arXiv subjects

A. Santangelo

Publications and source records attributed to A. Santangelo.

At least 199 records · Page 11Linked to original sources

arXiv2005

Observation of V0332+53 over the 2004/2005 outburst with Integral

We present the spectral and temporal analysis of the 2004/2005 outburst of the transient X-ray pulsar V0332+53 as observed with Integral. After the discovery of the third cyclotron line in phase averaged spectra (Kreykenbohm et al 2005, Pottschmidt et al 2005), detailed pulse phase spectroscopy revealed remarkably little variability of the cyclotron lines through the 4.4s X-ray pulse (Pottschmidt et al 2005). During the decline of the outburst, the flux was observed to decay exponentially until 2005 Feb 10 and linearly thereafter. The spectrum was found to become harder with time, while the folding energy remained constant. The energy of the fundamental cyclotron line increased with time from 26.5kev in the RXTE observation up to 29.5kev in the last Integral one indicating that the emission region is moving closer to the surface of the neutron star. For a detailed analysis, see Mowlavi et al (2005).

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arXiv2005

The BeppoSAX view on the galactic high-mass X-ray binary 4U 0114+65

A pointed observation on the galactic high-mass X-ray binary 4U 0114+65 was carried out with BeppoSAX in order to compare the X-ray spectral and timing characteristics observed by this satellite over the broadest range of energies ever (1.5-100 keV) with the information previously obtained with other spacecraft. The light curve of 4U 0114+65 shows a large flare at the beginning of the BeppoSAX pointing and no significant hardness evolution either during the flare or in the low state occurring after the flare itself. The modulation at about 2.7 hours, attributed to the accreting neutron star (NS) spin periodicity, is not significantly detected in our data, although fluctuations with timescales of about 3 hours can be seen in the 2-10 keV light curve. Shorter modulations down to timescales of minutes, are also found and interpreted as due to accretion of matter onto the NS. The flaring and the low state spectra of 4U 0114+65 can be equally well fitted either with a power law modulated by a high-energy exponential cutoff or with a Comptonization model. During the low state the presence, although tentative, of a thermal component (with kT around 0.3 keV) at low energies, possibly produced by an ionized plasma cloud around the NS, cannot be excluded. Contrary to previous claims, a cyclotron resonant feature in absorption at about 22 keV was not detected in the BeppoSAX spectroscopic data, whereas evidence for a Fe emission line around 6.4 keV is found only during the low state emission. Using all of the above information, a scenario for the system in which the NS is embedded in, and accreting from, a low angular momentum gas cloud is envisaged.

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arXiv2005

A large spin-up rate measured with INTEGRAL in the High Mass X-ray Binary Pulsar SAXJ2103.5+4545

The High Mass X-ray Binary Pulsar SAXJ2103.5+4545 has been observed with INTEGRAL several times during the last outburst in 2002-2004. We report a comprehensive study of all INTEGRAL observations, allowing a study of the pulse period evolution during the recent outburst. We measured a very rapid spin-up episode, lasting 130days, which decreased the pulse period by 1.8s. The spin-up rate, pdot=-1.5e-7 s/s, is the largest ever measured for SAXJ2103.5+4545, and it is among the fastest for an accreting pulsar. The pulse profile shows evidence for temporal variability, apparently not related to the source flux or to the orbital phase. The X-ray spectrum is hard and there is significant emission up to 150keV. A new derivation of the orbital period, based on RXTE data, is also reported.

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arXiv2005

INTEGRAL observations of V0332+53 in outburst

We present the analysis of a 100ksec Integral(3-100kev) observation of the transient X-ray pulsar V0332+53 inoutburst. The source is pulsating at P=4.3751+/-0.0002s with a clear double pulse from 6 kev to 60 kev. The average flux was ~550mCrab between 20 kev and 60 kev. We modeled the broad band continuum from 5 kev to 100 kev with a power-law modified by an exponential cut off. We observe three cyclotron lines: the fundamental line at 24.9+/-0.1 kev, the first harmonic at 50.5+/-0.1 kev as well as the second harmonic at71.7+/-0.8 kev, thus confirming the discovery of the harmonic lines by Coburn et al. (2005) in RXTE data.

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arXiv2004

GX 301-2 as seen by Integral

We present observations of the High Mass X-ray Binary GX 301-2 taken during the Galactic Plane Scan (GPS) with Integral following our RXTE observations (Kreykenbohm et al. 2004). The optical companion of GX 301-2 is the B1Ia+ hypergiant Wray 977 with a luminosity of 1.3 x 10^6 Lsun and a mass of ~48 Msun making the system one of the most massive X-ray binaries known. The system was observed 24 times during the GPS thus covering many orbital phases including the pre-periastron flare. The source is clearly detected in all Integral instruments in most pointings. The derived X-ray light curves show strong variability on all timescales. Depending on the orbital phase of the pointings, the luminosity changes by factor of more than 10 during the pre-periastron flare compared to other pointings. Furthermore, broad band spectra using all instruments available on Integral are compared with data taken by the RXTE instruments PCA and HEXTE (using the same data as in Kreykenbohm et al. 2004).

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arXiv2004

INTEGRAL broadband spectroscopy of Vela X-1

The wind-accreting X-ray binary pulsar and cyclotron line source Vela X-1 has been observed extensively during INTEGRAL Core Program observations of the Vela region in June-July and November-December 2003. In the latter set of observations the source showed intense flaring -- see also Staubert et al. (2004), these proceedings. We present early results on time averaged and time resolved spectra, of both epochs of observations. A cyclotron line feature at ~53 keV is clearly detected in the INTEGRAL spectra and its broad shape is resolved in SPI spectra. The remaining issues in the calibration of the instruments do not allow to resolve the question of the disputed line feature at 20-25 keV. During the first main flare the average luminosity increases by a factor of \~10, but the spectral shape remains very similar, except for a moderate softening.

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arXiv2004

The INTEGRAL LMXRB Monitoring Programme

Our collaboration is responsible for the study of a sample of 72 low mass X-ray binaries (LMXRB) using the INTEGRAL Core Programme data. In this paper we describe the monitoring programme we have started and the current variability and spectral results on a sample of 8 persistently bright LMXRBs hosting a neutron star (Z and Atoll sources). Current results show that among our sample of sources there seems to be no important difference in the variability of Z sources with respect to Atolls and the first colour-colour and hardness intensity diagrams built in the "traditional" energy bands display the expected patterns. Z sources seem to be harder than the bright Atolls of our sample (above 20 keV) and present no evident cut-off until about 50 keV. A hint of a non-thermal hard tail is seen in Sco X-1 with ISGRI and SPI, similarly to what was previously detected by D'Amico et al. (2001) with RXTE. These results, even if preliminary, show the importance of such a programme and the potential underlying it to understand these sources as a class.

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arXiv2004

First results on the HMXRB Pulsar SAXJ2103.5+4545 with INTEGRAL

We report on the preliminary timing and spectral analysis of the High Mass X-ray Binary Pulsar SAXJ2103.5+4545 as observed with INTEGRAL during the Galactic Pla n Scan of the Core Program. The source shows a hard spectrum, being detected up to 100 keV. The timing analysis performed on IBIS/ISGRI data shows evidence for a spin-up wi th respect to previous observations, performed in 1997 and 1999 with BeppoSAX an d RossiXTE.

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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

First IBIS results on the high energy emission of Cyg X-2

The bright low-mass X-ray binary Cyg X-2 was in the field of view of the IBIS telescope during the early Cygnus region observations, executed during the INTEGRAL Performance Verification Phase. The data presented are spanning about one week and cover the rising edge of one of the two peaks of the ~ 82 day cycle of the Cyg X-2 light curve. The IBIS data in the energy range 20-40 keV exhibit flux variation correlated with the RXTE/ASM light curve. Two different main exposures, separated by ~ 5 days are found to be characterized by sensibly different spectra, with significant softening and higher X-ray luminosity in the second part, coincident with the long-term cycle peak. At high energies, both measured spectra are very steep. The ratio of the 30-45 keV and 20-30 keV detected fluxes is ~ 0.30, against a value of 0.95 expected for a Crab-like spectrum. No positive detection exists for E > 45 keV, with a flux upper limit (5 sigma) of ~1.4E-10 erg cm-2 s-1$ in the 45-100 keV band.

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arXiv2003

First INTEGRAL Observations of Eight Persistent Neutron Star Low Mass X-ray Binaries

Early results from the INTEGRAL Core Program, for a sample of eight persistently bright neutron star low mass X-ray binaries in the energy range from 5 keV to 200 keV are presented. It is shown that INTEGRAL efficiently detects sources and that spectra may be obtained up to several hundreds of keV by combining data from three of the four INTEGRAL instruments: JEM-X, IBIS and SPI. For the source GX 17+2 it is shown that the spectrum extends well above 100 keV with a flattening of the spectrum above 30 keV. This might suggest a non-thermal comptonisation emission, but uncertainties in the current data reduction and background determination do not allow firm conclusions to be drawn.

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arXiv2002

A Puzzling Paucity of Double Peaked X-ray Pulsars

Accretion powered pulsars exhibit a variety of lightcurves. In this paper we propose to classify the observed lightcurves as single or double pulsed.We analyze the lightcurves of 86 accretion powered pulsars and assign them to these classes. We present three datasets: first in which the classification can be easily done, second for which the classification is more difficult and not certain, and third for which we were unable to classify the pulsar because of lack of published data. We analyze a simple model in which the angles between the magnetic and rotation axis $β$, and between the rotation axis and the line of sight $θ$ are random, and show that it is inconsistent with the data. We also present a model in whichthe angle between the magnetic axis and rotation axis is restricted and compare it with the data. This leads to an upper limit on the angle $β< 40^\circ$. We conclude that there must be a mechanism that leads to alignment of the magnetic and spin axis in X-ray pulsars.

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arXiv2001

The 1998 outburst of the X-ray transient XTE J2012+381 as observed with BeppoSAX

We report on the results of a series of X-ray observations of the transient black hole candidate XTE J2012+381 during the 1998 outburst performed with the BeppoSAX satellite. The observed broad-band energy spectrum can be described with the superposition of an absorbed disk black body, an iron line plus a high energy component, modelled with either a power law or a Comptonisation tail. The source showed pronounced spectral variability between our five observations. While the soft component in the spectrum remained almost unchanged throughout our campaign, we detected a hard spectral tail which extended to 200 keV in the first two observations, but became barely detectable up to 50 keV in the following two. A further re-hardening is observed in the final observation. The transition from a hard to a soft and then back to a hard state occurred around an unabsorbed 0.1-200 keV luminosity of 10^38 erg/s (at 10 kpc). This indicates that state transitions in XTE 2012+281 are probably not driven only by mass accretion rate, but additional physical parameters must play a role in the evolution of the outburst.

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arXiv2001

X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her

(Abridged) We report on ASCA and BeppoSAX observations of the X-ray source 4U 1700+24 and on (quasi-)simultaneous spectroscopy of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. Archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24 are also analyzed along with a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; a revised distance to the object of ~400 pc is inferred. While these spectra do not show either any change between the two epochs or any peculiar feature, the X-ray spectra reveal a complex and long-term variable shape, with a clear soft excess. The X-ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy(<1 keV) excess, which can be modeled with a blackbody emission with kT_BB ~ 1 keV; the latter component is not detected at the lowest source flux levels. The ratio between the two components varies substantially with the source flux. The X-ray emission from the object appears to become harder as its luminosity increases, and the RXTE data acquired during an outburst occurred during Fall 1997 display a hard tail detected up to 100 keV. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no significant pulsations or QPOs are found from the timing analysis of the X-ray light curves. With the new distance determination, the 2-10 keV X-ray luminosity range spanned in the considered observations lies between ~2x10^32 and ~1x10^34 erg/s. All this allows us to suggest a scenario consisting of a wide binary system in which a neutron star accretes matter from the wind of an M giant.

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arXiv2001

X-Ray Observations of Cygnus X-2

We report on a joint BeppoSAX/RossiXTE observation of the Z-type low mass X-ray binary Cyg X-2. The source was in the so-called high overall intensity state and in less than 24 hours went through all three branches of the Z-track. The continuum x-ray spectrum could be described by the absorbed sum of a soft thermal component, modeled as either a blackbody or a multicolor disk blackbody, and a Comptonized component. The timing power spectrum showed several components including quasiperiodic oscillations in the range 28-50 Hz while the source was on the horizontal branch (horizontal branch oscillation; HBO). We found that the HBO frequency was well correlated with the parameters of the soft thermal component in the x-ray spectrum. We discuss implications of this correlation for models of the HBO.

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arXiv2001

A measurement of the broad-band spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

We report on results of a target of opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the SAX satellite. The measured spectrum is a power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ~100 keV. The soft excess is consistent with a blackbody with temperature of ~40 eV and a low flux, while the cut-off power law is well fitted by thermal Comptonization in a plasma with an electron temperature of 100 keV and an optical depth of order of unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Though the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is about 10^{-3}, which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with location of the system in the halo.

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arXiv2001

A BeppoSAX observation of Her X-1 during the first main-on after an anomalous low-state: evidence for rapid spin-down

Results of a BeppoSAX observation of Her X-1 in 2000 October during the first main-on state after the longest recorded anomalous low-state are presented. The 0.1-30 keV spectrum, light curve and pulse profile are all consistent with those measured during previous main on-states, indicating that Her X-1 has resumed its regular 35 day cycle with similar on-state properties as before. However, from a comparison of the measured pulse period with that obtained close to the start of the anomalous low-state, it is evident that Her X-1 continued to spin-down strongly during the anomalous low-state such that the pulse period has returned to a similar value as ~15 years ago. Additionally, the occurrence time of the main-on states after the end of the anomalous low-state indicate that a change in the length, or phasing, of the 35-day cycle occurred during the anomalous low-state.

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arXiv2000

X-ray emission in the direction of the SNR G318.2+0.1

We report the discovery of three X-ray sources within the radio shell G318.2+0.1, one of which may be extended. Two of the sources were detected during the \sax Galactic Plane Survey and one was found in archival \ros data. The fainter \sax source is coincident with an ultra-compact galactic H {\sc ii} region, and we discuss the possibility that it can be a flaring young stellar object, while the other \sax source has no obvious counterpart. The PSPC source is consistent with emission from a foreground star. The hard spectrum of the brighter \sax source is consistent with a non-thermal origin, although a thermal nature cannot be formally excluded. If this source is associated with G318.2+0.1, then its hard spectrum suggests that it may be site of non-thermal electron acceleration.

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