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F. D'Amico

Publications and source records attributed to F. D'Amico.

At least 19 recordsLinked to original sources

Algorithmic thresholds in combinatorial optimization depend on the time scaling

In the last decades, many efforts have focused on analyzing typical-case hardness in optimization and inference problems. Some recent work has pointed out that polynomial algorithms exist, running with a time that grows more than linearly with the system size, which can do better than linear algorithms, finding solutions to random problems in a wider range of parameters. However, a theory for polynomial and superlinear algorithms is in general lacking. In this paper, we examine the performance of the Simulated Annealing algorithm, a standard, versatile, and robust choice for solving optimization and inference problems, in the prototypical random $K$-Sat problem. For the first time, we show that the algorithmic thresholds depend on the time scaling of the algorithm with the size of the system. Indeed, one can identify not just one, but different thresholds for linear, quadratic, cubic regimes (and so on). This observation opens new directions in studying the typical case hardness in optimization problems.

cond-mat.dis-nn

AGILE gamma-ray detection of the exceptional GRB 221009A

Gamma-ray emission in the MeV-GeV range from explosive cosmic events is of invaluable relevance to understanding physical processes related to the formation of neutron stars and black holes. Here we report on the detection by the AGILE satellite in the MeV-GeV energy range of the remarkable long-duration gamma-ray burst GRB 221009A. The AGILE onboard detectors have good exposure to GRB 221009A during its initial crucial phases. Hard X-ray/MeV emission in the prompt phase lasted hundreds of seconds, with the brightest radiation being emitted between 200 and 300 seconds after the initial trigger. Very intense GeV gamma-ray emission is detected by AGILE in the prompt and early afterglow phase up to 10,000 seconds. Time-resolved spectral analysis shows time-variable MeV-peaked emission simultaneous with intense power-law GeV radiation that persists in the afterglow phase. The coexistence during the prompt phase of very intense MeV emission together with highly nonthermal and hardening GeV radiation is a remarkable feature of GRB 221009A. During the prompt phase, the event shows spectrally different MeV and GeV emissions that are most likely generated by physical mechanisms occurring in different locations. AGILE observations provide crucial flux and spectral gamma-ray information regarding the early phases of GRB 221009A during which emission in the TeV range was reported.

astro-ph.HE

Revised ephemeris and orbital period derivative of the supersoft X-ray source CAL 87 based on 34 years of observations

In this study, we present an analysis of over 34 years of observational data from CAL 87, an eclipsing supersoft X-ray source. The primary aim of our study, which combines previously analysed measurements as well as unexplored publicly available datasets, is to examine the orbital period evolution of CAL 87. After meticulously and consistently determining the eclipse timings, we constructed an O$-$C (observed minus calculated) diagram using a total of 38 data points. Our results provide confirmation of a positive derivative in the system's orbital period, with a determined value of $\dot{P}=+ 8.18\pm1.46\times10^{-11}$ s/s. We observe a noticeable jitter in the eclipse timings and additionally identify a systematic delay in the X-ray eclipses compared to those observed in longer wavelengths. We discuss the interplay of the pertinent factors that could contribute to a positive period derivative and the inherent variability in the eclipses.

astro-ph.HE

Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission

Due to be launched in late 2021, the Imaging X-Ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission designed to perform polarization measurements in the 2-8 keV band, complemented with imaging, spectroscopy and timing capabilities. At the heart of the focal plane is a set of three polarization-sensitive Gas Pixel Detectors (GPD), each based on a custom ASIC acting as a charge-collecting anode. In this paper we shall review the design, manufacturing, and test of the IXPE focal-plane detectors, with particular emphasis on the connection between the science drivers, the performance metrics and the operational aspects. We shall present a thorough characterization of the GPDs in terms of effective noise, trigger efficiency, dead time, uniformity of response, and spectral and polarimetric performance. In addition, we shall discuss in detail a number of instrumental effects that are relevant for high-level science analysis -- particularly as far as the response to unpolarized radiation and the stability in time are concerned.

astro-ph.IM

Testing Comptonization as the origin of the continuum in nonmagnetic Cataclysmic Variables. The photon index of X-ray emission

X-ray spectra of nonmagnetic cataclysmic variables (nmCVs) in the ~ 0.3$-$15 keV energy band have been described either by one or several optically thin thermal plasma components, or by cooling flow models. We tested if the spectral continuum in nmCVs could be successfully described by Comptonization of soft photons off hot electrons presented in a cloud surrounding the source [the transition layer, (TL)]. We used publicly XMM-Newton Epic-pn, Chandra HETG/ACIS and LETG/HRC, and RXTE PCA and HEXTE observations of four Dwarf Novae (U~Gem, SS~Cyg, VW~Hyi and SS~Aur) observed in the quiescence and outburst states. In total, we analyzed 18 observations, including a simultaneous 0.4$-$150 keV Chandra/RXTE spectrum of SS~Cyg in quiescence. We fitted the spectral continuum with up to two thermal Comptonization components (compTT or compTB models in XSPEC), using only one thermal plasma temperature and one optical depth. In this framework the two seed photon components are coming presumably from the innermost and outer parts of the TL (or innermost part of the disk). We obtained that the thermal Comptonization can successfully describe the spectral continuum of these nmCV in the ~ 0.4$-$150 keV energy band. Moreover, we present the first principal radiative transfer model which explains the quasi-constancy of the spectral photon index observed around 1.8, which strongly supports the Comptonization framework in nmCVs.

astro-ph.HE

LECX: a cubesat experiment to detect and localize cosmic explosions in hard X rays

With the advent of the nanosat/cubesat revolution, new opportunities have appeared to develop and launch small ($\sim$\ts 1000 cm$^3$), low-cost ($\sim$\ts US\$ 1M) experiments in space in very short timeframes ($\sim$ 2\ts years). In the field of high-energy astrophysics, in particular, it is a considerable challenge to design instruments with compelling science and competitive capabilities that can fit in very small satellite buses such as a cubesat platform, and operate them with very limited resources. Here we describe a hard X-ray (30--200\ts keV) experiment, LECX ("Localizador de Explosões Cósmicas de Raios X" -- Locator of X-Ray Cosmic Explosions), that is capable of detecting and localizing within a few degrees events like Gamma-Ray Bursts and other explosive phenomena in a 2U-cubesat platform, at a rate of $\sim${\bf 5 events year$^{-1}$.} In the current gravitational wave era of astronomy, a constellation or swarm of small spacecraft carrying instruments such as LECX can be a very cost-effective way to search for electromagnetic counterparts of gravitational wave events produced by the coalescence of compact objects.

astro-ph.IM

AGILE Observations of the Gravitational-wave Source GW170817: Constraining Gamma-Ray Emission from a NS-NS Coalescence

The LIGO-Virgo Collaboration (LVC) detected, on 2017 August 17, an exceptional gravitational-wave (GW) event temporally consistent within $\sim\,1.7 \, \rm s$ with the GRB 1708117A observed by Fermi-GBM and INTEGRAL. The event turns out to be compatible with a neutron star-neutron star (NS-NS) coalescence that subsequently produced a radio/optical/X-ray transient detected at later times. We report the main results of the observations by the AGILE satellite of the GW170817 localization region (LR) and its electromagnetic (e.m.) counterpart. At the LVC detection time $T_0$, the GW170817 LR was occulted by the Earth. The AGILE instrument collected useful data before and after the GW-GRB event because in its spinning observation mode it can scan a given source many times per hour. The earliest exposure of the GW170817 LR by the gamma-ray imaging detector (GRID) started about 935 s after $T_0$. No significant X-ray or gamma-ray emission was detected from the LR that was repeatedly exposed over timescales of minutes, hours, and days before and after GW170817, also considering Mini-calorimeter and Super-AGILE data. Our measurements are among the earliest ones obtained by space satellites on GW170817 and provide useful constraints on the precursor and delayed emission properties of the NS-NS coalescence event. We can exclude with high confidence the existence of an X-ray/gamma-ray emitting magnetar-like object with a large magnetic field of $10^{15} \, \rm G$. Our data are particularly significant during the early stage of evolution of the e.m. remnant.

astro-ph.HE

AGILE Observations of the Gravitational Wave Source GW170104

The LIGO/Virgo Collaboration (LVC) detected on 2017 January 4, a significant gravitational-wave (GW) event (now named GW170104). We report in this Letter the main results obtained from the analysis of hard X-ray and gamma-ray data of the AGILE mission that repeatedly observed the GW170104 localization region (LR). At the LVC detection time $T_0$ AGILE observed about 36% of the LR. The gamma-ray imaging detector did not reveal any significant emission in the energy range 50 MeV--30 GeV. Furthermore, no significant gamma-ray transients were detected in the LR that was repeatedly exposed over timescales of minutes, hours, and days. We also searched for transient emission using data near $T_0$ of the omnidirectional detector MCAL operating in the energy band 0.4--100 MeV. A refined analysis of MCAL data shows the existence of a weak event (that we call "E2") with a signal-to-noise ratio of $4.4\,σ$ lasting about 32 ms and occurring $0.46\,\pm\,0.05 \,\rm s$ before $T_0$. A study of the MCAL background and of the false-alarm rate of E2 leads to the determinination of a post-trial significance between $2.4\,σ$ and $2.7\,σ$ for a temporal coincidence with GW170104. We note that E2 has characteristics similar to those detected from the weak precursor of the short GRB 090510. The candidate event E2 is worth consideration for simultaneous detection by other satellites. If associated with GW170104, it shows emission in the MeV band of a short burst preceding the final coalescence by 0.46 sec and involving $\sim 10^{-7}$ of the total rest mass energy of the system.

astro-ph.HE

Enhanced detection of terrestrial gamma-ray flashes by AGILE

At the end of March 2015 the onboard software configuration of the AGILE satellite was modified in order to disable the veto signal of the anticoincidence shield for the minicalorimeter instrument. The motivation for such a change was the understanding that the dead time induced by the anticoincidence prevented the detection of a large fraction of Terrestrial Gamma-Ray Flashes (TGFs). The configuration change was highly successful resulting in an increase of one order of magnitude in TGF detection rate. As expected, the largest fraction of the new events has short duration ($< 100 \mathrm {μs}$), and part of them has simultaneous association with lightning sferics detected by the World Wide Lightning Location Network (WWLLN). The new configuration provides the largest TGF detection rate surface density (TGFs/$\mathrm{km^2}$/year) to date, opening prospects for improved correlation studies with lightning and atmospheric parameters on short spatial and temporal scales along the equatorial region.

physics.space-ph

Telescope performance and image simulations of the balloon-borne coded-mask protoMIRAX experiment

In this work we present the results of imaging simulations performed with the help of the GEANT4 package for the protoMIRAX hard X-ray balloon experiment. The instrumental background was simulated taking into account the various radiation components and their angular dependence, as well as a detailed mass model of the experiment. We modeled the meridian transits of the Crab Nebula and the Galatic Centre region during balloon flights in Brazil ($\sim -23^{\circ}$ of latitude and an altitude of $\sim 40 \thinspace$ km) and introduced the correspondent spectra as inputs to the imaging simulations. We present images of the Crab and of three sources in the Galactic Centre region: 1E 1740.7-2942, GRS 1758-258 and GX 1+4. The results show that the protoMIRAX experiment is capable of making spectral and timing observations of bright hard X-ray sources as well as important imaging demonstrations that will contribute to the design of the MIRAX satellite mission.

astro-ph.HE

Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state

Hard X-ray spectra of black hole binaries in the low/hard state are well modeled by thermal Comptonization of soft seed photons by a corona-type region with $kT$\thinspace$\sim 50${\thinspace}keV and optical depth around 1. Previous spectral studies of 1E{\thinspace}1740.7$-$2942, including both the soft and the hard X-ray bands, were always limited by gaps in the spectra or by a combination of observations with imaging and non-imaging instruments. In this study, we have used three rare nearly-simultaneous observations of 1E{\thinspace}1740.7$-$1942 by both XMM-Newton and INTEGRAL satellites to combine spectra from four different imaging instruments with no data gaps, and we successfully applied the Comptonization scenario to explain the broadband X-ray spectra of this source in the low/hard state. For two of the three observations, our analysis also shows that, models including Compton reflection can adequately fit the data, in agreement with previous reports. We show that the observations can also be modeled by a more detailed Comptonization scheme. Furthermore, we find the presence of an iron K-edge absorption feature in one occasion, which confirms what had been previously observed by Suzaku. Our broadband analysis of this limited sample shows a rich spectral variability in 1E{\thinspace}1740.7$-$2942 at the low/hard state, and we address the possible causes of these variations. More simultaneous soft/hard X-ray observations of this system and other black-hole binaries would be very helpful in constraining the Comptonization scenario and shedding more light on the physics of these systems.

astro-ph.HE

INTEGRAL observations of Sco X-1: evidence for Comptonization up to 200 keV

We have analyzed a long-term database for Sco X-1 obtained with the telescope IBIS onboard the INTEGRAL satellite in order to study the hard X-ray behavior of Sco X-1 from 20 up to 200 keV. Besides the data used for producing of the INTEGRAL catalog of sources, this is the longest (412 ks) database of IBIS on Sco X-1 up to date. The production of hard X-ray tails in low-mass X-ray binaries is still a matter of debate. Since most of the fits to the high-energy part of the spectra are done with powerlaw models, the physical mechanism for the hard X-ray tail production is unclear. The purpose of this study is to better constrain those possible mechanisms. Our main result shows a strong correlation between the fluxes in the thermal and nonthermal part of Sco X-1 spectra. We thus suggest that Comptonization of lower energy photons is the mechanism for producing hard X-ray tails in Sco X-1.

astro-ph.HE

Differential photometry of 2MASS J09440940-5617117

2MASS J09440940-5617117 was identified as a cataclysmic variable in 2007 by its spectrum. In this article we present optical differential photometry in the Rc band obtained using the 0.6-m telescope at Observatorio do Pico dos Dias/Brazil. The complete light curves confirm the presence of a deep eclipse. We derive an orbital period of 0.1879340(5) d. The eclipse has a width of 0.112 +- 0.003 orbital cycles.

astro-ph.SR

Optical polarimetry and infrared photometry of two AM Her binaries: 1RXS J161008.0+035222 and 1RXS J231603.9-052713

We present the first optical circular and linear polarization measurements of two polar candidates from ROSAT: 1RXS J161008.0+035222 and 1RXS J231603.9-052713. We also present H band near-infrared photometry of the last object. The presence of strong circular polarization confirms them as AM Her systems. 1RXS J231603.9-052713 was observed in two different brightness states. The orbital phase dependence of the flux and polarization of 1RXS J161008.0+035222 is reasonably fitted with a simple model in which the binary is observed at a small inclination and the magnetic field axis is almost parallel to the white-dwarf rotation axis resulting in the accretion column axis being seen from top during the whole orbital revolution. The magnetic field is probably in the 10 to 20 MG range. The circular polarization of 1RXS J231603.9-052713 is complex and highly variable. The light-curves of that object have been fitted using a model which includes the white-dwarf, a heated secondary and a point-like accretion region. The secondary emission contributes significantly even in optical wavelengths. This model also reproduces the main features of the optical polarization of 1RXS J231603.9-052713. We estimate the main parameters of the binary, of the accretion region and the distance to the system. However, a more precise interpretation of this system should take in account an extended and inhomogeneous accreting region as well as non-radial accretion.

astro-ph

Long Term Studies of Z sources with HEXTE/RXTE

We have analyzed the long pointed observations of the Z sources in the Rossi X-Ray Timing Explorer (RXTE) public archive to study the high energy emission in those sources. Our analysis is concentrated on the High Energy X--Ray Timing Experiment (HEXTE) waveband, since we are primarily interested in studying the hard X-ray (i.e., E > 20 keV) production in those sources. We give here the preliminary results of this ongoing study. We have found no hard X-ray tails (besides Sco X-1) in our database from any of the Z sources, i.e., GX 349+2 (< 7.9 x 10^-5 photons cm^-2 s^-1, 3 sigma, 50-150 keV), Cyg X-2 (< 8.4 x 10^-5 photons cm^-2 s^-1, 3 sigma, 50-150 keV), GX 17+2 (< 4.2 x 10^-5 photons cm^-2 s^-1, 3 sigma, 50-150 keV), GX 5-1 (< 2.1 x 10^-5 photons cm^-2 s^-1, 3 sigma, 50-150 keV), and Gx 340+0 (< 6.0 x 10^-5 photons cm^-2 s^-1, 3 sigma, 50-150 keV). From the point of view of HEXTE/RXTE observations shown here, the production of hard X-ray tails in Z sources is a process triggered when special conditions are fulfilled. One of these conditions, as derived from our analysis, is a threshold of ~ 4 x 10^36 erg s^-1 for the luminosity of the source's thermal component.

astro-ph

Mirax: A Brazilian X-Ray Astronomy Satellite Mission

We describe the ``Monitor e Imageador de Raios-X'' (MIRAX), an X-ray astronomy satellite mission proposed by the high energy astrophysics group at the National Institute for Space Research (INPE) in Brazil to the Brazilian Space Agency. MIRAX is an international collaboration that includes, besides INPE, the University of California San Diego, the University of Tuebingen in Germany, the Massachusetts Institute of Technology and the Space Research Organization Netherlands. The payload of MIRAX will consist in two identical hard X-ray cameras (10 -200 keV) and one soft X-ray camera (2-28 keV), both with angular resolution of ~ 5-6 arcmin. The basic objective of MIRAX is to carry out continuous broadband imaging spectroscopy observations of a large source sample (~ 9 months/yr) in the central Galactic plane region. This will allow the detection, localization, possible identification, and spectral/temporal study of the entire history of transient phenomena to be carried out in one single mission. MIRAX will have sensitivities of ~ 5 mCrab/day in the 2-10 keV band (~2 times better than the All Sky Monitor on Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10-100 keV band (~40 times better than the Earth Occultation technique of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft will weigh about 200 kg and is expected to be launched in a low-altitude (~ 600 km) circular equatorial orbit around 2007/2008.

astro-ph

Studies of Hard X-ray Tails in Z sources with RXTE/HEXTE

We report RXTE results of spectral analyses of three (Sco X-1, GX 349+2, and Cyg X-2) out of the 6 known Z sources, with emphasis in the hard X-ray emission. No hard X-ray tails were found for Cyg X-2 (< 8.4E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma) and for GX 349+2 (< 7.9E-5 photons cm**-2 s**-1, 50-100 keV, 3 sigma). For Sco X-1 a variable hard X-ray tail (with an average flux of 2.0E-3 photons cm**-2 s**-1, 50-100 keV) has already been reported. We compare our results to reported detections of a hard component in the spectrum of Cyg X-2 and GX 349+2. We argue that, taking into account all the results on detections of hard X-ray tails in Sco X-1 and GX 349+2, the appearance of such a component is correlated with the brightness of the thermal component.

astro-ph

HEXTE Studies of Sco X-1 Spectra: Detections of Hard X-Ray Tails Beyond 200 keV

Using the HEXTE experiment on-board the RXTE satellite, we performed a search for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the presence of such a non-thermal component on several occasions. Using the PCA/RXTE we were able to track the position of the source along the Z diagram, and we observed that the presence of the hard X-ray tail is not confined to a particular region. However, we found a correlation between the power law index of the non-thermal component and the position of the source in the Z diagram, suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as the mass accretion rate increases. We were also able to study the temporal variation of the appearance/absence of the hard X-ray component. With our derived luminosities, we were also able to test the idea that X-ray luminosities can be used to distinguish between X-ray binary systems containing neutron stars and black holes.

astro-ph