arXiv subjects

G. Ingrosso

Publications and source records attributed to G. Ingrosso.

At least 37 records · Page 2Linked to original sources

arXiv2014

The puzzling symbiotic X-ray system 4U1700+24

Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems consisting of a neutron star accreting material from a red giant donor star via stellar wind or Roche lobe overflow. Only a few confirmed members are currently known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray object, possibly associated with the late-type star V934 Her. We analysed the archive {\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to have a uniform high-energy ($0.3-10$ keV) view of the source. We confirmed the existence of a red-shifted O VIII Ly-$α$ transition (already observed in the 2002 {\it XMM}-Newton data) in the high-resolution spectra collected via the RGS instruments. The red-shift of the line is found in all the analysed observations and, on average, it was estimated to be $\simeq 0.009$. We also observed a modulation of the centroid energy of the line on short time scales (a few days) and discuss the observations in the framework of different scenarios. If the modulation is due to the gravitational red-shift of the neutron star, it might arise from a sudden re-organization of the emitting $X$-ray matter on the scale of a few hundreds of km. Alternatively, we are witnessing a uni-polar jet of matter (with typical velocity of $1000-4000$ km s$^{-1}$) possibly emitted by the neutron star in an almost face-on system. The second possibility seems to be required by the apparent lack of any modulation in the observed $X$-ray light curve. We also note also that the low-resolution spectra (both {\it XMM}-Newton and Swift/XRT in the $0.3-10$ keV band) show the existence of a black body radiation emitted by a region (possibly associated with the neutron star polar cap) with typical size from a few tens to hundreds of meters. The size of this spot-like region reduces as the overall luminosity of 4U 1700+24 decreases.

astro-ph.HE↗

arXiv2013

Exoplanetary searches with gravitational microlensing: polarization issues

There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potentiality of detecting Earth-like planets at distances about a few astronomical units from their host star or near the so-called snow line with a temperature in the range $0-100^0$ C on a solid surface of an exoplanet. We emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source.

astro-ph.SR↗

arXiv2013

Polarization in binary microlensing events

The light received by source stars in microlensing events may be significantly polarized if both an efficient photon scattering mechanism is active in the source stellar atmosphere and a differential magnification is therein induced by the lensing system. The best candidate events for observing polarization are highly magnified events with source stars belonging to the class of cool, giant stars {in which the stellar light is polarized by photon scattering on dust grains contained in their envelopes. The presence in the stellar atmosphere of an internal cavity devoid of dust produces polarization profiles with a two peaks structure. Hence, the time interval between them gives an important observable quantity directly related to the size of the internal cavity and to the model parameters of the lens system.} We show that {during a microlensing event} the expected polarization variability can solve an ambiguity, that arises in some cases, related to the binary or planetary lensing interpretation of the perturbations observed near the maximum of the event light-curve. We consider a specific event case for which the parameter values corresponding to the two solutions are given. Then, assuming a polarization model for the source star, we compute the two expected polarization profiles. The position of the two peaks appearing in the polarization curves and the characteristic time interval between them allow us to distinguish between the binary and planetary lens solutions.

astro-ph.SR↗

arXiv2013

Polarization profiles for selected microlensing events towards the galactic bulge

Gravitational microlensing, in case of relevant finite source size effects, provides an unique tool for the study of stellar atmospheres through the enhancement of a characteristic polarization signal. Here, we consider a set of highly magnified events and show that for different types of source stars (as hot, late type main sequence and cool giants) showing that the polarization strength may be of $\simeq 0.04$ percent for late type stars and up to a few percent for cool giants.

astro-ph.GA↗

arXiv2013

An XMM-Newton search for X-ray sources in the Fornax dwarf galaxy

We report the results of a deep archive XMM-Newton observation of the Fornax spheroidal galaxy that we analyzed with the aim of fully characterizing the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected towards the target. A cross correlation with the available databases allowed us to find a source that may be associated with a variable star belonging to the galaxy. We also searched for X-ray sources in the vicinity of the Fornax globular clusters GC 3 and GC 4 and found two sources probably associated with the respective clusters. The deep X-ray observation was also suitable for the search of the intermediate-mass black hole (of mass $\simeq 10^{4}$ M$_{\odot}$) expected to be hosted in the center of the galaxy. In the case of Fornax, this search is extremely difficult since the galaxy centroid of gravity is poorly constrained because of the large asymmetry observed in the optical surface brightness. Since we cannot firmly establish the existence of an X-ray counterpart of the putative black hole, we put constraints only on the accretion parameters. In particular, we found that the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is as low as a few $10^{-5}$.

astro-ph.HE↗

arXiv2012

Polarization in microlensing towards the Galactic bulge

Gravitational microlensing, when finite size source effects are relevant, provides an unique tool for the study of source star stellar atmospheres through an enhancement of a characteristic polarization signal. This is due to the differential magnification induced during the crossing of the source star. In this paper we consider a specific set of reported highly magnified, both single and binary exoplanetary systems, microlensing events towards the Galactic bulge and evaluate the expected polarization signal. To this purpose, we consider several polarization models which apply to different types of source stars: hot, late type main sequence and cool giants. As a result we compute the polarization signal P,which goes up to P=0.04% for late type stars and up to a few percent for cool giants, depending on the underlying physical polarization processes and atmosphere model parameters. Given a I band magnitude at maximum magnification of about 12, and a typical duration of the polarization signal up to 1 day, we conclude that the currently available technology, in particular the polarimeter in FORS2 on the VLT, potentially may allow the detection of such signals. This observational programme may take advantage of the currently available surveys plus follow up strategy already routinely used for microlensing monitoring towards the Galactic bulge (aimed at the detection of exoplanets). In particular, this allows one to predict in advance for which events and at which exact time the observing resources may be focused to make intensive polarization measurements.

astro-ph.GA↗

arXiv2012

CMB as a possible new tool to study the dark baryons in galaxies

Baryons constitute about 4% of our universe, but most of them are missing and we do not know where and in what form they are hidden. This constitute the so-called missing baryon problem. A possibility is that part of these baryons are hidden in galactic halos. We show how the 7-year data obtained by the WMAP satellite may be used to trace the halo of the nearby giant spiral galaxy M31. We detect a temperature asymmetry in the M31 halo along the rotation direction up to about 120 kpc. This could be the first detection of a galactic halo in microwaves and may open a new way to probe hidden baryons in these relatively less studied galactic objects using high accuracy CMB measurements.

astro-ph.CO↗

arXiv2011

Possible detection of the M31 rotation in WMAP data

Data on the cosmic microwave background (CMB) radiation by the Wilkinson Microwave Anisotropy Probe (WMAP) had a profound impact on the understanding of a variety of physical processes in the early phases of the Universe and on the estimation of the cosmological parameters. Here, the 7-year WMAP data are used to trace the disk and the halo of the nearby giant spiral galaxy M31. We analyzed the temperature excess in three WMAP bands (W, V, and Q) by dividing the region of the sky around M31 into several concentric circular areas. We studied the robustness of the detected temperature excess by considering 500 random control fields in the real WMAP maps and simulating 500 sky maps from the best-fitted cosmological parameters. By comparing the obtained temperature contrast profiles with the real ones towards the M31 galaxy, we find that the temperature asymmetry in the M31 disk is fairly robust, while the effect in the halo is weaker. An asymmetry in the mean microwave temperature in the M31 disk along the direction of the M31 rotation is observed with a temperature contrast up to about 130 microK/pixel. We also find a temperature asymmetry in the M31 halo, which is much weaker than for the disk, up to a galactocentric distance of about 10 degrees (120 kpc) with a peak temperature contrast of about 40 microK/pixel. Although the confidence level of the signal is not high, if estimated purely statistically, which could be expected due to the weakness of the effect, the geometrical structure of the temperature asymmetry points towards a definite effect modulated by the rotation of the M31 halo. This result might open a new way to probe these relatively less studied galactic objects using high-accuracy CMB measurements, such as those with the Planck satellite or planned balloon-based experiments, which could prove or disprove our conclusions.

astro-ph.CO↗

arXiv2010

Estimating the parameters of the Sgr A* black hole

The measurement of relativistic effects around the galactic center may allow in the near future to strongly constrain the parameters of the supermassive black hole likely present at the galactic center (Sgr A*). As a by-product of these measurements it would be possible to severely constrain, in addition, also the parameters of the mass-density distributions of both the innermost star cluster and the dark matter clump around the galactic center.

astro-ph.GA↗

arXiv2010

M31 pixel lensing event OAB-N2: a study of the lens proper motion

We present an updated analysis of the M31 pixel lensing candidate event OAB-N2 previously reported in Calchi Novati et al. (2009). Here we take advantage of new data both astrometric and photometric. Astrometry: using archival 4m-KPNO and HST/WFPC2 data we perform a detailed analysis on the event source whose result, although not fully conclusive on the source magnitude determination, is confirmed by the following light curve photometry analysis. Photometry: first, unpublished WeCAPP data allows us to confirm OAB-N2, previously reported only as a viable candidate, as a well constrained pixel lensing event. Second, this photometry enables a detailed analysis in the event parameter space including the effects due to finite source size. The combined results of these analyses allow us to put a strong lower limit on the lens proper motion. This outcome favors the MACHO lensing hypothesis over self lensing for this individual event and points the way toward distinguishing between the MACHO and self-lensing hypotheses from larger data sets.

astro-ph.GA↗

arXiv2010

Search for exoplanets in M31 with pixel-lensing and the PA-99-N2 event revisited

Several exoplanets have been detected towards the Galactic bulge with the microlensing technique. We show that exoplanets in M31 may also be detected with the pixel-lensing method, if telescopes making high cadence observations of an ongoing microlensing event are used. Using a Monte Carlo approach we find that the mean mass for detectable planetary systems is about $2 M_{\rm {J}}$. However, even small mass exoplanets ($M_{\rm P} < 20 M_{\oplus}$) can cause significant deviations, which are observable with large telescopes. We reanalysed the POINT-AGAPE microlensing event PA-99-N2. First, we test the robustness of the binary lens conclusion for this light curve. Second, we show that for such long duration and bright microlensing events, the efficiency for finding planetary-like deviations is strongly enhanced with respect to that evaluated for all planetary detectable events.

astro-ph.SR↗

arXiv2010

Detection of Exoplanets in M31 with Pixel-Lensing: The Event Pa-99-N2 Case

We show that exoplanets in the M31 galaxy may be detected with the pixel-lensing method by using telescopes making high cadence observations of an ongoing microlensing event. Although the mean mass for detectable exoplanets is about $2 M_{\rm {J}}$, even small mass exoplanets ($M_{\rm P} < 20 M_{\oplus}$) can cause significant deviations, which are observable with large telescopes. We reanalysed the POINT-AGAPE microlensing event PA-99-N2. First, we test the robustness of the binary lens conclusion for this light curve. Second, we show that for such long duration and bright microlensing events, the efficiency for finding planetary-like deviations is strongly enhanced.

astro-ph.CO↗

arXiv2009

The X-ray eclipse of the dwarf nova HT CAS observed by the XMM-Newton satellite: spectral and timing analysis

A cataclysmic variable is a binary system consisting of a white dwarf that accretes material from a secondary object via the Roche-lobe mechanism. In the case of long enough observation, a detailed temporal analysis can be performed, allowing the physical properties of the binary system to be determined. We present an XMM-Newton observation of the dwarf nova HT Cas acquired to resolve the binary system eclipses and constrain the origin of the X-rays observed. We also compare our results with previous ROSAT and ASCA data. After the spectral analysis of the three EPIC camera signals, the observed X-ray light curve was studied with well known techniques and the eclipse contact points obtained. The X-ray spectrum can be described by thermal bremsstrahlung of temperature $kT_1=6.89 \pm 0.23$ keV plus a black-body component (upper limit) with temperature $kT_2=30_{-6}^{+8}$ eV. Neglecting the black-body, the bolometric absorption corrected flux is $F^{\rm{Bol}}=(6.5\pm 0.1)\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$, which, for a distance of HT Cas of 131 pc, corresponds to a bolometric luminosity of $(1.33\pm 0.02)\times10^{31}$ erg s$^{-1}$. The study of the eclipse in the EPIC light curve permits us to constrain the size and location of the X-ray emitting region, which turns out to be close to the white dwarf radius. We measure an X-ray eclipse somewhat smaller (but only at a level of $\simeq 1.5 σ$) than the corresponding optical one. If this is the case, we have possibly identified the signature of either high latitude emission or a layer of X-ray emitting material partially obscured by an accretion disk.

astro-ph.HE↗

arXiv2009

Pixel-lensing as a way to detect extrasolar planets in M31

We study the possibility to detect extrasolar planets in M31 through pixel-lensing observations. Using a Monte Carlo approach, we select the physical parameters of the binary lens system, a star hosting a planet, and we calculate the pixel-lensing light curve taking into account the finite source effects. Indeed, their inclusion is crucial since the sources in M31 microlensing events are mainly giant stars. Light curves with detectable planetary features are selected by looking for significant deviations from the corresponding Paczyński shapes. We find that the time scale of planetary deviations in light curves increase (up to 3-4 days) as the source size increases. This means that only few exposures per day, depending also on the required accuracy, may be sufficient to reveal in the light curve a planetary companion. Although the mean planet mass for the selected events is about $2 M_{\rm {Jupiter}}$, even small mass planets ($M_{\rm P} < 20 M_{\oplus}$) can cause significant deviations, at least in the observations with large telescopes. However, even in the former case, the probability to find detectable planetary features in pixel-lensing light curves is at most a few percent of the detectable events, and therefore many events have to be collected in order to detect an extrasolar planet in M31. Our analysis also supports the claim that the anomaly found in the candidate event PA-99-N2 towards M31 can be explained by a companion object orbiting the lens star.

astro-ph.SR↗

arXiv2009

Candidate microlensing events from M31 observations with the Loiano telescope

Microlensing observations towards M31 are a powerful tool for the study of the dark matter population in the form of MACHOs both in the Galaxy and the M31 halos, a still unresolved issue, as well as for the analysis of the characteristics of the M31 luminous populations. In this work we present the second year results of our pixel lensing campaign carried out towards M31 using the 152 cm Cassini telescope in Loiano. We have established an automatic pipeline for the detection and the characterisation of microlensing variations. We have carried out a complete simulation of the experiment and evaluated the expected signal, including an analysis of the efficiency of our pipeline. As a result, we select 1-2 candidate microlensing events (according to different selection criteria). This output is in agreement with the expected rate of M31 self-lensing events. However, the statistics are still too low to draw definitive conclusions on MACHO lensing.

astro-ph.GA↗

arXiv2008

XMM-Newton observation of MACHO 104.20906.960: a dwarf nova candidate with a 2 h period

The binaries known as cataclysmic variables are particular binary systems in which the primary star (a white dwarf) accretes material from a secondary via Roche-lobe mechanism. Usually, these objects have orbital period of a few hours so that a detailed temporal analysis can be performed. Here, we present Chandra ${\it XMM}$-Newton observations of a dwarf nova candidate identified in the past by optical observations towards the galactic Bulge and labeled as MACHO 104.20906.960. After a spectral analysis, we used the Lomb-Scargle technique for the period search and evaluated the confidence level using Monte-Carlo simulations. In this case, we found that the $X$-ray source shows a period of $2.03_{-0.07}^{+0.09} $ hours (3$σ$ error) so that it is most likely a system of interacting objects. The modulation of the signal was found with a confidence level of $>$99%. The spectrum can be described by a two thermal plasma components with X-ray flux in the 0.3--10 keV energy band of $1.77_{-0.19}^{+0.16}\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$. We find that the distance of the source is approximately 1 kpc thus corresponding to a luminosity $L_{X}\simeq 2\times 10^{31}$ erg s$^{-1}$.

astro-ph↗

arXiv2008

Gravitational waves from hyperbolic encounters

The emission of gravitational waves from a system of massive objects interacting on hyperbolic orbits is studied in the quadrupole approximation. Analytic expressions are derived for the gravitational radiation luminosity, the total energy output and the gravitational radiation amplitude. An estimation of the expected number of events towards different targets (i.e. globular clusters and the center of the Galaxy) is also given. In particular, for a dense stellar cluster at the galactic center, a rate up to one event per year is obtained.

gr-qc↗

arXiv2007

The globular cluster NGC 6388: $XMM$-Newton and $Chandra$ observations

By studying the optical brightness surface density of the globular cluster NGC 6388, it has been recently proposed that it harbors a central intermediate-mass black hole with mass $\simeq 5.7\times 10^3$ M$_{\odot}$. We expect that the compact object in the center of NGC 6388 emits radiation in the $X$-ray band as a consequence of the accretion from the surrounding matter. We searched for $XMM$-Newton and $Chandra$ observations towards NGC 6388 to test this hypothesis. The $Chandra$ satellite disentangles several point-like $X$-ray sources, probably low mass $X$-ray binaries, well within the core radius of the globular cluster. However, three of them, coinciding with the cluster center of gravity, remain unresolved. Their total luminosity is $L_X^{Obs}\simeq 2.7\times 10^{33}$ erg s$^{-1}$. If one of these sources is the $X$-ray counterpart of the intermediate-mass black hole in NGC 6388, the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is $3\times 10^{-9}$. This measurement could be tightened if moderately deep radio observations of the field were performed.

astro-ph↗