arXiv subjects

G. L. Israel

Publications and source records attributed to G. L. Israel.

At least 199 records · Page 11Linked to original sources

arXiv2002

The Anomalous X-ray Pulsars

The Anomalous X-ray Pulsars (AXPs) are a small class of pulsars with spin periods in the 6-12 s range, very soft X-ray spectra, secular spin down on time scales of ~10^3-10^5 years, and lack of bright optical counterparts. Two, possibly three, of them are close to the centre of shell-like supernova remnants. AXPs are one of the most enigmatic classes of galactic high-energy sources. Isolated neutron stars powered by the loss of rotational energy can be excluded on energetic grounds. The models based on neutron stars involve either accretion (perhaps from a fossil disk around an isolated neutron star) or the decay of a very strong magnetic field (10^14-10^15 G). We review the models and the observational properties of AXPs, including recent XMM-Newton and Chandra observations. We also present some unpublished ASCA data.

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arXiv2002

BeppoSAX observations of the young pulsar in the Kes 75 Supernova Remnant

We present the results of BeppoSAX observations of the young X-ray pulsar PSR J1846-0258, recently discovered at the center of the composite supernova remnant Kes 75. The pulsar (plus nebula) spectrum can be fitted by an absorbed power law with photon index alpha_ph=2.16+/-0.15, N_H=(4.7+/-0.8) x 10^22 cm^-2, and unabsorbed flux ~3.9x10^-11 erg cm^-2 s^-1 (2-10 keV). By joining two observations taken at an interval of two weeks we have been able to obtain a precise measurement of the spin period (P=324.818968+/-0.000006 ms). This value, when combined with previous measurements, cannot be fitted by a smooth frequency evolution with a canonical braking index n=3. With the hypothesis of no glitches and/or significant timing noise, the braking index would be n=1.86+/-0.08 and, assuming a short initial period, the pulsar age would be \~1700 years, closer to that of the supernova remnant than the simple estimate τ=P/2\pdot=723 years. Other likely possibilities involve the presence of glitches and lead to a wide range of acceptable ages. For example, we obtain n in the range 1.8-2.5, if a glitch occurred near MJD 51500, while for a glitch between October 1993 and March 1999 we can only get a lower limit of n>1.89.

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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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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

The X-ray Source at the Center of the Cassiopeia A Supernova Remnant

We present the first results of an XMM-Newton observation of the central X-ray source in the Cas A supernova remnant. The spectrum can be fit equally well with an absorbed steep power law (alpha~3, N_H~1.5x10^22 cm^-2) or with a bremsstrahlung with temperature kT~2.4 keV and N_H~10^22 cm^-2. A blackbody model (kT~0.7 keV) gives a slightly worse fit and requires a column density N_H~5x10^21 cm^-2 and an emitting area with radius ~0.3 km (for d=3.4 kpc). A search for pulsations for periods longer than 150 ms gave negative results. The 3 sigma upper limits on the pulsed fraction are ~13% for P>0.3 s and ~7% for P>3 s. The overall properties of the central X-ray source in Cas A are difficult to explain in terms of a rapidly spinning neutron star with a canonical magnetic field of ~10^12 G, and are more similar to those of slowly rotating neutron stars such as the Anomalous X-ray Pulsars.

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arXiv2001

Observations of Anomalous X-ray Pulsars

We review recent results obtained from observations of Anomalous X-ray Pulsars at different wavelengths (X-rays, Optical, IR and Radio) with particular emphasis on results obtained by BeppoSAX. Proposed models for AXPs are briefly presented and critically discussed in the light of these results.

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arXiv2001

The optical afterglow of GRB 000911: evidence for an associated supernova?

We present photometric and spectroscopic observations of the late afterglow of GRB 000911, starting ~1 day after the burst event and lasting ~8 weeks. We detect a moderately significant re-brightening in the R, I and J lightcurves, associated with a sizable reddening of the spectrum. This can be explained through the presence of an underlying supernova, outshining the afterglow ~30 days after the burst event. Alternative explanations are discussed.

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arXiv2001

The pulse phase-dependent spectrum of the anomalous X-ray pulsar 1RXS J170849-400910

We report on the results of a 50ks BeppoSAX observation of 1RXS J170849-400910, one of the five (plus a candidate) known anomalous X-ray pulsars. The BeppoSAX data are consistent with a power-law plus blackbody spectral decomposition, making 1RXS J170849-400910 the fourth source of this class for which such a spectral decomposition was found. The inferred power-law slope and blackbody temperature are Gamma~2.6 and kT_BB~0.46keV, respectively. We found significant energy-dependence of the pulse profile, a remarkable feature for an AXP. By using the power-law plus blackbody decomposition we detected a significant variation in at least one spectral parameter, the power-law photon index, as a function of the pulse phase. This is the first significant detection of spectral parameter variation in an AXP. The implications of these results are briefly discussed.

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arXiv2001

The transient X-ray pulsar 4U 0115+63 from quiescence to outburst through the centrifugal transition

We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63 close to periastron. This led to the discovery of a dramatic luminosity variation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15 hr. The variation was accompanied by only minor (if any) changes in the emitted spectrum and pulse fraction. On the contrary an observation near apastron detected the source in a nearly constant state at a level of ~2x10^33 erg/s. Direct accretion onto the neutron star surface encounters major difficulties in explaining the source variability properties. When the different regimes expected for a rotating magnetic neutron star subject to a variable inflow of matter from its companion are taken into consideration, the results of BeppoSAX observations of 4U 0115+63 can be explained naturally. In particular close to apastron, the regime of centrifugal inhibition of accretion applies, whereas the dramatic source flux variability observed close to periastron is readily interpreted as the transition regime between direct neutron star accretion and the propeller regime. In this centrifugal transition regime small variations of the mass inflow rate give rise to very large luminosity variations. We present a simple model for this transition, which we successfully apply to the X-ray flux and pulse fraction variations measured by BeppoSAX.

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arXiv2001

BMW: a ROSAT-HRI source catalogue obtained with a wavelet transform detection algorithm

In collaboration with the Observatories of Palermo and Rome and the SAX-SDC we are constructing a multi-site interactive archive system featuring specific analysis tools. In this context we developed a detection algorithm based on the Wavelet Transform (WT) and performed a systematic analysis of all ROSAT-HRI public data (~3100 observations +1000 to come). The WT is specifically suited to detect and characterize extended sources while properly detecting point sources in very crowded fields. Moreover, the good angular resolution of HRI images allows the source extension and position to be accurately determined. This effort has produced the BMW (Brera Multiscale Wavelet) catalogue, with more than 19,000 sources detected at the ~4.2sigma level. For each source detection we have information on the position, X-ray flux and extension. This allows for instance to select complete samples of extended X-ray sources such as candidate clusters of galaxies or SNR's. Details about the detection algorithm and the catalogue can be found in Lazzati et al. (1999), Campana et al. (1999) and Panzera et al. (2001). Here we shall present an overview of first results from several undergoing projects which make use of the BMW catalogue.

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arXiv2001

Detection of a Hard Tail in the X-ray Spectrum of the Z Source GX 349+2

We present the results of a BeppoSAX observation of the Z source GX 349+2 covering the energy range 0.1-200 keV. The presence of flares in the light curve indicates that the source was in the flaring branch during the BeppoSAX observation. We accumulated energy spectra separately for the non-flaring intervals and the flares. In both cases the continuum is well described by a soft blackbody ($k T_{BB} \sim 0.5$ keV) and a Comptonized spectrum corresponding to an electron temperature of $k T_e \sim 2.7$ keV, optical depth $τ\sim 10$ (for a spherical geometry), and seed photon temperature of $k T_W \sim 1$ keV. All temperatures tend to increase during the flares. In the non-flaring emission a hard tail dominates the spectrum above 30 keV. This can be fit by a power law with photon index $\sim 2$, contributing $\sim 2%$ of the total source luminosity over the BeppoSAX energy range. A comparison with hard tails detected in some soft states of black hole binaries suggests that a similar mechanism could originate these components in black hole and neutron star systems.

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arXiv2000

The BMW (Brera-Multiscale-Wavelet) Catalogue of Serendipitous X-ray Sources

In collaboration with the Observatories of Palermo and Rome and the SAX-SDC we are constructing a multi-site interactive archive system featuring specific analysis tools. In this context we developed a detection algorithm based on the Wavelet Transform (WT) and performed a systematic analysis of all ROSAT-HRI public data (~3100 observations +1000 to come). The WT is specifically suitable to detect and characterize extended sources while properly detecting point sources in very crowded fields. Moreover, the good angular resolution of HRI images allows the source extension and position to be accurately determined. This effort has produced the BMW (Brera Multiscale Wavelet) catalogue, with more than 19,000 sources detected at the 4.2 sigma level. For each source detection we have information on the X-ray flux and extension, allowing for instance to select complete samples of extended X-ray sources such as candidate clusters of galaxies or SNR's. Here we present an overview of first results from several undergoing projects which make use of the BMW catalogue.

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arXiv2000

The Discovery of a State Dependent Hard Tail in the X-ray Spectrum of the Luminous Z-source GX 17+2

We report results of a BeppoSAX (0.1-200 keV) observation of the Z-type low mass X-ray binary GX 17+2. The source was on the so-called Horizontal and Normal branches. Energy spectra were selected based on the source position in the X-ray hardness-intensity diagram. The continuum could be fairly well described by the sum of a ~0.6 keV blackbody, contributing ~10% of the observed 0.1-200 keV flux, and a Comptonized component, resulting from upscattering of \~1 keV seed photons by an electron cloud with temperature of ~3 keV and optical depth of ~10. Iron K-line and edge were also present at energies 6.7 and 8.5 keV, respectively. In the spectra of the Horizontal branch a hard tail was clearly detected at energies above ~30 keV. It could be fit by a power law of photon index ~2.7, contributing ~8% of the source flux. This component gradually faded as the source moved towards the Normal branch, where it was no longer detectable. We discuss the possible origin of this component and the similarities with the spectra of Atoll sources and black hole X-ray binaries.

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arXiv2000

X-ray emission from the galactic center region Supernova Remnant G0.9+0.1

We report the results of a BeppoSAX observation of the composite supernova remnant G0.9+0.1, located in the Galactic Center region. This new observation, in which the core of G0.9+0.1 is observed on-axis, confirms our previous results obtained from a pointing in which the supernova remnant was serendipitously observed at a large off-axis angle. The better quality of the new data allows us to confirm the non-thermal nature of the X-ray source coincident with the central plerion, to significantly improve the estimate of its spectral parameters, to put more stringent upper limits on the presence of coherent pulsations (in the range of periods from 4ms to ~ 4\times10^4 s), to obtain marginal evidence of the possible extended nature of the X-ray plerion and to put a better upper limit on the 2--10 keV X-ray emission from the 8' diameter radio shell (L$_{\rm S}<3.4\times 10^{34} {\rm d}_{10}^{2}$ erg s^{-1}). The X-ray spectrum is best fit with a power-law model with a photon index Γ~ 2, N_{\rm H} ~ 10^{23} \hcm and a 2-10 keV luminosity of 7.5\times 10^{34} {\rm d}_{10}^{2} erg s^{-1}. The broad band (radio-X-ray) properties of the composite supernova remnant G0.9+0.1 are indicative of the likely presence of a young (~ 2,700 yr) neutron star in its central radio core.

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arXiv2000

A Search for the Optical/Infrared Counterpart of the Anomalous X-ray Pulsar 1E 1841-045

We have carried out a search for the optical and infrared counterpart of the Anomalous X-ray Pulsar 1E 1841-045, which is located at the center of the supernova remnant Kes73. We present the first deep optical and infrared images of the field of 1E 1841-045, as well as optical spectroscopy results that exclude the brightest objects in the error circle as possible counterparts. A few of the more reddened objects in this region can be considered as particularly interesting candidates, in consideration of the distance and absorption expected from the association with Kes73. The strong interstellar absorption in the direction of the source does not allow to completely exclude the presence of main sequence massive companions.

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arXiv2000

BeppoSAX Observations of the Galactic Center Region: Soft x-rays from the radio halo of SgrAEast

The BeppoSAX satellite performed a survey of the Galactic Center Region in the 1-10 keV energy band with its Narrow Field Instruments. Several bright X-ray sources containing neutron stars and black holes have been observed and studied, including the possible counterpart of SgrA*. Here we report the results on the diffuse emission coming from the SgrA Complex. The emission from within 8 arcmin from SgrA* has a double-temperature thermal spectrum (kT1~0.6 keV and kT2~8 keV) and an energy-dependent morphology: the hard emission (5-10 keV) is elongated along the galactic plane, while the soft one (2-5 keV) shows a triangular shape, very similar to the radio halo of SgrAEast. This spatial correlation and the physical parameters of the lower temperature component support the interpretation of the radio halo of the Sgr~A~East shell as a SNR.

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arXiv2000

The discovery of quiescent X-ray emission from SAX J1808.4-3658, the transient 2.5ms pulsar

We report on a 20 ks BeppoSAX observation of the transient 2.5 ms X-ray pulsar SAX J1808.4-3658 in its quiescent state. A source at a level of ~ 3 x 10^-3 c/s was detected with the BeppoSAX/MECS at a position consistent with that of SAX J1808.4-3658. The inferred 0.5-10 keV luminosity was to that measured from other quiescent low mass X-ray transients hosting an old neutron star. This result is discussed in the light of the propeller and radio pulsar shock emission models for the quiescent emission of these systems.

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arXiv2000

BeppoSAX and Chandra Observations of SAXJ0103.2-7209=2E0101.5-7225: a new Persistent 345s X-ray Pulsar in the SMC

We report the results of a 1998 July BeppoSAX observation of a field in the SMC which led to the discovery of 345s pulsations in the X-ray flux of SAXJ0103.2-7209. The BeppoSAX X-ray spectrum is well fit by an absorbed power-law with photon index 1.0 plus a black body component with kT=0.1keV. The unabsorbed luminosity in the 2-10 keV energy range is 1.2x10^{36} erg/s. In a very recent Chandra observation the 345s pulsations are also detected. The available period measurements provide a constant period derivative of -1.7s/yr over the last three years making SAXJ0103.2-7209 one of the most rapidly spinning-up X-ray pulsars known. The BeppoSAX position is consistent with that of the Einstein source 2E0101.5-7225 and the ROSAT source RXJ0103.2-7209. This source was detected at a luminosity level of few 10^{35}-10^{36} erg/s in all datasets of past X-ray missions since 1979. The ROSAT HRI and Chandra positions are consistent with that of a m_V=14.8 Be spectral type star already proposed as the likely optical counterpart of 2E0101.5-7225. We conclude that SAXJ0103.2-7209 and 2E0101.5-7225 are the same source, a relatively young and persistent X-ray pulsar in the SMC.

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