arXiv subjects

Andrea Santangelo

Publications and source records attributed to Andrea Santangelo.

At least 109 records · Page 6Linked to original sources

arXiv2019

ESA Voyage 2050 white paper: A Polarized View of the Hot and Violent Universe

Since the birth of X-ray Astronomy, spectacular advances have been seen in the imaging, spectroscopic and timing studies of the hot and violent X-ray Universe, and further leaps forward are expected in the future. On the other hand, polarimetry is very much lagging behind: after the measurements of the Crab Nebula and Scorpius X-1, obtained by OSO-8 in the 70s, no more observations have been performed in the classical X-ray band, even if some interesting results have been obtained in hard X-rays and in soft gamma-rays. The NASA/ASI mission IXPE, scheduled for the launch in 2021, is going to provide for the first time imaging X-ray polarimetry in the 2-8 keV band thanks to its photoelectric polarimeter, coupled with ~25'' angular resolution X-ray mirrors. Its orders of magnitude improvement in sensitivity with respect to the OSO-8 Bragg polarimeter implies scientifically meaningful polarimetric measurements for at least the brightest specimens of most classes of X-ray sources. In 2027, the Chinese-led mission eXTP should also be launched. In addition to timing and spectroscopic instruments, eXTP will have on board photoelectric polarimeters very similar to those of IXPE, but with a total effective area 2-3 times larger. Building on IXPE results, eXTP will increase the number of sources for which significant polarimetric measurements could be obtained. However, further progresses, such as exploring a broader energy range, considering a larger effective area, improving the angular resolution, and performing wide-field polarization measurements, are needed to reach a mature phase for X-ray polarimetry. In the first part of this White Paper we will discuss a few scientific cases in which a next generation X-ray Polarimetry mission can provide significant advances. In the second part, a possible concept for a medium-class Next Generation X-ray Polarimetry (NGXP) mission will be sketched.

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arXiv2019

Insight-HXMT observations of Swift J0243.6+6124 during its 2017-2018 outburst

The recently discovered neutron star transient Swift J0243.6+6124 has been monitored by {\it the Hard X-ray Modulation Telescope} ({\it Insight-\rm HXMT). Based on the obtained data, we investigate the broadband spectrum of the source throughout the outburst. We estimate the broadband flux of the source and search for possible cyclotron line in the broadband spectrum. No evidence of line-like features is, however, found up to $\rm 150~keV$. In the absence of any cyclotron line in its energy spectrum, we estimate the magnetic field of the source based on the observed spin evolution of the neutron star by applying two accretion torque models. In both cases, we get consistent results with $B\rm \sim 10^{13}~G$, $D\rm \sim 6~kpc$ and peak luminosity of $\rm >10^{39}~erg~s^{-1}$ which makes the source the first Galactic ultraluminous X-ray source hosting a neutron star.

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arXiv2019

Evidence for recent GeV brightening of the SN 1987A region

We report on a recent (2016-2018) enhancement of the GeV emission from the SN 1987A region as observed with Fermi/LAT. The observed signal is characterised by a power-law spectrum with a slope of 2.1 +/- 0.2 and is detected only at energies >1 GeV. The Fermi/LAT data constrain the position of the signal to within 0.15 degree around SN 1987A. Although a recent increase in the gamma-ray emission from SN 1987A seems to be a natural explanation for the detected emission, given the youth of the source and its rapid evolution, the Fermi/LAT location also overlaps with several other potential gamma-ray sources: 30 Dor C, Honeycomb nebula, RX J0536.9-6913, and a hypothetical, previously unknown transient source. We argue that multiwavelength observations of the region performed during the next few years can clarify the nature of the signal and encourage such observations. We also present upper limits on the time-averaged flux of SN 1987A based on 10 years of Fermi/LAT exposure, which can be used to better constrain the particle acceleration models of this source.

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arXiv2019

Long-term evolutions of the cyclotron line energies in Her X-1, Vela X-1 and Cen X-3 as observed with Swift/BAT

We study the long-term evolution of the centroid energy of cyclotron lines - often referered to as Cyclotron Resonance Scattering Features (CRSF) - in Her X-1, Vela X-1 and Cen X-3, using survey observations of the Burst Alert Telescope (BAT) onboard Swift. We find a significant decrease of the fundamental CRSF energy in Her X-1 and the first harmonic line energy in Vela X-1, since the launch of Swift in 2004 and until 2010 and 2012, respectively.In both sources the decreases stopped at some time, with a quite stable centroid energy thereafter. Unlike in Her X-1 and Vela X-1, the CRSF energy in Cen X-3 does not show a long-term decrease. It is observed not to change for at least the past 14 years. The long-term variation of the line energy is a direct way to investigate the magnetic field structure in the polar regions of pulsars. Our results may stimulate the development of theoretical models, especially regarding to how the accreted mass accumulates in the accretion mound or how the magnetic field distorts around the polar cap.

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arXiv2018

Physics and astrophysics of strong magnetic field systems with eXTP

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies of strongly magnetized objects. We will focus on the physics and astrophysics of strongly magnetized objects, namely magnetars, accreting X-ray pulsars, and rotation powered pulsars. We also discuss the science potential of eXTP for QED studies. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

Dense matter with eXTP

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

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arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

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arXiv2018

The enhanced X-ray Timing and Polarimetry mission - eXTP

In this paper we present the enhanced X-ray Timing and Polarimetry mission - eXTP. eXTP is a space science mission designed to study fundamental physics under extreme conditions of density, gravity and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring effects of QED, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, eXTP will be a very powerful observatory for astrophysics that will provide observations of unprecedented quality on a variety of galactic and extragalactic objects. In particular, its wide field monitoring capabilities will be highly instrumental to detect the electro-magnetic counterparts of gravitational wave sources. The paper provides a detailed description of: (1) the technological and technical aspects, and the expected performance of the instruments of the scientific payload; (2) the elements and functions of the mission, from the spacecraft to the ground segment.

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arXiv2018

Spectral and timing analysis of the bursting pulsar GRO J1744-28 with RXTE observations

We analyzed RXTE/PCA observations of the bursting pulsar GRO J1744-28 during its two outbursts in 1995-1997. We have found a significant transition, i.e., a sharp change, of both spectral and timing properties around a flux of $\rm 10^{-8}\ ergs\ cm^{-2}\ s^{-1}$ (3-30 keV), which corresponds to a luminosity of 2--8$\rm \times 10^{37}\ ergs\ s^{-1}$ for a distance of 4--8 kpc. We define the faint (bright) state when the flux is smaller (larger) than this threshold. In the faint state, the spectral hardness increases significantly with the increasing flux, while remaining almost constant in the bright state. In addition, we find that the pulsed fraction is positively related to both the flux and the energy (<30keV) in both states. Thanks to the very stable pulse profile shape in all energy bands, a hard X-ray lag could be measured. This lag is only significant in the faint state (reaching $\sim$ 20\,ms between 3-4 keV and 16-20 keV), and much smaller in the bright state ($\lesssim$2ms). We speculate that the discovered transition might be caused by changes of the accretion structure on the surface of the neutron star, and the hard X-ray lags are due to a geometry effect.

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arXiv2018

The Large Area Detector onboard the eXTP mission

The eXTP (enhanced X-ray Timing and Polarimetry) mission is a major project of the Chinese Academy of Sciences (CAS) and China National Space Administration (CNSA) currently performing an extended phase A study and proposed for a launch by 2025 in a low-earth orbit. The eXTP scientific payload envisages a suite of instruments (Spectroscopy Focusing Array, Polarimetry Focusing Array, Large Area Detector and Wide Field Monitor) offering unprecedented simultaneous wide-band X-ray spectral, timing and polarimetry sensitivity. A large European consortium is contributing to the eXTP study and it is expected to provide key hardware elements, including a Large Area Detector (LAD). The LAD instrument for eXTP is based on the design originally proposed for the LOFT mission within the ESA context. The eXTP/LAD envisages a deployed 3.4 m2 effective area in the 2-30 keV energy range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to 200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper we provide an overview of the LAD instrument design, including new elements with respect to the earlier LOFT configuration.

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arXiv2018

Infrared and X-ray study of the Galactic SNR G15.9+0.2

G15.9+0.2 is a Galactic shell-type supernova remnant (SNR), which was detected in radio and has been confirmed in X-rays based on Chandra observations. An X-ray point source CXOUJ181852.0-150213 has been detected and suggested to be an associated neutron star. In a recent study, we have confirmed the source to be a central compact object (CCO). We have studied the SNR using high-resolution X-ray data taken with Chandra in combination with infrared (IR) data in order to understand its emission and to derive its physical parameters. This will also help to constrain, e.g., the age of the CCO and the environment in which it was born. The spectral analysis of the X-ray emission using the new Chandra data and the comparison to the IR data have shown that the SNR is relatively young with an age of a few thousand years and that its emission is dominated by that of shocked interstellar medium (ISM). However, the analysis of the spectrum of the bright eastern shell shows that there is an additional emission component with enhanced abundances of α elements and Fe, suggesting ejecta emission. The multi-wavelength emission is consistent with SNR G15.9+0.2 expanding in an ISM with a density gradient, while there is also colder material located in front of the SNR, which absorbs its thermal X-ray emission in the softer bands.

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arXiv2018

Changes in the cyclotron line energy on short and long timescales in V 0332+53

We present the results of the pulse-amplitude-resolved spectroscopy of the accreting pulsar V 0332+53 using the NuSTAR observations of the source in 2015 and 2016. We investigate the dependence of the energy of the cyclotron resonant scattering feature (CRSF) as a function of X-ray luminosity on timescales comparable with the spin period of the pulsar within individual observations, and the behavior on longer timescales within and between the two observed outbursts. We confirm that in both cases the CRSF energy is negatively correlated with flux at luminosities higher than the critical luminosity and is positively correlated at lower luminosities. We also confirm the recently reported gradual decrease in the line energy during the giant outburst in 2015. Using the NuSTAR data, we find that this decrease was consistent with a linear decay throughout most of the outburst, and flattened or even reversed at the end of the 2015 outburst, approximately simultaneously with the transition to the subcritical regime. We also confirm that by the following outburst in 2016 the line energy rebounded to previous values. The observed behavior of the CRSF energy with time is discussed in terms of changes in the geometry of the CRSF forming region caused by changes in the effective magnetospheric radius.

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arXiv2017

Orbit and intrinsic spin-up of the newly discovered transient X-ray pulsar Swift J0243.6+6124

We present the orbital solution for the newly discovered transient Be X-ray binary Swift J0243.6+6124 based on the data from gamma-ray burst monitor onboard Fermi obtained during the Oct 2017 outburst. We model the Doppler induced and intrinsic spin variations of the neutron star assuming that the later is driven by accretion torque and discuss the implications of the observed spin variations for the parameters of the neutron star and the binary. In particular we conclude that the neutron star must be strongly magnetized, and estimate the distance to the source at $\sim$5 kpc.

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arXiv2017

Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+53

We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotron-line energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-induced decay of the neutron stars magnetic field. At lower luminosities, we find for the first time a hint of departure from the anticorrelation of line energy with flux, which we interpret as a transition from super- to sub-critical accretion associated with the disappearance of the accretion column. Finally, we confirm and briefly discuss the orbital modulation observed in the outburst light curve of the source.

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arXiv2016

Possible regular phenomena in EXO 2030+375

In the last 10 years, since its last giant outburst in 2006, regular X-ray outbursts (type I) were detected every periastron passage in the Be X-ray binary EXO 2030+375. Recently, however, it was reported that the source started to show a peculiar behavior: its X-ray flux decreased significantly and type I outbursts were missed in several cases. At the same time, the spin frequency of the neutron star, which had been increasing steadily since the end of the 2006 giant outburst, reached a plateau. Very recent observations indicate that the source is now starting to spin down. These observed phenomena have a striking similarity with those which took place 20 years ago, just before the source displayed a sudden orbital phase shift of the outburst peak (1995). This historical event occurred at the time exactly between the two giant outbursts (1985 and 2006). These phenomena suggest the system to have an underlying periodicity of 10.5 years between orbital phase shifts and/or giant outbursts. The suggested periodicity may reflect some long-period dynamics in the circumstellar disk of the Be star, due, e.g., to the Kozai-Lidov effect. A model generating such a periodic change of the Be disk, namely Kozai-Lidov oscillations in the Be disk, is discussed. If this behavior is really periodical, another phase shift of the X-ray outburst peak is predicted to occur around 2016 December.

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arXiv2016

Large Observatory for x-ray Timing (LOFT-P): A Probe-classs Mission Concept Study

LOFT-P is a concept for a NASA Astrophysics Probe-Class (<$1B) X-ray timing mission, based on the LOFT concept originally proposed to ESAs M3 and M4 calls. LOFT-P requires very large collecting area (>6 m^2, >10x RXTE), high time resolution, good spectral resolution, broad-band spectral coverage (2-30 keV), highly flexible scheduling, and an ability to detect and respond promptly to time-critical targets of opportunity. It addresses science questions such as: What is the equation of state of ultra dense matter? What are the effects of strong gravity on matter spiraling into black holes? It would be optimized for sub-millisecond timing to study phenomena at the natural timescales of neutron star surfaces and black hole event horizons and to measure mass and spin of black holes. These measurements are synergistic to imaging and high-resolution spectroscopy instruments, addressing much smaller distance scales than are possible without very long baseline X-ray interferometry, and using complementary techniques to address the geometry and dynamics of emission regions. A sky monitor (2-50 keV) acts as a trigger for pointed observations, providing high duty cycle, high time resolution monitoring of the X-ray sky with ~20 times the sensitivity of the RXTE All-Sky Monitor, enabling multi-wavelength and multi-messenger studies. A probe-class mission concept would employ lightweight collimator technology and large-area solid-state detectors, technologies which have been recently greatly advanced during the ESA M3 study. Given the large community interested in LOFT (>800 supporters, the scientific productivity of this mission is expected to be very high, similar to or greater than RXTE (~2000 refereed publications). We describe the results of a study, recently completed by the MSFC Advanced Concepts Office, that demonstrates that LOFT-P is feasible within a NASA probe-class mission budget.

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arXiv2016

Evidence for a binary origin of a central compact object

Central compact objects (CCOs) are thought to be young thermally emitting isolated neutron stars that were born during the preceding core-collapse supernova explosion. Here we present evidence that at least in one case the CCO could have been formed within a binary system. We show that the highly reddened optical source IRAS~17287$-$3443, located $25^{\prime \prime}$ away from the CCO candidate XMMUJ173203.3$-$344518 and classified previously as a post asymptotic giant branch star, is indeed surrounded by a dust shell. This shell is heated by the central star to temperatures of $\sim90$\,K and observed as extended infrared emission in 8-160\,$μ$m band. The dust temperature also increases in the vicinity of the CCO which implies that it likely resides within the shell. We estimate the total dust mass to be $\sim0.4-1.5\,M_\odot$ which significantly exceeds expected dust yields by normal stars and thus likely condensed from supernova ejecta. Taking into account that both the age of the supernova remnant and the duration of active mass loss phase by the optical star are much shorter than the total lifetime of either object, the supernova and the onset of the active mass loss phase of the companion have likely occurred approximately simultaneously. This is most easily explained if the evolution of both objects is interconnected. We conclude, therefore, that both stars were likely members of the same binary system disrupted by a supernova.

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arXiv2015

Detection of Earth-skimming UHE tau neutrino with the JEM-EUSO detector

The ultra high energy cosmic neutrinos are powerful astrophysical probes for both astrophysical mechanisms of particle acceleration and fundamental interactions. They open a window into the very distant and high-energy Universe that is difficult to access by any human means and devices. The possibility of detecting them in large exposure space-based apparatus, like JEM-EUSO, is an experimental challenge. In this paper we present an estimation of the feasibility of detection of UHE tau neutrino by the JEM-EUSO telescope. The interactions of tau-neutrino in sea water and Earth's crust have been investigated. The estimation of the propagation length and energy of the outgoing tau-lepton shows that if its decay occurs in the atmosphere close enough to the Earth's surface, e.g. below $\sim$ $5 km$ altitude, the cascade is intensive enough and the generated light can be detected from space. We have evaluated the geometrical aperture of the JEM-EUSO detector for the Earth-skimming (horizontal and upward-going) tau-neutrinos by making specific modifications to the standard CORSIKA code and developing an interface to the existing ESAF (EUSO Simulation and Analysis Framework) software.

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