arXiv subjects

A. Sanna

Publications and source records attributed to A. Sanna.

At least 73 records · Page 4Linked to original sources

arXiv2021

The HERMES-Technologic and Scientific Pathfinder

HERMES-TP/SP (High Energy Rapid Modular Ensemble of Satellites Technologic and Scientific Pathfinder) is a constellation of six 3U nano-satellites hosting simple but innovative X-ray detectors, characterized by a large energy band and excellent temporal resolution, and thus optimized for the monitoring of Cosmic High Energy transients such as Gamma Ray Bursts and the electromagnetic counterparts of Gravitational Wave Events, and for the determination of their positions. The projects are funded by the Italian Ministry of University and Research and by the Italian Space Agency, and by the European Union Horizon 2020 Research and Innovation Program under Grant Agreement No. 821896. HERMES-TP/SP is an in-orbit demonstration, that should be tested starting from 2022. It is intrinsically a modular experiment that can be naturally expanded to provide a global, sensitive all sky monitor for high-energy transients.

astro-ph.HE↗

arXiv2021

Timing techniques applied to distributed modular high-energy astronomy: the HERMES project

The HERMES-TP/SP (High Energy Rapid Modular Ensemble of Satellites -- Technologic and Scientific Pathfinder) is an in-orbit demonstration of the so-called distributed astronomy concept. Conceived as a mini-constellation of six 3U nano-satellites hosting a new miniaturized detector, HERMES-TP/SP aims at the detection and accurate localisation of bright high-energy transients such as Gamma-Ray Bursts. The large energy band, the excellent temporal resolution and the wide field of view that characterize the detectors of the constellation represent the key features for the next generation high-energy all-sky monitor with good localisation capabilities that will play a pivotal role in the future of Multi-messenger Astronomy. In this work, we will describe in detail the temporal techniques that allow the localisation of bright transient events taking advantage of their almost simultaneous observation by spatially spaced detectors. Moreover, we will quantitatively discuss the all-sky monitor capabilities of the HERMES Pathfinder as well as its achievable accuracies on the localisation of the detected Gamma-Ray Bursts.

astro-ph.HE↗

arXiv2020

Evidence of a non-conservative mass transfer in the ultra-compact X-ray source XB 1916-053

The dipping source XB 1916-053 is a compact binary system with an orbital period of 50 min harboring a neutron star. Using ten new {\it Chandra} observations and one {\it Swift/XRT} observation, we are able to extend the baseline of the orbital ephemeris; this allows us to exclude some models that explain the dip arrival times. The Chandra observations provide a good plasma diagnostic of the ionized absorber and allow us to determine whether it is placed at the outer rim of the accretion disk or closer to the compact object. From the available observations we are able to obtain three new dip arrival times extending the baseline of the orbital ephemeris from 37 to 40 years. From the analysis of the dip arrival times we confirm an orbital period derivative of $\dot{P}=1.46(3) \times 10^{-11}$ s s$^{-1}$. We show that the $\dot{P}$ value and the luminosity values are compatible with a mass accretion rate lower than 10\% of the mass transfer rate. We show that the mass ratio $q=m_2/m_1$ of 0.048 explains the apsidal precession period and the nodal precession period. The observed absorption lines are associated with the presence of \ion{Ne}{x}, \ion{Mg}{xii}, \ion{Si}{xiv}, \ion{S}{xvi,} and \ion{Fe}{xxvi} ions. We observe a redshift in the absorption lines between $1.1 \times 10^{-3}$ and $1.3 \times 10^{-3}$. By interpreting it as gravitational redshift, as recently discussed in the literature, we find that the ionized absorber is placed at a distance of $10^8$ cm from the neutron star with a mass of 1.4 M$_{\odot}$ and has a hydrogen atom density greater than $10^{15}$ cm$^{-3}$. (Abstract abridged)

astro-ph.HE↗

arXiv2020

The INTEGRAL view of the pulsating hard X-ray sky: from accreting and transitional millisecond pulsars to rotation-powered pulsars and magnetars

In the last 25 years, a new generation of X-ray satellites imparted a significant leap forward in our knowledge of X-ray pulsars. The discovery of accreting and transitional millisecond pulsars proved that disk accretion can spin up a neutron star to a very high rotation speed. The detection of MeV-GeV pulsed emission from a few hundreds of rotation-powered pulsars probed particle acceleration in the outer magnetosphere, or even beyond. Also, a population of two dozens of magnetars has emerged. INTEGRAL played a central role to achieve these results by providing instruments with high temporal resolution up to the hard X-ray/soft gamma-ray band and a large field of view imager with good angular resolution to spot hard X-ray transients. In this article, we review the main contributions by INTEGRAL to our understanding of the pulsating hard X-ray sky, such as the discovery and characterization of several accreting and transitional millisecond pulsars, the generation of the first catalog of hard X-ray/soft gamma-ray rotation-powered pulsars, the detection of polarization in the hard X-ray emission from the Crab pulsar, and the discovery of persistent hard X-ray emission from several magnetars.

astro-ph.HE↗

arXiv2020

The SEDIGISM survey: Molecular clouds in the inner Galaxy

We use the 13CO(2-1) emission from the SEDIGISM high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the SCIMES algorithm. This work compiles a cloud catalogue with a total of 10663 molecular clouds, 10300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases, and thus require further follow up work in order to be confirmed.

astro-ph.GA↗

arXiv2020

NICER observations reveal that the X-ray transient MAXI J1348-630 is a Black Hole X-ray binary

We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348-630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348-630 is similar to that commonly observed in black hole transients. The source evolved from the hard state, through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the hard state in reverse during the outburst decay. At the end of the outburst, MAXI J1348-630 underwent two reflares with peak fluxes ~1 and ~2 orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the hard state only, without undergoing any state transitions, which is similar to the so-called "failed outbursts". Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348-630 is a black hole candidate.

astro-ph.HE↗

arXiv2020

ESA Voyage 2050 white paper -- GrailQuest: hunting for Atoms of Space and Time hidden in the wrinkle of Space-Time

GrailQuest (Gamma Ray Astronomy International Laboratory for QUantum Exploration of Space-Time) is a mission concept based on a constellation (hundreds/thousands) of nano/micro/small-satellites in low (or near) Earth orbits. Each satellite hosts a non-collimated array of scintillator crystals coupled with Silicon Drift Detectors with broad energy band coverage (keV-MeV range) and excellent temporal resolution ( below or equal 100 nanoseconds) each with effective area around 100 cm2. This simple and robust design allows for mass-production of the satellites of the fleet. This revolutionary approach implies a huge reduction of costs, flexibility in the segmented launching strategy, and an incremental long-term plan to increase the number of detectors and their performance: a living observatory for next-generation, space-based astronomical facilities. GrailQuest is conceived as an all-sky monitor for fast localisation of high signal-to-noise ratio transients in the X/gamma-ray band, e.g. the elusive electromagnetic counterparts of gravitational wave events. Robust temporal triangulation techniques will allow unprecedented localisation capabilities, in the keV-MeV band, of a few arcseconds or below, depending on the temporal structure of the transient event. The ambitious ultimate goal of this mission is to perform the first experiment, in quantum gravity, to directly probe space-time structure down to the minuscule Planck scale, by constraining or measuring a first order dispersion relation for light in vacuo. This is obtained by detecting delays between photons of different energies in the prompt emission of Gamma-ray Bursts.

astro-ph.IM↗

arXiv2020

Ab initio theory of plasmonic superconductivity within the Eliashberg and density-functional formalisms

We extend the two leading methods for the \emph{ab initio} computational descrip tion of phonon-mediated superconductors, namely Eliashberg theory and density fu nctional theory for superconductors (SCDFT), to include plasmonic effects. Furth ermore, we introduce a hybrid formalism in which the Eliashberg approximation fo r the electron-phonon coupling is combined with the SCDFT treatment of the dynam ically screened Coulomb interaction. The methods have been tested on a set of we ll-known conventional superconductors by studying how the plasmon contribution a ffects the phononic mechanism in determining the critical temperature (\tc). Our simulations show that plasmonic SCDFT leads to a good agreement between predict ed and measured \tc's, whereas Eliashberg theory considerably overestimates the plasmon-mediated pairing and, therefore, \tc. The hybrid approach, on the other hand, gives results close to SCDFT and overall in excellent agreement with exper iments.

cond-mat.supr-con↗

arXiv2020

Reflection component in the Bright Atoll Source GX 9+9

GX 9+9 (4U 1728-16) is a low mass X-ray binary (LMXB) source harboring a neutron star. Although it belongs to the subclass of the bright Atoll sources together with GX 9+1, GX 3+1, and GX 13+1, its broadband spectrum is poorly studied and apparently does not show reflection features in the spectrum. To constrain the continuum well and verify whether a relativistic smeared reflection component is present, we analyze the broadband spectrum of GX 9+9 using {\it BeppoSAX} and \textit{XMM-Newton} spectra covering the 0.3-40 keV energy band. We fit the spectrum adopting a model composed of a disk-blackbody plus a Comptonized component whose seed photons have a blackbody spectrum (Eastern Model). A statistically equivalent model is composed of a Comptonized component whose seed photons have a disk-blackbody distribution plus a blackbody that mimics a saturated Comptonization likely associated with a boundary layer (Western model). Other trials did not return a good fit. The spectrum of GX 9+9 was observed in a soft state and its luminosity is $2.3 \times 10^{37}$ erg s$^{-1}$ assuming a distance to the source of 5 kpc. In the Eastern Model scenario, we find the seed-photon temperature and electron temperature of the Comptonized component to be $1.14^{+0.10}_{-0.07}$ keV and $2.80^{+0.09}_{-0.04}$ keV, respectively, while the optical depth of the Comptonizing corona is $8.9\pm0.4$. The color temperature of the inner accretion disk is $0.86^{+0.08}_{-0.02}$ keV and $0.82 \pm 0.02$ keV for the {\it BeppoSAX} and \textit{XMM-Newton} spectrum, respectively. In the Western Model scenario, instead, we find that the seed-photon temperature is $0.87 \pm 0.07$ keV and $1.01 \pm 0.08$ keV for the {\it BeppoSAX} and \textit{XMM-Newton} spectrum, respectively. (Abridged abstract)

astro-ph.HE↗

arXiv2020

Timing of the accreting millisecond pulsar IGR J17591-2342: evidence of spin-down during accretion

We report on the phase-coherent timing analysis of the accreting millisecond X-ray pulsar IGR J17591-2342, using Neutron Star Interior Composition Explorer (NICER) data taken during the outburst of the source between 2018 August 15 and 2018 October 17. We obtain an updated orbital solution of the binary system. We investigate the evolution of the neutron star spin frequency during the outburst, reporting a refined estimate of the spin frequency and the first estimate of the spin frequency derivative ($\dotν \sim -7\times 10^{-14}$ Hz s$^{-1}$), confirmed independently from the modelling of the fundamental frequency and its first harmonic. We further investigate the evolution of the X-ray pulse phases adopting a physical model that accounts for the accretion material torque as well as the magnetic threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. From this analysis we estimate the neutron star magnetic field $B_{eq} = 2.8(3)\times10^{8}$ G. Finally, we investigate the pulse profile dependence on energy finding that the observed behaviour of the pulse fractional amplitude and lags as a function of energy are compatible with a thermal Comptonisation of the soft photons emitted from the neutron star caps.

astro-ph.HE↗

arXiv2019

Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way

We compile and analyze ~200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars. These measurements strongly suggest that the Milky Way is a four-arm spiral. Fitting log-periodic spirals to the locations of the masers, allows us to significantly expand our view of the structure of the Milky Way. We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms. Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center, Ro = 8.15 +/- 0.15 kpc, the circular rotation speed at the Sun's position, To = 236 +/- 7 km/s, and the nature of the rotation curve. Our data strongly constrain the full circular velocity of the Sun, To + Vsun = 247 +/- 4 km/s, and its angular velocity, (To + Vsun)/Ro = 30.32 +/- 0.27 km/s/kpc. Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy, we find non-circular velocity components typically about 10 km/s. However, near the Galactic bar and in a portion of the Perseus arm, we find significantly larger non-circular motions. Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc and, thus, are well suited to define the Galactic plane. We find that the orientation of the plane is consistent with the IAU-defined plane to within +/-0.1 deg., and that the Sun is offset toward the north Galactic pole by Zsun = 5.5 +/- 5.8 pc. Accounting for this offset places the central supermassive black hole, Sgr A*, in the midplane of the Galaxy. Using our improved Galactic parameters, we predict the Hulse-Taylor binary pulsar to be at a distance of 6.54 +/- 0.24 kpc, assuming its orbital decay from gravitational radiation follows general relativity.

astro-ph.GA↗

arXiv2019

Broadband spectral analysis of MXB 1659-298 in its soft and hard state

The X-ray transient eclipsing source MXB 1659-298 went into outburst in 1999 and 2015. During these two outbursts the source was observed by XMM-Newton, nuSTAR, and Swift/XRT. Using these observations, we studied the broadband spectrum of the source to constrain the continuum components and to verify whether it had a reflection component, as is observed in other X-ray eclipsing transient sources. We combined the available spectra to study the soft and hard state of the source in the 0.45-55 keV energy range. We report a reflection component in the soft and hard state. The direct emission in the soft state can be modeled with a thermal component originating from the inner accretion disk plus a Comptonized component associated with an optically thick corona surrounding the neutron star. On the other hand, the direct emission in the hard state is described only by a Comptonized component with a temperature higher than 130 keV; this component is associated with an optically thin corona. We observed narrow absorption lines from highly ionized ions of oxygen, neon, and iron in the soft spectral state. We investigated where the narrow absorption lines form in the ionized absorber. The equivalent hydrogen column density associated with the absorber is close to $6 \times 10^{23}$ cm$^{-2}$ and $1.3 \times 10^{23}$ cm$^{-2}$ in the soft and hard state, respectively.

astro-ph.HE↗

arXiv2019

Measuring the ionisation fraction in a jet from a massive protostar

It is important to determine if massive stars form via disc accretion, like their low-mass counterparts. Theory and observation indicate that protostellar jets are a natural consequence of accretion discs and are likely to be crucial for removing angular momentum during the collapse. However, massive protostars are typically rarer, more distant and more dust enshrouded, making observational studies of their jets more challenging. A fundamental question is whether the degree of ionisation in jets is similar across the mass spectrum. Here we determine an ionisation fraction of $\sim5-12\%$ in the jet from the massive protostar G35.20-0.74N, based on spatially coincident infrared and radio emission. This is similar to the values found in jets from lower-mass young stars, implying a unified mechanism of shock ionisation applies in jets across most of the protostellar mass spectrum, up to at least $\sim10$ solar masses.

astro-ph.SR↗

arXiv2019

Sub-arcsecond (sub)millimeter imaging of the massive protocluster G358.93-0.03: Discovery of 14 new methanol maser lines associated with a hot core

We present (sub)millimeter imaging at 0.5'' resolution of the massive star-forming region G358.93-0.03 acquired in multiple epochs at 2 and 3 months following the recent flaring of its 6.7 GHz methanol maser emission. Using SMA and ALMA, we have discovered 14 new Class II methanol maser lines ranging in frequency from 199 GHz to 361 GHz, which originate mostly from vt=1 torsionally-excited transitions and include one vt=2 transition. The latter detection provides the first observational evidence that Class II maser pumping involves levels in the vt=2 state. The masers are associated with the brightest continuum source (MM1), which hosts a line-rich hot core. The masers present a consistent curvilinear spatial velocity pattern that wraps around MM1, suggestive of a coherent physical structure 1200 au in extent. In contrast, the thermal lines exhibit a linear pattern that crosses MM1 but at progressive position angles that appear to be a function of either increasing temperature or decreasing optical depth. The maser spectral profiles evolved significantly over one month, and the intensities dropped by factors of 3.0 to 7.2, with the vt=2 line showing the largest change. A small area of maser emission from only the highest excitation lines closest to MM1 has disappeared. There are seven additional dust continuum sources in the protocluster, including another hot core (MM3). We do not find evidence for a significant change in (sub)millimeter continuum emission from any of the sources during the one month interval, and the total protocluster emission remains comparable to prior single dish measurements.

astro-ph.SR↗

arXiv2019

Broadband spectral analysis of MXB 1659-298 in its soft and hard state

The X-ray transient eclipsing source MXB 1659-298 went in outburst in 1999 and 2015, respectively, during which it was observed by XMM-Newton, NuSTAR and Swift. Using these observations we studied the broadband spectrum of the source to constrain the continuum components and to verify the presence of a reflection component. We analysed the soft and hard state of the source, finding that the soft state can be modelled with a thermal component associated with the inner accretion disc plus a Comptonised component. A smeared reflection component and the presence of an ionised absorber are also requested in the best-fit model. On the other hand, the direct continuum emission in the hard state can be described by a Comptonised component with a temperature larger than 150 keV. Also in this case a reflection component and a ionised absorber are observed.

astro-ph.HE↗

arXiv2019

Pulsating in unison at optical and X-ray energies: simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038

PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the visible band; such a detection took place when the pulsar was surrounded by an accretion disk and also showed X-ray pulsations. We report on the first high time resolution observational campaign of this transitional pulsar in the disk state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray (XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and X-ray pulsations were detected simultaneously in the X-ray high intensity mode in which the source spends $\sim$ 70% of the time, and both disappeared in the low mode, indicating a common underlying physical mechanism. In addition, optical and X-ray pulses were emitted within a few km, had similar pulse shape and distribution of the pulsed flux density compatible with a power-law relation $F_ν \propto ν^{-0.7}$ connecting the optical and the 0.3-45 keV X-ray band. Optical pulses were detected also during flares with a pulsed flux reduced by one third with respect to the high mode; the lack of a simultaneous detection of X-ray pulses is compatible with the lower photon statistics. We show that magnetically channeled accretion of plasma onto the surface of the neutron star cannot account for the optical pulsed luminosity ($\sim 10^{31}$ erg/s). On the other hand, magnetospheric rotation-powered pulsar emission would require an extremely efficient conversion of spin-down power into pulsed optical and X-ray emission. We then propose that optical and X-ray pulses are instead produced by synchrotron emission from the intrabinary shock that forms where a striped pulsar wind meets the accretion disk, within a few light cylinder radii away, $\sim$ 100 km, from the pulsar.

astro-ph.HE↗

arXiv2019

X-ray dips and a complex UV/X-ray cross-correlation function in the black hole candidate MAXI J1820+070

MAXI J1820+070, a black hole candidate first detected in early March 2018, was observed by XMM-Newton during the outburst rise. In this letter we report on the spectral and timing analysis of the XMM-Newton X-ray and UV data, as well as contemporaneous X-ray data from the Swift satellite. The X-ray spectrum is well described by a hard thermal Comptonization continuum. The XMM-Newton X-ray light curve shows a pronounced dipping interval, and spectral analysis indicates that it is caused by a moderately ionized partial covering absorber. The XMM-Newton/OM U-filter data does not reveal any signs of the 17 hr orbital modulation that was seen later on during the outburst decay. The UV/X-ray cross correlation function shows a complex shape, with a peak at positive lags of about 4 seconds and a pre-cognition dip at negative lags, which is absent during the X-ray dipping episode. Such shape could arise if the UV emission comes partially from synchrotron self-Compton emission near the black hole, as well as from reprocessing of the X-rays in the colder accretion disc further out.

astro-ph.HE↗

arXiv2019

Indications of non-conservative mass-transfer in AMXPs

Context. Since the discovery of the first Accreting Millisecond X-ray Pulsar SAX J1808.4-3658 in 1998, the family of these sources kept growing on. Currently, it counts 22 members. All AMXPs are transients with usually very long quiescence periods, implying that mass accretion rate in these systems is quite low and not constant. Moreover, for at least three sources, a non-conservative evolution was also proposed. Aims. Our purpose is to study the long term averaged mass-accretion rates in all the Accreting Millisecond X-ray Pulsars discovered so far, to investigate a non-conservative mass-transfer scenario. Methods. We calculated the expected mass-transfer rate under the hypothesis of a conservative evolution based on their orbital periods and on the (minimum) mass of the secondary (as derived from the mass function), driven by gravitational radiation and/or magnetic braking. Using this theoretical mass-transfer, we determined the expected accretion luminosity of the systems. Thus, we achieved the lower limit to the distance of the sources by comparing the computed theoretical luminosity and the observed flux averaged over a time period of 20 years. Then, the lower limit to the distance of the sources has been compared to the value of the distance reported in literature to evaluate how reasonable is the hypothesis of a conservative mass-transfer. Results. Based on a sample of 18 sources, we found strong evidences of a non-conservative mass-transfer for five sources, for which the estimated distance lower limits are higher than their known distances. We also report hints for mass outflows in other six sources. The discrepancy can be fixed under the hypothesis of a non-conservative mass-transfer in which a fraction of the mass transferred onto the compact object is swept away from the system, likely due to the (rotating magnetic dipole) radiation pressure of the pulsar.

astro-ph.HE↗