the observational feasibility of extragalactic sources and on astrophysical processes that best complement and expand the ASTRI Mini-Array core science, presenting the most relevant examples that are at reach of detection over long-term time scales and whose observation can provide breakthrough achievements in the very-high energy extragalactic science. Such examples cover a wide range of $γ$-ray emitters, including the study of AGN low states in the multi-TeV energy range, the possible detection of Seyfert galaxies with long exposures and the searches of dark matter lines above 10 TeV. Simulations of the presented objects show that the instrument performance will be competitive at multi-TeV energies with respect to current arrays of Cherenkov telescopes.

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arXiv2022

Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Mini-Array will be composed of nine imaging atmospheric Cherenkov telescopes at the Observatorio del Teide site. The array will be best suited for astrophysical observations in the 0.3-200 TeV range with an angular resolution of few arc-minutes and an energy resolution of 10-15\%. A core-science programme in the first four years will be devoted to a limited number of key targets, addressing the most important open scientific questions in the very-high energy domain. At the same time, thanks to a wide field of view of about 10 degrees, ASTRI Mini-Array will observe many additional field sources, which will constitute the basis for the long-term observatory programme that will eventually cover all the accessible sky. In this paper, we review different astrophysical Galactic environments, e.g. pulsar wind nebulae, supernova remnants, and gamma-ray binaries, and show the results from a set of ASTRI Mini-Array simulations of some of these field sources made to highlight the expected performance of the array (even at large offset angles) and the important additional observatory science that will complement the core-science program.

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arXiv2022

A transient ultraluminous X-ray source in NGC 55

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10$^{39}$ erg s$^{-1}$. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 10$^{38}$ erg s$^{-1}$ in a 2010 XMM-Newton observation. Thanks to new and deeper observations ($\sim$ 130 ks each), we show that the source reaches a luminosity peak $>1.6 \times 10^{39}$ erg s$^{-1}$. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well described using a model with two thermal components, as often found in ULXs. The time scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.

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arXiv2021

Multiple accelerated particle populations in the Cygnus Loop with Fermi-LAT

The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been studied in detail from radio to $γ$-ray emission. The $γ$ -rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds. We present an analysis of the $γ$-ray emission detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope over 11 years in the region of the Cygnus Loop. We performed detailed morphological and spectral studies of the $γ$-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ray, UV, optical, and radio images. The higher statistics with respect to the previous studies enabled us to decompose the emission from the remnant into two morphological components to model its nonthermal multiwavelength emission. The extended $γ$-ray emission is well correlated with the thermal X-ray and UV emission of the SNR. Our morphological analysis reveals that a model considering two contributions from the X-ray and the UV emission regions is the best description of the $γ$-ray data. Both components show a curved spectrum, but the X-ray component is softer and more curved than the UV component, suggesting a different physical origin. The multiwavelength modeling of emission toward the SNR suggests that the nonthermal radio and $γ$-ray emission associated with the UV component is mostly due to the reacceleration of preexisting cosmic rays by radiative shocks in the adjacent clouds, while the nonthermal emission associated with the X-ray component arises from freshly accelerated cosmic rays.

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arXiv2021

Swift/UVOT follow-up of Gravitational Wave Alerts in the O3 era

In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the Gravitational Wave alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and analysis of candidate optical/UV sources. For the O3 period, we also provide a statistical overview and report on serendipitous sources discovered by Swift/UVOT. Swift followed 18 gravitational-wave candidate alerts, with UVOT observing a total of 424 deg^2. We found 27 sources that changed in magnitude at the 3 sigma level compared with archival u or g-band catalogued values. Swift/UVOT also followed up a further 13 sources reported by other facilities during the O3 period. Using catalogue information, we divided these 40 sources into five initial classifications: 11 candidate active galactic nuclei (AGN)/quasars, 3 Cataclysmic Variables (CVs), 9 supernovae, 11 unidentified sources that had archival photometry and 6 uncatalogued sources for which no archival photometry was available. We have no strong evidence to identify any of these transients as counterparts to the GW events. The 17 unclassified sources are likely a mix of AGN and a class of fast-evolving transient, and one source may be a CV.

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arXiv2021

Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

It is thought that ultraluminous X-ray sources (ULXs) are mainly powered by super-Eddington accreting neutron stars or black holes as shown by recent discovery of X-ray pulsations and relativistic winds. This work presents a follow up study of the spectral evolution over two decades of the pulsing ULX NGC 1313 X-2, in order to understand the structure of the accretion disc. The primary objective is to determine the shape and nature of the dominant spectral components by investigating their variability with the changes in the source luminosity. We have performed a spectral analysis over the canonical 0.3-10 keV energy band of all the high signal-to-noise XMM-Newton observations, and we have tested a number of different spectral models, which should approximate super-Eddington accretion discs. The baseline model consists of two thermal blackbody components with different temperatures plus an exponential cutoff powerlaw. In particular, the hotter and brighter thermal component describes the emission from the super-Eddington inner disc and the cutoff powerlaw the contribution from the accretion column of the neutron star. Instead, the cooler component describes the emission from the outer region of the disc close to the spherisation radius and the wind. The luminosity-temperature relation for the cool component follows a negative trend, which is not consistent with L$\propto$T$^4$, as expected from a sub-Eddington thin disc of Shakura-Sunayev, nor with L$\propto$T$^2$, as expected for advection-dominated disc, but would rather agree with a wind-dominated X-ray emitting region. Instead, the (L,T) relation for the hotter component is somewhere in between the first two theoretical scenarios. Our findings agree with the super-Eddington scenario and provide further detail on the disc structure. The source spectral evolution is qualitatively similar to that seen in NGC1313 X-1 and HolmbergIX X-1.

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arXiv2020

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, and the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg (assuming typical GRB parameters).

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arXiv2020

A search for optical and near-infrared counterparts of the compact binary merger GW190814

We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on August 14th, 2019. This signal has the best localisation of any observed gravitational wave (GW) source, with a 90% probability area of 18.5 deg$^2$, and an estimated distance of ~ 240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88% of the probability area down to a limiting magnitude of $w$ = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multi-colour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet's axis is disfavoured by the multi-wavelength dataset, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1 M$_{\odot}$) fast-moving (mean velocity >= 0.3c) wind ejecta for a possible kilonova associated with this merger.

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arXiv2020

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 x 10^{-12} erg cm^{-2} s^{-1} over 0.3-10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent.

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arXiv2020

Swift unveils the orbital period of IGR J18214-1318

We analysed 13 years of the Neil Gehrels Swift Observatory survey data collected on the High Mass X-ray Binary IGR J18214-1318. Performing the timing analysis we detected a periodic signal of 5.42 d. From the companion star characteristics we derived an average orbital separation of $\sim 41 \rm R_{\odot}\simeq 2 R_{\star}$. The spectral type of the companion star (O9) and the tight orbital separation suggest that IGR~J18214-1318 is a wind accreting source with eccentricity lower than 0.17. The intensity profile folded at the orbital period shows a deep minimum compatible with an eclipse of the source by the companion star. In addition, we report on the broad-band 0.6--100 keV spectrum using data from XMM-Newton, NuSTAR, and Swift, applying self-consistent physical models. We find that the spectrum is well fitted either by a pure thermal Comptonization component, or, assuming that the source is a neutron star accreting above the critical regime, by a combined thermal and bulk-motion Comptonization model. In both cases, the presence of a local neutral absorption (possibly related to the thick wind of the companion star) is required.

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arXiv2020

Scientific prospects for a mini-array of ASTRI telescopes: a gamma-ray TeV data challenge

ASTRI is a project aiming at the realization of a gamma-ray imaging Cherenkov telescope that observes the sky in the TeV band. Recently, the development of a mini-array (MA) of ASTRI telescopes has been funded by the Istituto Nazionale di Astrofisica. The ASTRI Comprehensive Data Challenge (ACDC) project aims at optimizing the scientific exploitation and analysis techniques of the ASTRI MA, by performing a complete end-to-end simulation of a tentative scientific program, from the generation of suitable instrument response functions to the proposal, selection, analysis, and interpretation of the simulated data. We assumed that the MA will comprise nine ASTRI telescopes arranged in a (almost) square geometry (mean distance between telescopes of ~250m). We simulated three years of observations, adopting a realistic pointing plan that takes into account, for each field, visibility constraints for an assumed site in Paranal (Chile) and observational time slots in dark sky conditions. We simulated the observations of nineteen Galactic and extragalactic fields selected for their scientific interest, including several classes of objects (such as pulsar wind nebulae, supernova remnants, gamma-ray binaries etc), for a total of 81 point-like and extended sources. Here we present an overview of the ACDC project, providing details on the different software packages needed to carry out the simulated three-years operation of the ASTRI MA. We discuss the results of a systematic analysis applied on the whole simulated data, by making use of prototype science tools widely adopted by the TeV astronomical community. Furthermore, particular emphasis is also given to some targets used as benchmarks.

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arXiv2020

First detection of the Crab Nebula at TeV energies with a Cherenkov telescope in a dual-mirror Schwarzschild-Couder configuration: the ASTRI-Horn telescope

We report on the first detection of very high-energy (VHE) gamma-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Couder (SC) configuration. The result has been achieved by means of the 4 m size ASTRI-Horn telescope, operated on Mt. Etna (Italy) and developed in the context of the Cherenkov Telescope Array Observatory preparatory phase. The dual-mirror SC design is aplanatic and characterized by a small plate scale, allowing us to implement large field of view cameras with small-size pixel sensors and a high compactness. The curved focal plane of the ASTRI camera is covered by silicon photo-multipliers (SiPMs), managed by an unconventional front-end electronics based on a customized peak-sensing detector mode. The system includes internal and external calibration systems, hardware and software for control and acquisition, and the complete data archiving and processing chain. The observations of the Crab Nebula were carried out in December 2018, during the telescope verification phase, for a total observation time (after data selection) of 24.4 h, equally divided into on- and off-axis source exposure. The camera system was still under commissioning and its functionality was not yet completely exploited. Furthermore, due to recent eruptions of the Etna Volcano, the mirror reflection efficiency was reduced. Nevertheless, the observations led to the detection of the source with a statistical significance of 5.4 sigma above an energy threshold of ~3 TeV. This result provides an important step towards the use of dual-mirror systems in Cherenkov gamma-ray astronomy. A pathfinder mini-array based on nine large field-of-view ASTRI-like telescopes is under implementation.

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arXiv2019

Swift-XRT Follow-up of Gravitational Wave Triggers in the Second Advanced LIGO/Virgo Observing Run

The Neil Gehrels Swift Observatory carried out prompt searches for gravitational wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW 170814 and the epochal GW 170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW 170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2, and how these are being used to improve the \swift\ follow-up of GW events. In particular, we simulate a population of GRB afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncatalogued X-ray sources. We find that $\approx$60-70% of afterglows whose jets are oriented towards Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.

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arXiv2019

Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout

The Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based very-high-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions.

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arXiv2018

X-ray, UV, and optical observations of the accretion disk and boundary layer in the symbiotic star RT Cru

Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer material to white dwarfs (WDs) has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the WD to hide them. RT Cru is the prototype of such non-burning symbiotics, with its hard (δ-type) X-ray emission providing a view of its innermost accretion structures. In the past 20 yr, RT Cru has experienced two similar optical brightening events, separated by 4000 days and with amplitudes of ΔV 1.5 mag. After Swift became operative, the Burst Alert Telescope (BAT) detector revealed a hard X-ray brightening event almost in coincidence with the second optical peak. Spectral and timing analyses of multi-wavelength observations that we describe here, from NuSTAR, Suzaku, Swift/X-Ray Telescope (XRT) + BAT + UltraViolet Optical Telescope (UVOT) (photometry) and optical photometry and spectroscopy, indicate that accretion proceeds through a disk that reaches down to the WD surface. The scenario in which a massive, magnetic WD accretes from a magnetically truncated accretion disk is not supported. For example, none of our data show the minute-time-scale periodic modulations (with tight upper limits from X-ray data) expected from a spinning, magnetic WD. Moreover, the similarity of the UV and X-ray fluxes, as well as the approximate constancy of the hardness ratio within the BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout the brightening event, during which the rate of accretion onto the WD increased to 6.7 $\times$ 10-9 Msun yr^{-1} (d/2 kpc)^2. (Abridged abstract version)

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arXiv2018

Dramatic change in the boundary layer in the symbiotic recurrent nova T Coronae Borealis

A sudden increase in the rate at which material reaches the most internal part of an accretion disk, i.e. the boundary layer, can change its structure dramatically. We have witnessed such change for the first time in the symbiotic recurrent nova T CrB. Our analysis of XMM-Newton, Swift Burst Alert Telescope (BAT)/ X-Ray Telescope (XRT) / UltraViolet Optical Telescope (UVOT) and American Association of Variable Stars Observers (AAVSO) V and B-band data indicates that during an optical brightening event that started in early 2014 ($Δ$ V$\approx$1.5): (i) the hard X-ray emission as seen with BAT almost vanished; (ii) the XRT X-ray flux decreased significantly while the optical flux remained high; (iii) the UV flux increased by at least a factor of 40 over the quiescent value; and (iv) the X-ray spectrum became much softer and a bright, new, blackbody-like component appeared. We suggest that the optical brightening event, which could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946, is due to a disk instability

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