L. Burderi

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-Ray Burst

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superposed onto the host galaxy in our first infrared JHK observations, carried out ~9 hr after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude MV = -19.75 ± 0.15.

Probing the Inner Region of Cygnus X-1 in the Low/Hard State through Its X-Ray Broadband Spectrum

We present the broadband X-ray spectrum of Cyg X-1 in the low/hard state as observed by the instruments on board BeppoSAX. The spectrum spans from 0.1 to 200 keV, allowing the total accretion luminosity to be observed rather than extrapolated, corresponding to ~2% of the Eddington limit for a 10 M☉ black hole. The broad bandpass allows us to determine the continuum shape with great accuracy. Simple models of Compton upscattering of seed photons from the accretion disk do not adequately match the spectrum. At low energies an additional continuum component is required, giving a complex soft excess which extends up to ~4 keV, in line with previous results from ASCA. Moreover, we clearly detect a reflected component from the accretion disk that is smeared, probably because of relativistic and Doppler effects. The reflecting material is not strongly ionized and does not subtend a large solid angle as seen from the corona (Ω/2π ~ 0.1-0.3). The estimated inner radius of the disk, which depends on the inclination of the system, is most probably between 10 and 70 gravitational radii (Rg). An unsmeared reprocessed component, probably originating from the companion star or the outer disk, could also be present. In this case, the inner radius of the disk, as inferred from the smeared reflection, is smaller, between 6Rg and 20Rg.

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 nonflaring intervals and for the flares. In both cases, the continuum is well described by a soft blackbody (kTBB ~ 0.5 keV) and a Comptonized spectrum corresponding to electron temperature kTe ~ 2.7 keV, optical depth τ ~ 10 (for a spherical geometry), and seed-photon temperature kTW ~ 1 keV. All temperatures tend to increase during the flares. In the nonflaring emission, a hard tail dominates the spectrum above 30 keV. This can be fit by a power law with photon index ~2, contributing ~2% of the total source luminosity over the BeppoSAX energy range. A comparison with hard tails that are 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.

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 of ~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 toward 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.

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 that assessed its quiescent luminosity and spectrum with a good signal-to-noise ratio. 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 with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of LX = 5 ? 1031 ergs s-1, 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 (? ~ 1.5) power law with only a minor contribution (10%) from a soft blackbody 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 an X-ray luminosity conversion efficiency of ? ~ 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 neutron star (M 1.7 M?).

Where May Ultrafast Rotating Neutron Stars Be Hidden

The existence of ultrafast rotating neutron stars (spin period P 1 ms) is expected on the basis of current models for the secular evolution of interacting binaries, although they have not been detected yet. Their formation depends on the quantity of matter accreted by the neutron star which, in turn, is limited by the mechanism of mass ejection from the binary. An efficient mass ejection can avoid the formation of ultrafast pulsars or their accretion-induced collapse to a black hole. We propose that significant reductions of the mass transfer rate may cause the switch-on of a radio pulsar phase, whose radiation pressure may be capable of ejecting out of the system most of the matter transferred by the companion. This can prevent, for long orbital periods and if a sufficiently fast spin has been reached, any further accretion, even if the original transfer rate is restored, thus limiting the minimum spin period attainable by the neutron star. We show that close systems (orbital periods Porb ~ 1 hr) are the only possible hosts for ultrafast spinning neutron stars. This could explain why ultrafast radio pulsars have not been detected so far, as the detection of pulsars with very short spin periods in close systems is hampered, in current radio surveys, by strong Doppler modulation and computational limitations.

On the spectral evolution of Cygnus X-2 along its color-color diagram

We report on the results of a broad band (0.1{200 keV) spectral study of Cyg X{2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total eective on-source time of100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around dierent positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are t to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at 1k eV and 6.6 keV, respectively. The addition of a hard power-law tail with photon index2, contributing1.5% of the source luminosity, improves the t of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, kTin, varying between0.8 and1.7 keV and inner radius, Rin, varying between26 and11 km (assuming an inclination angle of the system of 60). The Comptonization component is probably emitted by hot plasma (electron temperature kTe varying between 3a nd20 keV, optical depth 11 0:4, seed-photon temperature kTW 1 2:4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change signicantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.

A Hard Tail in the X-Ray Broadband Spectrum of Circinus X-1 at the Periastron: A Peculiar Z Source

We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A flare lasting ~6 × 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 × 1038 ergs s-1, while during the flare it is 2 × 1038 ergs s-1. We produced broadband (0.1-100 keV) energy spectra during the flare and the persistent emission. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of ~0.4 keV, by electrons at a temperature of ~1 keV. After the flare, a power-law component with photon index ~3 is dominant at energies higher than 10 keV. This component contributes ~4% of the total luminosity. During the flare its addition is not statistically significant. An absorption edge at ~8.4 keV, with optical depth ~1, corresponding to the K edge of Fe XXIII-Fe XXV, and an iron emission line at 6.7 keV are also present. The iron-line energy is in agreement with the ionization level inferred from the absorption edge. The hydrogen column deduced from the absorption edge is ~1024 cm-2, 2 orders of magnitude larger than the low-energy absorption measured in this source. We calculated the radius of the region originating the Comptonized seed photons, RW ~ 150 km. We propose a scenario where RW (the Wien radius) is the inner disk radius, a highly ionized torus surrounds the accretion disk, and a magnetosphere is present up to RW. The absorption edge and the emission line could originate in the photoionized torus, while the Comptonized component originates in an inner region of the disk.

THE 0.1¨100 keV SPECTRUM OF LMC X-4 IN THE HIGH STATE: EVIDENCE FOR A HIGH-ENERGY CYCLOTRON ABSORPTION LINE

We report the spectral analysis of the X-ray pulsar LMC X-4 in its high state, out of eclipse, observed by BeppoSAX. During this observation no coherent pulsations are detected. The primary continuum is well described by a power law with a high-energy cutoff Ecutoff ~ Efold ~ 18 keV. The addition of a cyclotron absorption line at ~100 keV improves the fit significantly. The inferred magnetic moment is 1.1 × 1031 G cm3, in agreement with the value estimated, assuming that the neutron star is at the spin equilibrium, as has been proposed for this source. The remaining excess at low energies can be fitted by a Comptonization of soft photons by moderately hot electrons (kT ~0.9 keV) with an optical depth τ ~ 16. The seed photons for this Comptonization are consistent with blackbody emission from the accretion disk at the magnetospheric radius. Another possibility is to fit the soft excess with blackbody and thermal bremsstrahlung. In this case, the blackbody would originate from cold plasma at the magnetosphere while the bremsstrahlung component would be produced by the strong stellar wind from the companion star, ionized by the X-ray emission from the pulsar.

The Broadband Spectrum of MXB 1728–34 Observed by BeppoSAX

We report on the results of a broadband (0.1-100 keV) spectral analysis of the bursting atoll source MXB 1728-34 (4U 1728-34) observed by the BeppoSAX satellite. Three bursts were present during this observation. The spectrum during the bursts can be fitted by a blackbody with a temperature of ~2 keV. The radius of the blackbody emitting region is compatible with the radius of the neutron star if we correct for the difference between the observed color temperature and the effective temperature. From the bursts we also estimate a distance to the source of ~5.1 kpc. MXB 1728-34 was in a rather soft state during the BeppoSAX observation. The persistent spectrum is well fitted by a continuum consisting of a soft blackbody emission and a Comptonized spectrum. We interpreted the soft component as the emission from the accretion disk. Taking into account a spectral hardening factor of ~1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin ~ 20 km, where i is the inclination angle. The Comptonized component could originate in a spherical corona, with temperature ~10 keV and optical depth ~5, surrounding the neutron star. A broad Gaussian emission line at ~6.7 keV is observed in the spectrum, probably emitted in the ionized corona or in the inner part of the disk. Another emission line is present at ~1.66 keV. No reflection component is detected with high statistical significance, probably because of the low temperature of the corona in this state of the source. If the iron emission line is caused by reflection of the Comptonized spectrum by the accretion disk, it requires a ionized disk (ξ ~ 280) and a solid angle of ~0.2 (in units of 2π) subtended by the reflector as seen from the corona.