Alessandro Riggio

Swings between rotation and accretion power in a binary millisecond pulsar

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.

A relativistically smeared spectrum in the neutron star X-ray binary 4U 1705−44: looking at the inner accretion disc with X-ray spectroscopy

Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kα transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent XMM―Newton observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high-velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile, we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kα transition of highly ionized iron, Fe xxv, the inner disc radius is R in = 14 ± 2 R g (where Rg is the Gravitational radius, GM/c 2 ), the emissivity dependence from the disc radius is r ―2.27±0.08 , the inclination angle with respect to the line of sight is i = 39° ± 1 °. Finally, the XMM―Newton spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.

Orbital evolution of an accreting millisecond pulsar: witnessing the banquet of a hidden black widow?

We report here on the orbital evolution of the accreting millisecond pulsar SAX J1808.4{3658. In particular, we nd for this source the rst estimate of the orbital period derivative in an accreting millisecond pulsar, _ Porb = (3:40 0:12) 10 12 s/s, and a rened estimate of the orbital period, Porb = 7249:156499 (1:2 10 5 ) s. This derivative is positive and 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. In the hypothesis that the measured derivative of the orbital period reects the secular evolution of the system, we propose a simple explanation of this puzzling result assuming that during X-ray quiescence the source is ejecting matter (and angular momentum) from the inner Lagrangian point. The proposed orbital evolution of the system suggests a degenerate or fully convective companion star and indicates that this kind of sources are capable to ecien tly ablate the companion star, and therefore are black widows visible in X-rays during transient mass accretion episodes.

The spin and orbit of the newly discovered pulsar IGR J17480-2446

We present an analysis of the spin and orbital properties of the newly discovered accreting pulsar IGR J17480-2446, located in the globular cluster Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary system. The binary mass function is estimated to be 0.021275(5) M o , indicating a companion star with a mass larger than 0.4 M ⊙ . The X-ray pulsar spins up while accreting at a rate of between 1.2 and 1.7 x 10 -12 Hz s -1 , in agreement with the accretion of disc matter angular momentum given the observed luminosity. We also report the detection of pulsations at the spin period of the source during a Swift observation performed ~2 d before the beginning of the RXTE coverage. Assuming that the inner disc radius lies in between the neutron star radius and the corotation radius while the source shows pulsations, we estimate the magnetic field of the neutron star to be within ~2 x 10 8 G and ~2.4 x 10 10 G. From this estimate, the value of the spin period and of the observed spin-up rate, we associate this source with the still poorly sampled population of slow, mildly recycled, accreting pulsars.

Timing of the accreting millisecond pulsar XTE J1814−338

We present a precise timing analysis of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst, observed by RXTE. A full orbital solution is given for the first time; Doppler effects induced by the motion of the source in the binary system were corrected, leading to a refined estimate of the orbital period, P orb = 15 388.7229(2) s, and of the projected semimajor axis, a sin i/c = 0.390633(9) light-second. We could then investigate the spin behaviour of the accreting compact object during the outburst. We report here a refined value ·of the spin frequency (v = 314.356 108 79(1) Hz) and the first estimate of the spin frequency derivative of this source while accreting [ν = (-6.7 ± 0.7) x 10 -14 Hz s -1 ]. This spin-down behaviour arises when both the fundamental frequency and the second harmonic are taken into consideration. We discuss this in the context of the interaction between the disc and the quickly rotating magnetosphere, at accretion rates sufficiently low to allow a threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. We also present indications of a jitter of the pulse phases around the mean trend, which we argue results from movements of the accreting hotspots in response to variations of the accretion rate.

A self-consistent approach to the hard and soft states of 4U 1705-44

Context. High-resolution spectroscopy has recently revealed in many low-mass X-ray binaries hosting a neutron star that the shape of the broad iron line observed in the 6.4-6.97 keV range is consistently well-fitted by a relativistically smeared line profile. Aims. The presence of other broad features, besides the iron line, together with a high S/N of the spectra offer the possibility of testing a self-consistent approach to the overall broadband reflection spectrum and evaluating the impact of the reflection component in the formation of the broadband X-ray spectra. Methods. We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a hard low flux state, the second during a soft, high-flux state. Both the spectra show broad iron emission lines. We fit the spectra using a two-component model, together with a reflection model specifically suited to the case of a neutron star, where the incident spectrum has a blackbody shape. Results. In the soft state, the reflection model, convolved with a relativistic smearing component, consistently describes the broad features present in the spectrum, and we find a clear relation between the temperature of the incident flux and the temperature of the harder X-ray component that we interpret as the boundary layer emission. In this state we find converging evidence that the boundary layer outer radius is � 2 times the neutron star radius. In the low flux state, we observe a change in the continuum shape of the spectrum with respect to the soft state. Still, the broad local emission features can be associated with a disk reflecting matter, but in a lower ionization state, and possibly produced in an accretion disk truncated at greater distance. Conclusions. Our analysis provides strong evidence that the reflection component in soft states of LMXBs comes from to hard X-ray thermal irradiation, which we identify with the boundary layer emission, also present in the continuum model. In the hard state, the broad iron line if also produced by reflection, and the continuum disk emission can be self-consistently accounted if the disk is truncated at a greater distance than the soft state.

Timing an accreting millisecond pulsar : Measuring the accretion torque in IGR j00291+5934

We performed a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of 2004 December. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we are able to study, for the first time in this class of sources, the spin-up of a millisecond pulsar as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.

ORDER IN THE CHAOS: SPIN-UP AND SPIN-DOWN DURING THE 2002 OUTBURST OF SAX J1808.4-3658

We present a timing analysis of the 2002 outburst of the accreting millisecond pulsar SAX J1808.4-3658. A study of the phase delays of the entire pulse profile shows a behavior that is surprising and difficult to interpret: superposed to a general trend, a big jump by about 0.2 in phase is visible, starting at day 14 after the beginning of the outburst. An analysis of the pulse profile indicates the presence of a significant first harmonic. Studying the fundamental and the first harmonic separately, we find that the phase delays of the first harmonic are more regular, with no sign of the jump observed in the fundamental. The fitting of the phase delays of the first harmonic with a model that takes into account the observed exponential decay of the X-ray flux (and therefore of the mass accretion rate onto the neutron star) gives important information on the torque acting on the neutron star during the outburst. We find that the source shows spin-up in the first part of the outburst, while a spin-down dominates at the end. From these results we derive an estimate of the neutron star magnetic field strength.

Spin down during quiescence of the fastest known accretion-powered pulsar

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. To achieve the highest possible accuracy in the measurement of the spin frequency variation experienced by the source in-between the 2008 August outburst and the last outburst exhibited in 2004, we re-analysed the latter considering the whole data set available. We find that the source spins down during quiescence at an average rate of u νsd = (−4.1 ± 1.2) × 10 −15 Hz s −1 . We discuss possible scenarios that can account for the long-term neutron star spin-down in terms of either magneto-dipole emission, emission of gravitational waves, and a propeller effect. If interpreted in terms of magneto-dipole emission, the measured spin down translates into an upper limit to the neutron star magnetic field, B < 3 × 10 8 G, while an upper limit to the average neutron star mass quadrupole moment of Q < 2 × 10 36 gc m 2 is set if the spin down is interpreted in terms of the

New ephemeris of the ADC source 2A 1822-371: a stable orbital-period derivative over 30 years

We report on a timing of the eclipse arrival times of the low mass X-ray binary and X-ray pulsar 2A 1822-371 performed using all available observations of the Proportional Counter Array on board the Rossi X-ray Timing Explorer, XMM-Newton pn, and Chandra. These observations span the years from 1996 to 2008. Combining these eclipse arrival time measurements with those already available covering the period from 1977 to 1996, we obtain an orbital solution valid for more than thirty years. The time delays calculated with respect to a constant orbital period model show a clear parabolic trend, implying that the orbital period in this source constantly increases with time at a rate P orb = 1.50(7) × 10 ―10 s/s. This is 3 orders of magnitude larger than what is expected from conservative mass transfer driven by magnetic braking and gravitational radiation. From the conservation of the angular momentum of the system we find that to explain the high and positive value of the orbital period derivative the mass transfer rate must not be less than 3 times the Eddington limit for a neutron star, suggesting that the mass transfer has to be partially non-conservative. With the hypothesis that the neutron star accretes at the Eddington limit we find a consistent solution in which at least 70% of the transferred mass has to be expelled from the system.