arXiv subjects

Ruben Salvaterra

Publications and source records attributed to Ruben Salvaterra.

At least 37 records · Page 2Linked to original sources

arXiv2021

Properties of High-Redshift GRBs

The immense power of gamma-ray bursts (GRBs) make them ideal probes of the early universe. By using absorption lines in the afterglows of high-redshift GRBs, astronomers can study the evolution of metals in the early universe. With an understanding of the nature of GRB progenitors, the rate and properties of GRBs observed at high redshift can probe the star formation history and the initial mass function of stars at high redshift. This paper presents a detailed study of the metallicity- and mass-dependence of the properties of long-duration GRBs under the black-hole accretion disk paradigm to predict the evolution of these properties with redshift. These models are calibrated on the current GRB observations and then used to make predictions for new observations and new missions (e.g. the proposed Gamow mission) studying high-redshift GRBs.

astro-ph.HE↗

arXiv2021

Scientific simulations and optimization of the XGIS instrument on board THESEUS

The XGIS (X and Gamma Imaging Spectrometer) is one of the three instruments onboard the THESEUS mission (ESA M5, currently in Phase-A). Thanks to its wide field of view and good imaging capabilities, it will efficiently detect and localize gamma-ray bursts and other transients in the 2-150 keV sky, and also provide spectroscopy up to 10 MeV. Its current design has been optimized by means of scientific simulations based on a Monte Carlo model of the instrument coupled to a state-of-the-art description of the populations of long and short GRBs extending to high redshifts. We describe the optimization process that led to the current design of the XGIS, based on two identical units with partially overlapping fields of view, and discuss the expected performance of the instrument.

astro-ph.IM↗

arXiv2019

A science gateway for Exploring the X-ray Transient and variable sky using EGI Federated Cloud

Modern soft X-ray observatories can yield unique insights into time domain astrophysics, and a huge amount of information is stored - and largely unexploited - in data archives. Like a treasure-hunt, the EXTraS project harvested the hitherto unexplored temporal domain information buried in the serendipitous data collected by the European Photon Imaging Camera instrument onboard the ESA XMM-Newton, in 16 years of observations. All results have been released to the scientific community, together with new software analysis tools. This paper presents the architecture of the EXTraS science gateway, that has the goal to provide the software to the scientific community through a Web based portal using the EGI Federated Cloud infrastructure. The main focus is on the light software architecture of the portal and on the technological insights for an effective use of the EGI ecosystem.

astro-ph.IM↗

arXiv2019

A citizen science exploration of the X-ray transient sky using the EXTraS science gateway

Modern soft X-ray observatories can yield unique insights into time domain astrophysics, and a huge amount of information is stored - and largely unexploited - in data archives. Like a treasure-hunt, the EXTraS project harvested the hitherto unexplored temporal domain information buried in the serendipitous data collected by the European Photon Imaging Camera instrument onboard the XMM- Newton satellite in 20 years of observations. The result is a vast catalogue, describing the temporal behaviour of hundreds of thousands of X-ray sources. But the catalogue is just a starting point because it has to be, in its turn, further analysed. During the project an education activity has been defined and run in several workshops for high school students in Italy, Germany and UK. The final goal is to engage the students, and in perspective citizen scientists, to go through the whole validation process: they look into the data and try to discover new sources, or to characterize already known sources. This paper describes how the EXTraS science gateway is used to accomplish these tasks and highlights the first discovery, a flaring X-ray source in the globular cluster NGC 6540.

astro-ph.IM↗

arXiv2019

Diffuse X-ray emission around an ultraluminous X-ray pulsar

Ultraluminous X-ray sources (ULXs) are extragalactic X-ray emitters located off-center of their host galaxy and with a luminosity in excess of a few ${10^{39}\text{ erg s}^{-1}}$, if emitted isotropically. The discovery of periodic modulation revealed that in some ULXs the accreting compact object is a neutron star, indicating luminosities substantially above their Eddington limit. The most extreme object in this respect is ${NGC 5907~ULX-1}$ (ULX1), with a peak luminosity that is 500 times its Eddington limit. During a Chandra observation to probe a low state of ULX1, we detected diffuse X-ray emission at the position of ULX1. Its diameter is $2.7 \pm 1.0$ arcsec and contains 25 photons, none below 0.8 keV. We interpret this extended structure as an expanding nebula powered by the wind of ULX1. Its diameter of about ${200\text{ pc}}$, characteristic energy of ${\sim 1.9\text{ keV}}$, and luminosity of ${\sim 2\times10^{38}\text{ erg s}^{-1}}$ imply a mechanical power of ${1.3\times10^{41}\text{ erg s}^{-1}}$ and an age ${\sim 7 \times 10^{4}\text{ yr}}$. This interpretation suggests that a genuinely super-Eddington regime can be sustained for time scales much longer than the spin-up time of the neutron star powering the system. As the mechanical power from a single ULX nebula can rival the injection rate of cosmic rays of an entire galaxy, ULX nebulae could be important cosmic ray accelerators.

astro-ph.HE↗

arXiv2019

Cosmic Dawn and Reionization: Astrophysics in the Final Frontier

The cosmic dawn and epoch of reionization mark the time period in the universe when stars, galaxies, and blackhole seeds first formed and the intergalactic medium changed from neutral to an ionized one. Despite substantial progress with multi-wavelength observations, astrophysical process during this time period remain some of the least understood with large uncertainties on our existing models of galaxy, blackhole, and structure formation. This white paper outlines the current state of knowledge and anticipated scientific outcomes with ground and space-based astronomical facilities in the 2020s. We then propose a number of scientific goals and objectives for new facilities in late 2020s to mid 2030s that will lead to definitive measurements of key astrophysical processes in the epoch of reionization and cosmic dawn.

astro-ph.GA↗

arXiv2018

The first orbital period of a very bright and fast Nova in M31: M31N 2013-01b

We present the first X-ray and UV/optical observations of a very bright and fast nova in the disc of M31, M31N 2013-01b. The nova reached a peak magnitude $R\sim$15 mag and decayed by 2 magnitudes in only 3 days, making it one of the brightest and fastest novae ever detected in Andromeda. From archival multi-band data we have been able to trace its fast evolution down to $U>21$ mag in less than two weeks and to uncover for the first time the Super-Soft X-ray phase, whose onset occurred 10-30 days from the optical maximum. The X-ray spectrum is consistent with a blackbody with a temperature of $\sim$50 eV and emitting radius of $\sim$4$\times 10^{9}$ cm, larger than a white dwarf radius, indicating an expanded region. Its peak X-ray luminosity, 3.5$\times 10^{37}$ erg s$^{-1}$, locates M31N 2013-01b among the most luminous novae in M31. We also unambiguously detect a short 1.28$\pm$0.02 h X-ray periodicity that we ascribe to the binary orbital period, possibly due to partial eclipses. This makes M31N 2013-01b the first nova in M31 with an orbital period determined. The short period also makes this nova one of the few known below the 2-3 h orbital period gap. All the observed characteristics strongly indicate that M31N 2013-01b harbours a massive white dwarf and a very low-mass companion, consistent with being a nova belonging to the disc population of the Andromeda Galaxy.

astro-ph.HE↗

arXiv2018

EXTraS discovery of a peculiar flaring X-ray source in the Galactic globular cluster NGC 6540

We report the discovery of a flaring X-ray source in the globular cluster NGC 6540, obtained during the EXTraS project devoted to a systematic search for variability in archival data of the XMM-Newton satellite. The source had a quiescent X-ray luminosity of the order of ~10^32 erg/s in the 0.5-10 keV range (for a distance of NGC 6540 of 4 kpc) and showed a flare lasting about 300 s. During the flare, the X-ray luminosity increased by more than a factor 40, with a total emitted energy of ~10^36 erg. These properties, as well as Hubble Space Telescope photometry of the possible optical counterparts, suggest the identification with a chromospherically active binary. However, the flare luminosity is significantly higher than what commonly observed in stellar flares of such a short duration, leaving open the possibility of other interpretations.

astro-ph.HE↗

arXiv2016

The dependence of gamma-ray burst X-ray column densities on the model for Galactic hydrogen

We study the X-ray absorption of a complete sample of 99 bright Swift gamma-ray bursts. Over the last few years, a strong correlation between the intrinsic X-ray absorbing column density (N_H(z)) and the redshift was found. This absorption excess in high-z GRBs is now thought to be due to the overlooked contribution of the absorption along the intergalactic medium, by means of both intervening objects and the diffuse warm-hot intergalactic medium along the line of sight. In this work we neglect the absorption along the IGM, because our purpose is to study the eventual effect of a radical change in the Galactic absorption model on the N_H(z) distribution. Therefore, we derive the intrinsic absorbing column densities using two different Galactic absorption models, the Leiden Argentine Bonn HI survey and the more recent model including molecular hydrogen. We find that, if on the one hand the new Galactic model considerably affects the single column density values, on the other hand there is no drastic change in the distribution as a whole. It becomes clear that the contribution of Galactic column densities alone, no matter how improved, is not sufficient to change the observed general trend and it has to be considered as a second-order correction. The cosmological increase of N_H(z) as a function of redshift persists and, in order to explain the observed distribution, it is necessary to include the contribution of both the diffuse intergalactic medium and the intervening systems along the line of sight of the GRBs.

astro-ph.HE↗

arXiv2016

Updated analysis of near-infrared background fluctuations

The power spectrum of Near InfraRed Background (NIRB) fluctuations measured at 3.6 $μ$m by {\tt Spitzer} shows a clustering excess over the known galaxies signal that has been interpreted in terms of early ($z\simgt 13$), accreting (direct collapse) black holes (DCBH) or low-$z$ intrahalo light (IHL). In addition, these fluctuations correlate with the cosmic X-ray background (CXB) measured at (0.5-2) keV, supporting the black hole explanation. This scenario has been questioned by the recent detection of a correlation between the two {\tt CIBER} 1.1/1.6 $μ$m bands with the 3.6 $μ$m {\tt Spitzer} one. This correlation is hardly explained by early DCBHs that, due to intergalactic absorption, cannot contribute to the shortest wavelength bands. Here we show that the new correlation is caused instead by a Diffuse Galactic Light (DGL) component arising from Galactic stellar light scattered by dust. The black hole interpretation of the excess remains perfectly valid and, actually, the inclusion of DGL allows less demanding (by up to about 30\%) requirements on the DCBH abundance/mass.

astro-ph.CO↗

arXiv2015

Results from DROXO IV. EXTraS discovery of an X-ray flare from the Class I protostar candidate ISO-Oph 85

X-ray emission from Young Stellar Objects (YSOs) is crucial to understand star formation. A very limited amount of X-ray results is available for the protostellar (ClassI) phase. A systematic search of transient X-ray phenomena combined with a careful evaluation of the evolutionary stage offer a widely unexplored window to our understanding of YSOs X-ray properties. Within the EXTraS project, a search for transients and variability in the whole XMM-Newton archive, we discover transient X-ray emission consistent with ISO-Oph 85, a strongly embedded YSO in the rho Ophiuchi region, not detected in previous time-averaged X-ray studies. We extract an X-ray light curve for the flare and determine its spectral parameters from XMM-Newton/EPIC (European Photon Imaging Camera) data using quantile analysis. The X-ray flare ($2500\,s$), the only one detected in the XMM-Newton archive for ISO-Oph 85, has a luminosity of $LogL_X[erg/s]=31.1$ and a spectrum consistent with a highly-absorbed one-component thermal model ($N_H=1.0^{+1.2}_{-0.5}10^{23}\,cm^{-2}$, $kT=1.15^{+2.35}_{-0.65}\,keV)$. We set an upper limit of $LogL_X[erg/s]<29.5$ to the quiescent X-ray luminosity. We build a SED with IR to mm photometry drawn from literature and mid-IR Spitzer and sub-mm Herschel photometry analysed by us, and compare it with pre-computed models. The sub-mm emission peak in the Herschel data suggests that the object is a ClassI protostar. However, the Herschel/IR position offset is larger than for other YSOs in the region, leaving some doubt on the association. This is the first X-ray flare from a YSO recognised as a candidate ClassI YSO via the analysis of its complete SED. This work shows how the analysis of the whole SED is fundamental for the classification of YSOs, and how the X-ray source detection techniques we developed can open a new era in time-resolved analysis of the X-ray emission from stars.

astro-ph.SR↗

arXiv2015

High redshift AGNs and HI reionisation: limits from the unresolved X-ray background

The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI reionisation by z>6. A galaxy dominated background, however, may require that the escape fraction of Lyman continuum radiation from high redshift galaxies is as high as 10%, a value somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we question the latter hypothesis, and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z=0, can provide a fraction of the HII filling factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies >10%.

astro-ph.CO↗

arXiv2014

Ultra-faint high-redshift galaxies in the Frontier Fields

By combining cosmological simulations with Frontier Fields project lens models we find that, in the most optimistic case, galaxies as faint as $m \approx 33 - 34$ (AB magnitude at $1.6 \rm μm$) can be detected in the Frontier Fields. Such faint galaxies are hosted by dark matter halos of mass $\sim10^9 M_\odot$ and dominate the ionizing photon budget over currently observed bright galaxies, thus allowing for the first time the investigation of the dominant reionization sources. In addition, the observed number of these galaxies can be used to constrain the role of feedback in suppressing star formation in small halos: for example, if galaxy formation is suppressed in halos with circular velocity $v_c < 50$ km s$^{-1}$, galaxies fainter than $m=31$ should not be detected in the FFs.

astro-ph.CO↗

arXiv2014

The brief era of direct collapse black hole formation

It has been proposed that the first, intermediate-mass ($\approx 10^{5-6}~M_\odot$) black holes might form through direct collapse of unpolluted gas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton-thick, photons above 13.6 eV are largely absorbed and re-processed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the "DCBH era" at $z \approx 20$ when a large fraction of atomic-cooling halos are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations we show that in this scenario the DCBH mass density rises from $\sim 5$~$M_\odot$ Mpc$^{-3}$ at $z\sim 30$ to the peak value $\sim5\times10^5 M_\odot$ Mpc$^{-3}$ at $z \sim 14$ in our fiducial model. However, the abundance of \textit{active} (accreting) DCBHs drops after $z \sim 14$, as gas in the potential formation sites (unpolluted halos with virial temperature slightly above $10^4$~K) is photoevaporated. This effect almost completely suppresses DCBH formation after $z\sim 13$. The DCBH formation era lasts only $\approx 150$ Myr, but it might crucially provide the seeds of the supermassive black holes (SMBHs) powering $z\sim6$ quasars.

astro-ph.CO↗

arXiv2013

GRB 081007 and GRB 090424: the surrounding medium, outflows and supernovae

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Γ~ 200, while for GRB 090424 a lower limit of Γ> 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.

astro-ph.HE↗

arXiv2013

The Hot and Energetic Universe: The formation and growth of the earliest supermassive black holes

A crucial challenge in astrophysics over the coming decades will be to understand the origins of supermassive black holes (SMBHs) that lie at the centres of most, if not all, galaxies. The processes responsible for the initial formation of these SMBHs and their early growth via accretion - when they are seen as Active Galactic Nuclei (AGN) - remain unknown. To address this challenge, we must identify low luminosity and obscured z>6 AGNs, which represent the bulk of early SMBH growth. Sensitive X-ray observations are a unique signpost of accretion activity, uncontaminated by star formation processes, which prevent reliable AGN identification at other wavelengths (e.g. optical, infrared). The Athena+ Wide Field Imager will enable X-ray surveys to be carried out two orders of magnitude faster than with Chandra or XMM-Newton, opening a new discovery space and identifying over 400 z>6 AGN, including obscured sources. Athena+ will also play a fundamental role to enhance the scientific return of future multiwavelength facilities that will probe the physical conditions within the host galaxies of early SMBHs, which is vital for understanding how SMBHs form, what fuels their subsequent growth, and to assess their impact on the early Universe. Follow-up of samples of z>6 galaxies with the Athena+ X-ray Integral Field Unit could also reveal the presence of highly obscured AGNs, thanks to the detection of strong iron lines. Thus, Athena+ will enable the first quantitative measurements of the extent and distribution of SMBH accretion in the early Universe.

astro-ph.HE↗

arXiv2013

Infrared background signatures of the first black holes

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are observed up to scales $\simlt 1^{\ensuremath{^{\circ}}}$. Their interpretation is challenging as even after removing the contribution from detected sources, the residual signal is $>10$ times higher than expected from distant galaxies below the detection limit and first stars. We propose here a novel interpretation in which early, intermediate mass, accreting direct collapse black holes (DCBH), which are too faint to be detected individually in current surveys, could explain the observed fluctuations. We find that a population of highly obscured ($N_{\rm H}\simgt 10^{25} \rm cm^{-2}$) DCBHs formed in metal-free halos with virial temperature $10^4$ K at $z\simgt 12$, can explain the observed level $\approx 10^{-3}$ (nW m$^{-2}$ sr$^{-1})^2$ of the 3.6 and 4.5 $μ$m fluctuations on scales $>100''$. The signal on smaller scales is instead produced by undetected galaxies at low and intermediate redshifts. Albeit Compton-thick, at scales $θ> 100''$ DCBHs produce a CXB (0.5-2 keV)-NIRB ($4.5 \rm μm$) cross-correlation signal of $\simeq 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ nW m$^{-2}$ sr$^{-1}$ slightly dependent on the specific value of the absorbing gas column ($N_{\rm H} \approx 10^{25} \rm cm^{-2}$) adopted and in agreement with the recent measurements by \cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is dominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same low-$z$, undetected galaxies accounting for small scale NIRB fluctuations. These results outline the great potential of the NIRB as a tool to investigate the nature of the first galaxies and black holes.

astro-ph.CO↗

arXiv2013

The contribution of high redshift galaxies to the Near-Infrared Background

Several independent measurements have confirmed the existence of fluctuations ($δF_{\rm obs}\approx 0.1 \rm nW/m^{2}/sr$ at $3.6 \rm μm$) up to degree angular scales in the source-subtracted Near InfraRed Background (NIRB) whose origin is unknown. By combining high resolution cosmological N-body/hydrodynamical simulations with an analytical model, and by matching galaxy Luminosity Functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-$z$ ($z > 5$) galaxies (some of which harboring Pop III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of ionizing photon escape fraction: we find $f_{\rm esc} = 1$ at $z \ge 11$, decreasing to $\approx 0.05$ at $z = 5$. In the wavelength range $1.0-4.5 \rm μm$, the predicted cumulative flux is $F =0.2-0.04 \rm nW/m^2/sr$. However, we find that the radiation from high-$z$ galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at $l=2000$, the fluctuation level due to $z > 5$ galaxies is $δF = 0.01-0.002 \rm nW/m^2/sr$, with a relative wavelength-independent amplitude $δF/F = 4$%. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-$z$ galaxies, dominates the source-subtracted NIRB fluctuation signal.

astro-ph.CO↗