It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star’s rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superimposed to the host galaxy in our first infrared JHK observations, carried out ∼ 9 hours 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 V RI bands, reaching an absolute magnitude M V = −19.75 ± 0.15.

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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 D2% of the Eddington limit for a 10 black hole. The broad bandpass allows us to determine the continuum shape with great accuracy. M _ 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 D4 keV, in line with previous results from ASCA. Moreover, we clearly detect a reÑected component from the accretion disk that is smeared, probably because of relativistic and Doppler e †ects. The reÑecting material is not strongly ionized and does not subtend a large solid angle as seen from the corona ()/2n D 0.1È0.3). The estimated inner radius of the disk, which depends on the inclination of the system, is most probably between 10 and 70 gravitational radii An unsmeared (R g ). 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 reÑection, is smaller, between and 6R g 20R g .

SAX J1808.4Ϫ3658 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 J1808.4Ϫ3658 in quiescence, the first that assessed its quiescent luminosity and spectrum with a good signal-tonoise ratio. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4Ϫ3658, 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 ergs s Ϫ1 , a relatively low value 31 L p 5 # 10 X compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4Ϫ3658 was dominated by a hard ( ) power law with only a minor G ∼ 1.5 contribution (Շ10%) 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 is derived; this is in line with the value estimated from the eclipsing radio pulsar Ϫ3h ∼ 10 PSR J1740Ϫ5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4Ϫ3658 likely hosts a massive neutron star ( ). M տ 1.7 M ,

We have performed a timing analysis of all the four X-ray outbursts from the accreting millisecond pulsar SAX J1808.4-3658 observed so far by the Proportional Counter Array on board the Rossi X-ray Timing Explorer. For each of the outbursts, we derived the best-fitting value of the time of ascending node passage. We find that these times follow a parabolic trend, which gives an orbital-period derivative P(orb) = (3.40 +/- 0.18) x 10(-12)ss(-1), and a refined estimate of the orbital period, P(orb) = 7249.156 499 +/- 1.8 x 10(-5) s (reference epoch T(0) = 509 14.8099 MJD). This derivative is positive, suggesting a degenerate or fully convective companion star, but is more than one order of magnitude higher than what is expected from secular evolution driven by angular momentum losses caused by gravitational radiation under the hypothesis of conservative mass transfer. Using simple considerations on the angular momentum of the system, we propose an explanation of this puzzling result assuming that during X-ray quiescence the source is ejecting matter (and angular momentum) from the inner Lagrangian point. We have also verified that this behaviour is in agreement with a possible secular evolution of the system under the hypothesis of highly non-conservative mass transfer. In this case, we find stringent constraints on the masses of the two components of the binary system and its inclination. The proposed orbital evolution indicates that in this kind of sources the neutron star is capable to efficiently ablate the companion star, suggesting that this kind of objects are part of the population of the so-called black widow pulsars, still visible in X-rays during transient mass-accretion episodes

The existence of ultra-fast rotating neutron stars (spin period P ∼ < 1 ms) is expected on the basis of current models for the secular evolution of interacting binaries, though 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 ultra-fast 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 P orb ∼ 1 hr) are the only possible hosts for ultra-fast spinning neutron stars. This could explain why ultra-fast 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.