arXiv subjects

Sergio Campana

Publications and source records attributed to Sergio Campana.

124 records · Page 7Linked to original sources

arXiv2000

On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients

Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs) display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact that the swing in the mass inflow rate is likely in the same range. Advection dominated accretion flows (ADAFs) were proposed to interpret such a difference. The residual optical/UV emission of quiescent LMXRTs, after subtraction of the companion star spectrum, is produced by synchrotron radiation in the (latest version) of ADAF and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of BHCs is consistent with that of NSs. We explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star takes place in the quiescence intervals. The residual optical/UV emission of BHCs are expected to derive from the energy released by the matter transferred from the companion star at radii comparable to the circularisation radius. The quiescent X-ray luminosity originates either from accretion onto the BH at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, the NSs in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission and most of the residual optical/UV. The soft, thermal-like X-ray component may arise from the cooling of the NS surface. This scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs. (ABRIDGED).

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arXiv2000

BeppoSAX observation of Cen X-4 in quiescence

We report on a 61 ks BeppoSAX observation of the soft X-ray transient Cen X-4 during quiescence which allowed to study the source X-ray spectrum from ~ 0.3 keV to ~ 8 keV. A two-component spectral model was required, consisting of a black body with temperature of ~ 0.1 keV and a power law with photon index ~ 2. These values are compatible with earlier ASCA results indicating that Cen X-4 may be stable, within a factor of a few, over a 5 year baseline.

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arXiv2000

Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field

Starting from the observation that kilohertz Quasi Period Oscillations (kHz QPO) occur in a very narrow range of X-ray luminosities in neutron star low mass X-ray binaries, we try to link the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changing mass inflow rate. At low luminosities, the drop off of kHz QPOs activity may be explained by the onset of the centrifugal barrier, when the magnetospheric radius reaches the corotation radius. At the opposite side, at higher luminosities, the magnetospheric radius may reach the neutron star and the vanishing of the magnetosphere may led to the stopping of the kHz QPOs activity. If we apply these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1) and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits naturally apply in the framework of beat frequency models but can also work in the case of general relativistic models.

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arXiv1999

Evidence for an ~80 day periodicity in the X-ray transient pulsar XTE J1946+274

We report evidence for an ~ 80 d periodicity in the X-ray flux of the hard transient pulsar XTE J1946+274. The 1.3-12 keV light curve obtained with the RossiXTE All Sky Monitor shows five regularly spaced flares over a ~ 1 year baseline starting from the outburst onset in Sept. 1998. The first and strongest flare is somewhat longer than the subsequent four flares, which recur in a fairly periodic fashion. This suggests that the profile of the first flare is dominated by the time variability of the Be star ejection episode, while the following four flares are primarily caused by the neutron star motion along an eccentric orbit.

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arXiv1999

GRB Redshift determination in the X-ray band

If gamma-ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X-ray spectrum. Such features can be detected by existing and planned X-ray satellites, as long as the X-ray afterglow is observed after a few minutes from the burst. Detection of these X-ray features will make possible the determination of the redshift of gamma-ray bursts even when their optical afterglows are severely dimmed by extinction.

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arXiv1999

Iron line in the afterglow: a key to the progenitor

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectrum of GRB 970508 and GRB 970828, if confirmed, would be a major breakthrough for understanding the nature the progenitor of gamma-ray bursts. We show that a large mass of iron very close to the burster is necessary to produce the emission line. This in itself strongly limits the possible progenitor of the gamma-ray event, suggesting the former explosion of a supernova, as predicted in the Supranova model (Vietri & Stella 1998). The line emission process and the line intensity depend strongly on the age, density and temperature of the remnant. The simultaneous observation of the iron line and of a power-law optical afterglow lasted for one year strongly suggest that the burst emission is isotropic. Recent observations of GRB 990123 are also discussed.

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arXiv1999

Iron line afterglows: general constraints

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors, strongly suggesting the presence of a large, iron rich, mass in the vicinity of the burst event. Model-independent limits to the size and the mass of the the iron line emitting region are derived and discussed. We also discuss how these results can be used to constrain the amount of beaming or anisotropy of the burst emission.

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arXiv1999

Iron line afterglows: how to produce them

We discuss how a powerful iron line emission can be produced if ~1-5 iron rich solar masses are concentrated in the close vicinity of the burst. Recombination, thermal and fluorescent reflection are discussed. We find that recombination suffers the high Compton temperature of the plasma while the other two scenarios are not mutually exclusive and could account for the claimed iron line detected in two afterglows.

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arXiv1999

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm

We present a new detection algorithm based on the wavelet transform for the analysis of high energy astronomical images. The wavelet transform, due to its multi-scale structure, is suited for the optimal detection of point-like as well as extended sources, regardless of any loss of resolution with the off-axis angle. Sources are detected as significant enhancements in the wavelet space, after the subtraction of the non-flat components of the background. Detection thresholds are computed through Monte Carlo simulations in order to establish the expected number of spurious sources per field. The source characterization is performed through a multi-source fitting in the wavelet space. The procedure is designed to correctly deal with very crowded fields, allowing for the simultaneous characterization of nearby sources. To obtain a fast and reliable estimate of the source parameters and related errors, we apply a novel decimation technique which, taking into account the correlation properties of the wavelet transform, extracts a subset of almost independent coefficients. We test the performance of this algorithm on synthetic fields, analyzing with particular care the characterization of sources in poor background situations, where the assumption of Gaussian statistics does not hold. For these cases, where standard wavelet algorithms generally provide underestimated errors, we infer errors through a procedure which relies on robust basic statistics. Our algorithm is well suited for the analysis of images taken with the new generation of X-ray instruments equipped with CCD technology which will produce images with very low background and/or high source density.

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arXiv1999

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. II: application to the HRI and first results

The wavelet detection algorithm (WDA) described in the accompanying paper by Lazzati et al. is made suited for a fast and efficient analysis of images taken with the High Resolution Imager (HRI) instrument on board the ROSAT satellite. An extensive testing is carried out on the detection pipeline: HRI fields with different exposure times are simulated and analysed in the same fashion as the real data. Positions are recovered with few arcsecond errors, whereas fluxes are within a factor of two from their input values in more than 90% of the cases in the deepest images. At variance with the ``sliding-box'' detection algorithms, the WDA provides also a reliable description of the source extension, allowing for a complete search of e.g. supernova remnant or cluster of galaxies in the HRI fields. A completeness analysis on simulated fields shows that for the deepest exposures considered (~120 ks) a limiting flux of \~3x10^{-15} erg/cm2/s can be reached over the entire field of view. We test the algorithm on real HRI fields selected for their crowding and/or presence of extended or bright sources (e.g. cluster of galaxies and of stars, supernova remnants). We show that our algorithm compares favorably with other X-ray detection algorithms such as XIMAGE and EXSAS. A complete catalog will result from our analysis: it will consist of the Brera Multi-scale Wavelet Bright Source Catalog (BMW-BSC) with sources detected with a significance >4.5 sigma and of the Faint Source Catalog (BMW-FSC) with sources at >3.5 sigma. A conservative estimate based on the extragalactic log(N)-log(S) indicates that at least 16000 sources will be revealed in the complete analysis of the whole HRI dataset.

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arXiv1999

Further evidence that 1RXS J170849.0-400910 is an Anomalous X-ray pulsar

We report the results of two ROSAT HRI observations of the recently discovered 11s X-ray pulsar 1RXS J170849.0-400910. A refined position with a smaller error radius (10" uncertainty) and a new spin period measurement were obtained. These results allowed to derive a period derivative of about 7 times 10^-4 s yr^-1 and to perform a photometric and spectroscopic study of the possible optical counterparts of the source. The limits derived from the optical to X-ray flux ratio exclude the presence of a massive OB companion. These findings, together with the nearly constant X-ray flux, the stability of the pulse shape and pulsed fraction across observations spanning three years, strongly support the inclusion of this 11s pulsar in the class of Anomalous X-ray Pulsars (AXPs).

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arXiv1999

Iron line in the afterglow: a key to unveil Gamma-Ray Burst progenitors

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors. Piro et al. (1999) and Yoshida et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB 970828, respectively. We discuss how such a strong line could be produced in the various scenarios proposed for the event progenitor. We show that the observed line intensity requires a large iron mass, concentrated in the vicinity of the burst. The previous explosion of a supernova, predicted in the Supranova scenario, is the most straightforward way to account for such a large amount of matter. We discuss three different physical processes that could account for the line: recombination, reflection and thermal emission. Among these, reflection and thermal emission may explain the observed line features: reflection should be important if the remnant is optically thick, while thermal lines can be produced only in a thin plasma. The recombination process requires extremely high densities to efficiently reprocess the burst photons, whereas this process could work during the X-ray afterglow. Future key observations for discriminating the actual radiating process are discussed.

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arXiv1996

Search for Old Neutron Stars in molecular clouds: Cygnus Rift and Cygnus OB7

We present the results of a systematic search for old isolated neutron stars (ONSs) in the direction of two giant molecular clouds in Cygnus (Rift and OB7). From theoretical calculations, we expect the detection of a large number of ONSs with the PSPC on board ROSAT. By analyzing the PSPC pointings in the direction of the clouds, we find four sources characterized by count rates ($\sim 10^{-3}$ c s$^{-1}$) and spectral properties consistent with the hypothesis that the X--ray radiation is produced by ONSs and also characterized by the absence of any measurable optical counterpart within their error circle in the digitized red plates of the Palomar All Sky Survey. The importance of follow--up deep observations in the direction of these ONS candidates is discussed. The observational and theoretical approach presented here could be fruitfully applied also to the systematic search for ONSs in other regions of the Galaxy.

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arXiv1996

ROSAT observations of A0538--66 during quiescence

We discuss serendipitous ROSAT observations of the Be transient A0538-66, which contains the fastest known accreting X-ray pulsar (69 ms). We show that this transient is detected several times during quiescence at a level of about 10^34-35 erg/s. Considering this low luminosity it is more likely that X-rays are produced by matter falling onto the neutron star magnetosphere, rather than onto the star surface. This could be the first example of an X-ray source in the "propeller regime".

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arXiv1994

Reverberation by a Relativistic Accretion Disk

We calculate the response of a line emitted from a relativistic accretion disk to a continuum variation of the central illuminating source. This model might be relevant to the very broad and sometimes redshifted emission lines which have been observed at optical and X-ray energies in Active Galactic Nuclei and X-ray binaries. We consider separately the cases of a point-like and of an extended central source. Based on the first three moments of the line energy distribution (i.e. the line intensity, centroid energy and width) and the two-dimensional transfer function, we identify a number of characteristic features, which can help assessing the presence of a relativistic accretion disk and estimating its parameters. In particular, by combining an absolute time measurement of the maximum of line response function with a relative measurement in units of light crossing time the mass of the central object can be derived.

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arXiv1994

Microlensing and Halo Mass in Form of Stars

Paczyński (1986) suggested that ``dark" objects in the halo of our Galaxy could enhance the luminosity of foreground stars, acting as gravitational microlenses. Such events has been recently reported by different collaborations. We assume that these microlensing events are produced by baryonic objects in the halo of our Galaxy. Rather than adopting a mean mass for the lensing objects we consider them distributed along a mass function similar to the disk one. Even if the number of microlensing events so far detected is not large, we show that it is possible to constrain the fraction of mass density in the halo of our Galaxy in form of stars. We estimate that this fraction varies between 0.1 and 0.8, if halo stars trace the total halo mass density; in the opposite case this range becomes narrower. If lensing objects have not been formed apart from the other halo stars, but in a more general context of star formation, neutron stars must also be present. We discuss these results in relation to galactic halo models for gamma-ray bursts.

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