arXiv subjects

S. Campana

Publications and source records attributed to S. Campana.

At least 397 records · Page 22Linked to original sources

arXiv2004

Accurate X-ray position of the Anomalous X-ray Pulsar XTE J1810-197 and identification of its likely IR counterpart

We report the accurate sub-arcsec X-ray position of the new Anomalous X-ray Pulsar (AXP) XTE J1810-197, derived with a Chndra-HRC Target of Opportunity observation carried out in November 2003. We also report the discovery of a likely IR counterpart based on a VLT (IR band) Target of Opportunity observation carried out in October 2003. Our proposed counterpart is the only IR source (Ks=20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio, are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn-i band images obtained at the 3.6m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27th and September 8th, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.

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arXiv2003

X-ray/optical observations of A0535+26/HDE 245770 in quiescence

We present the result of three BeppoSAX observations of the X-ray binary pulsar A0535+26 in its quiescent state. The source is quite well detected up to 200 keV (6 sigma detection in the stronger observation). No Iron line is detected in the MECS data (3 sigma upper limit on its equivalent width of 150 eV). There is evidence of a soft excess below 2 keV. Pulsation is detected in all energy bands up to 10 keV, with a pulsed fraction of ~ 0.5, not varying with energy. There is a marginal detection (4 sigma) of a cyclotron resonance feature (CRF) at 118 +/- 20 keV in the PDS spectrum. During the BeppoSAX observations HDE 245770, the optical counterpart to A0535+26, was monitored spectroscopically and photometrically. These observations show that the Halpha and Hbeta lines previously observed in absorption returned in emission, indicating that the Be disk formed again. The presence of pulsation and a CRF is a clear indication that accretion onto the polar caps is occurring, and that the propeller mechanism is leaking. The presence of a soft excess could be explained either in terms of thermal emission (kT ~ 1.2 keV) from the neutron star surface, as observed in other accreting X-ray pulsars, or in terms of an overabundance of Mg in the circumstellar matter.

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arXiv2003

The variable quiescence of Cen X-4

Cen X-4 is one of the best studied low-mass neutron star transients in quiescence. Thanks to XMM-Newton large throughput, Cen X-4 was observed at the highest signal to noise ever. This allowed us to disclose rapid (>100 s), large (45+/-7 rms in the 10^{-4}-1 Hz range) intensity variability, especially at low energies. In order to highlight the cause of this variability, we divided the data into intensity intervals and fit the resulting spectra with the canonical model for neutron star transients in quiescence, i.e. an absorbed power law plus a neutron star atmosphere. The fit is consistent with a variable column density plus variability in (at least) one of the spectral models. Variations in the neutron star atmosphere might suggest that accretion onto the neutron star surface is occuring in quiescence; variations in the power law tail should support the view of an active millisecond radio pulsar emitting X-rays at the shock between a radio pulsar wind and inflowing matter from the companion star.

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arXiv2003

SIMBOL-X : a new generation hard X-ray telescope

SIMBOL-X is a hard X-ray mission, operating in the 0.5-70 keV range, which is proposed by a consortium of European laboratories for a launch around 2010. Relying on two spacecraft in a formation flying configuration, SIMBOL-X uses a 30 m focal length X-ray mirror to achieve an unprecedented angular resolution (30 arcsec HEW) and sensitivity (100 times better than INTEGRAL below 50 keV) in the hard X-ray range. SIMBOL-X will allow to elucidate fundamental questions in high energy astrophysics, such as the physics of accretion onto Black Holes, of acceleration in quasar jets and in supernovae remnants, or the nature of the hard X-ray diffuse emission. The scientific objectives and the baseline concepts of the mission and hardware design are presented.

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arXiv2003

Unveiling the Nature of the 321 s Modulation in RX J0806.3+1527: Near-Simultaneous Chandra and VLT Observations

We report on the first near-simultaneous X-ray and optical observations of RX J0806.3+1527. The source is believed to be a 321s orbital period ultra-compact binary system hosting an X-ray emitting white dwarf. Data were obtained with Chandra and the ESO Very Large Telescope (VLT) in November 2001. We found an optical/X-ray phase-shift in the periodic modulation of about 0.5, strongly favoring the existance of two distinct emission regions in the two bands (for the pulsed fluxes). The Chandra data allow us to study, for the first time, the spectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by a blackbody spectrum with kT about 65eV and hydrogen column density of N_H about 5 x 10^20 cm^-2. The average (unabsorbed) source 0.1-2.5 keV luminosity during the modulation-on is L_X about 5 x 10^32 erg s^-1 (assuming a distance of 500pc). Such a value is lower than the luminosity expected if stable mass transfer between two white dwarfs were driven by gravitational radiation. Evidence for absorption-like features are present in the phase-averaged spectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavy elements (C and N). We compare and discuss these findings with other binary systems hosting an accreting white dwarf.

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arXiv2003

XMM-Newton observation of the 5.25 ms transient millisecond pulsar XTE J1807-294 in outburst

We report on the results obtained for the millisecond transient X-ray pulsar XTE J1807-294 in a 40 min orbital period system, based on an XMM-Newton ToO observation carried out during March 2003. The source was found at a luminosity level of about 2x10^36 erg/s in the 0.5-10 keV range (assuming a distance of 8 kpc). We confirm the presence of the 5.25 ms pulsations (after accounting for the orbital modulation) and find a pulsed fraction of 5.8% in the 0.3-10 keV band. The pulse shape in nearly sinusoidal. The spectral continuum of the source is well fitted by an absorbed Comptonization model plus a soft component. No emission or absorption lines have been detected in the 0.5-10 keV range with upper limits of 10-40 eV. The reported analysis represent the first detailed study of this source, the fourth belonging to the ultra-compact binary system class hosting an accreting neutron star.

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arXiv2003

The IR counterpart to the Anomalouos X-ray Pulsar 1RXS J1708-4009

We report the discovery of the likely IR counterpart to the Anomalous X-ray Pulsar (AXP) 1RXS J1708-4009, based on the combination of Chandra HRC-I X-ray position, and deep optical/IR observations carried out from ESO telescopes and the Canada France Hawaii Telescope (CFHT) during 1999-2002. Within the narrow uncertainty region we found two relatively faint (K'=20.0 and K'=17.53) IR objects. Based on their color and position in the J-K' versus J-H diagram only the brighter object is consistent with the known IR properties of the counterparts to other AXPs. No variability was detected for this source, similarly to what is observed in the case of 4U 0142+614. Like in other AXPs, we found that the IR flux of 1RXSJ1708-4009 is higher then expected for a simple blackbody component extrapolated from the X-ray data. If confirmed, this object would be the fourth IR counterpart to a source of the AXP class, and would make the IR excess a likely new characteristic of AXPs.

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arXiv2003

Discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40

We report the discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40 during the 100 ks observation performed by BeppoSAX on 1999 August 15-16. Six X-ray bursts have been observed. The unabsorbed 2-10 keV fluxes of the bursts range from ~ (3-9)x10^(-10) erg cm^(-2)s^(-1). A correlation between peak flux and fluence of the bursts is found, in agreement with the behaviour observed in other similar sources. There is a trend of the burst flux to increase with the time interval from the previous burst. From the value of the persistent flux we infer a mass accretion rate Mdot~7x10^(-11) Msun/yr, that may correspond to the mixed hydrogen/helium burning regime triggered by thermally unstable hydrogen. We have also analysed a BeppoSAX observation performed on 2001 August 22 and previous RXTE observations of 4U 1708-40, where no bursts have been observed; we found persistent fluxes of more than a factor of 7 higher than the persistent flux observed during the BeppoSAX observation showing X-ray bursts.

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arXiv2003

The resolved fraction of the Cosmic X-ray Background

We present the X-ray source number counts in two energy bands (0.5-2 and 2-10 keV) from a very large source sample: we combine data of six different surveys, both shallow wide field and deep pencil beam, performed with three different satellites (ROSAT, Chandra and XMM-Newton). The sample covers with good statistics the largest possible flux range so far: [2.4*10^-17 - 10^-11] cgs in the soft band and [2.1*10^-16 - 8*10^{-12}]cgs in the hard band. Integrating the flux distributions over this range and taking into account the (small) contribution of the brightest sources we derive the flux density generated by discrete sources in both bands. After a critical review of the literature values of the total Cosmic X--Ray Background (CXB) we conclude that, with the present data, the 94.3%, and 88.8% of the soft and hard CXB can be ascribed to discrete source emission. If we extrapolate the analytical form of the Log N--Log S distribution beyond the flux limit of our catalog in the soft band we find that the flux from discrete sources at ~3*10^-18 cgs is consistent with the entire CXB, whereas in the hard band it accounts for only 93% of the total CXB at most, hinting for a faint and obscured population to arise at even fainter fluxes.

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arXiv2002

The Brera Multi-scale Wavelet ROSAT HRI source catalogue (BMW-HRI)

We present the Brera Multi-scale Wavelet ROSAT HRI source catalogue (BMW-HRI) derived from all ROSAT HRI pointed observations with exposure time longer than 100 s available in the ROSAT public archives. The data were analyzed automatically using a wavelet detection algorithm suited to the detection and characterization of both point-like and extended sources. This algorithm is able to detect and disentangle sources in very crowded fields and/or in presence of extended or bright sources. Images have been also visually inspected after the analysis to ensure verification. The final catalogue, derived from 4,303 observations, consists of 29,089 sources detected with a detection probability of greater or equal 4.2 sigma. For each source, the primary catalogue entries provide name, position, count rate, flux and extension along with the relative errors. In addition, results of cross-correlations with existing catalogues at different wavelengths (FIRST, IRAS, 2MASS and GSC2) are also reported. All these information are available on the web via the DIANA Interface. As an external check, we compared our catalogue with the previously available ROSHRICAT catalogue (both in its short and long versions) and we were able to recover, for the short version, \~90% of the entries. We computed the sky coverage of the entire HRI data set by means of simulations. The complete BMW-HRI catalogue provides a sky coverage of 732 square degrees down to a limiting flux of ~1E-12 erg/s/cm2 and of 10 square degrees down to ~1E-14 erg/s/cm2. We were able to compute the cosmological log(N)-log(S) distribution down to a flux of about 1.2x1E-14 erg/s/cm2.

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arXiv2002

The detection of variability from the candidate IR counterpart to the Anomalous X-ray Pulsar 1E1048.1-5937

We report on the detection of variability from the proposed IR counterpart to the Anomalous X-ray Pulsar (AXP) 1E1048.1-5937 based on Chandra and ESO optical/IR deep observations carried out in 2001-2002. Within the narrow Chandra uncertainty region for 1E1048.1-5937 we found only one relatively faint (J=22.1+/-0.3, J-Ks=2.4) source, while the recently proposed IR counterpart was not detected down to a limiting Ks magnitude of about 20.7 (3sigma confidence level). This implies a remarkable IR brightening of this object, Delta Ks>1.3, on a timescale of about 50 days. Although our knowledge of the IR properties of AXPs is rather limited (there is only another source, 1E2259+586, for which IR variability has been detected), the observed IR variability of the proposed counterpart strengthens its association with 1E1048.1-5937. Our results make the IR (and presumably optical) variability a likely common characteristic of AXPs, and provide new constraints on this class of objects.

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arXiv2002

The 2002 outburst of the microquasar XTE J1550-564

We present results of spectral and timing analysis based on 11 RXTE PCA/HEXTE observations of the microquasar XTE J1550-564 during its last outburst in January 2002. The observed behaviour is comparable to most Black-Hole Candidates in the low/hard state. This is unlike the 1998-99 outburst, when it showed a much more complex feature, probably because of the higher luminosity. For each of the 11 observations we extracted energy spectra and power density spectra, finding typical features of a low/hard state.

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arXiv2002

The quiescent X-ray emission of three transient X-ray pulsars

We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U 0115+63 and V 0332+53. These observations allowed us to study these transients at the faintest flux levels thus far. Spectra are remarkably different from the ones obtained at luminosities a factor >10 higher, testifying that the quiescent emission mechanism is different. Pulsations were not detected in any of the sources, indicating that accretion of matter down to the neutron star surface has ceased. We conclude that the quiescent emission of the three X-ray transients likely originates from accretion onto the magnetospheric boundary in the propeller regime and/or from deep crustal heating resulting from pycnonuclear reactions during the outbursts.

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arXiv2002

An XMM-Newton study of the 401 Hz accreting pulsar SAX J1808.4-3658 in quiescence

SAX J1808.4-3658 is a unique source being the first Low Mass X-ray Binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first which assessed its quiescent luminosity and spectrum with good signal to noise. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658 likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even if with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5x10^{31} erg/s, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (Gamma~1.5) power law with only a minor contribution (<10%) from a soft black body component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to X-ray luminosity conversion efficiency of eta~10^{-3} is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutronstar (M>1.7 solar masses).

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arXiv2002

A Low/Hard State Outburst of XTE J1550-564

We present the results of the analysis of 11 RXTE/PCA observations of the January 2002 outburst of the black-hole transient XTE J1550-564. The outburst was rather short, with an e-folding time of ~11 days in the PCA band 3-25 keV. The source was seen in an extreme low/hard state. The energy spectra could be fitted with a power law slightly steepening (Gamma ~ 1.4 to ~1.5) in time, with a high-energy cutoff decreasing from ~300 keV to ~100 keV and some additional features between 6 and 7 keV, while the 3-150 keV flux decreased by a factor of 17. The power spectra were fitted with two broad Lorentzian components, whose characteristic frequencies decreased with the source flux, and whose fractional rms contribution was rather constant. The timing parameters derived can be identified with those observed in other systems. These results show that, although during previous, more luminous, outbursts, a very complex behavior was observed, at low luminosities XTE J1550-564 behaves in a way comparable to most Black-Hole Candidates.

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arXiv2002

RXJ0806.3+1527: a double degenerate binary with the shortest known orbital period (321s)

We carried out optical observations of the field of the X-ray pulsator RXJ0806.3+1527. A blue V=21.1 star was found to be the only object consistent with the X-ray position. VLT FORS spectra revealed a blue continuum with no intrinsic absorption lines. Broad (v~1500 km/s), low equivalent width (about -1/-6A) emission lines from the HeII Pickering series were clearly detected. B, V and R time-resolved photometry revealed the presence of about 15% pulsations at the 321s X-ray period, confirming the identification. These findings, together with the period stability and absence of any additional modulation in the 1min-5hr period range, argue in favour of the orbital interpretation of the 321s pulsations. The most likely scenario is thus that RXJ0806.3+1527 is a double degenerate system of the AM CVn class. This would make RXJ0806.3+1527 the shortest orbital period binary currently known and one of the best candidates for gravitational wave detection.

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arXiv2002

A BeppoSAX Observation of KS 1731-260 in its Quiescent State: Constraints on the NS Magnetic Field

We report here the results of a 90 ks BeppoSAX observation of the low mass X-ray binary and atoll source KS 1731-260 during a quiescent phase. From this observation we derive a source X-ray luminosity of \sim 10^{33} ergs/s (for a source distance of 7 kpc). If the neutron star is spinning at a period of a few milliseconds, as inferred from the nearly-coherent oscillations observed during type-I X-ray bursts, the quiescent X-ray luminosity constrains the neutron star magnetic field strength to less than \sim 10^9 G. If the quiescent emission originates from the release of thermal energy accumulated through deep crust heating in the previous accretion phase, then the measured quiescent luminosity requires a neutron star mass of more than 1.7 M_\odot or a very long outbust recurrence time (\sim 10^3 yr). The former possibility would be in agreement with the expectation of the recycling scenario, namely that the neutron star has accreted a significant amount of mass, enough to spin it up to millisecond periods.

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arXiv2002

The BMW Detection Algorithm applied to the Chandra Deep Field south: deeper and deeper

Chandra deep fields represent the deepest look at the X-ray sky. We analyzed the Chandra Deep Field South (CDFS) with the aid of a dedicated wavelet-based algorithm. Here we present a detailed description of the procedures used to analyze this field, tested and verified by means of extensive simulations. We show that we can safely reconstruct the LogN-Log S source distribution of the CDFS down to limiting fluxes of 2.4x10^-17 and 2.1x10^-16 erg s^-1 cm^-2 in the soft (0.5-2 keV) and hard (2-10 keV) bands, respectively, fainter by a factor ~ 2 than current estimates. At these levels we can account for ~ 90% of the 1-2 keV and 2-10 keV X-ray background.

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