M. T. Menna

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.

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.

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.

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.

The accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1−2455, as observed by XMM–Newton and RXTE

We present a study of the accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1-2455, based on observations performed simultaneously by XMM-Newton and RXTE. The 0.33-50 keV spectrum is described by the sum of a hard Comptonized component originated in an optically thin {\tau}~1 corona, a soft kTin~0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter is identified as K{\alpha} transition of Fe XXIII-XXV. A simultaneous detection in EPIC-pn, EPIC-MOS2, and RGS spectra favours an astrophysical origin also for the former, which has an energy compatible with Fe-L{\alpha} and helium-like Ne-K{\alpha} transitions. Broadness of the two features suggests a common origin, resulting from reflection in an accretion disc with inclination of (30+4{\deg}), and extending down to Rin=25(+16,-11) gravitational radii from the compact object. However, the strength of the feature at lower energy measured by EPIC-pn cannot be entirely reconciled with the amplitude of the Fe K{\alpha} line, hampering the possibility of describing it in terms of a broad-band reflection model, and preventing a firm identification. Pulsations at the 377.3 Hz spin frequency could not be detected, with an upper limit of 0.4% at 3-{\sigma} c.l. on the fractional amplitude. We interpret the inner disc radius estimated from spectral modelling and the lack of significant detection of coherent X-ray pulsations as an indication of a disc accretion flow truncated by some mechanism connected to the overall evolution of the accretion disc, rather than by the neutron star magnetic field. This is compatible with the extremely close similarity of spectral and temporal properties of this source with respect to other, non pulsing atoll sources in the hard state.

Timing of the 2008 outburst of SAX J1808.4-3658 with XMM-Newton: A stable orbital-period derivative over ten years

We report on a timing analysis performed on a 62-ks longXMM-Newton observation of the accreting millisecond pulsar SAX J1808.4– 3658 during the latest X-ray outburst that started on September 21, 2008. By connecting the time of arrivals of the pulses observed during the XMM-Newton observation, we derived the best-fit orbital solution and a best-fit value of the spin period for the 2008 outburst. Comparing this new set of orbital parameters and, in particular, the value of the time of ascending-node passage with the orbital parameters derived for the previous four X-ray outbursts of SAX J1808.4–3658 observed by the PCA onboard RXTE ,w e fi nd an updated value of the orbital period derivative, which turns out to be u Porb = (3.89 ± 0.15) × 10 −12 s/s. This new value of the orbital period derivative agrees with the previously reported value, demonstrating that the orbital period derivative in this source has remained stable over the past ten years. Although this timespan is not sufficient yet for confirming the secular evolution of the system, we again propose an explanation of this behavior in terms of a highly non-conservative mass transfer in this system, where the accreted mass (as derived from the X-ray luminosity during outbursts) accounts for a mere 1% of the mass lost by the companion.

Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294

We have performed a timing analysis of the 2003 outburst of the accreting X-ray millisecond pulsar XTE J1807–294 as observed by the Rossi X-Ray Timing Explorer. Using recently refined orbital parameters, we report for the first time a precise estimate of the spin frequency and of the spin frequency derivative. The phase delays of the pulse profile show a strong erratic behavior superposed on what appears to be a global spin-up trend. The erratic behavior of the pulse phases is strongly related to rapid variations of the light curve, making it very difficult to fit these phase delays with a simple formula. As in previous cases, we therefore separately analyze the phase delays of the first harmonic and of the second harmonic of the spin frequency, finding that the phases of the second harmonic are far less affected by the erratic behavior. Under the hypothesis that the second-harmonic pulse phase delays are a good tracer of the spin frequency evolution, we give for the first time an estimate of the spin frequency derivative for this source. XTE J1807–294 shows a clear spin-up of = 2.5(7) × 10−14 Hz s−1 (1 σ confidence level). The majority of the uncertainty in the value of the spin-up rate is due to the uncertainties in the source position on the sky. We discuss the effect of this systematic error on the spin frequency and its derivative.

Measuring the spin up of the Accreting Millisecond Pulsar XTE J1751-305

ABSTRACT We perform a timing analysis on RXTE data of the accreting millisecond pulsarXTE J1751–305 observed during the April 2002 outburst. After having correctedfor Doppler effects on the pulse phases due to the orbital motion of the source, weperformed a timing analysis on the phase delays, which gives, for the first time for thissource, an estimate of the averagespin frequency derivative ˙ = (3.7±1.0)×10 −13 Hz/s. We discuss the torque resulting from the spin-up of the neutron star deriving adynamical estimate of the mass accretion rate and comparing it with the one obtainedfrom X-ray flux. Constraints on the distance to the source are discussed, leading to alower limit of ∼ 6.7 kpc.Key words: stars: neutron – stars: magnetic fields – pulsars: general – pulsars:individual: XTE J1751–305– X-ray: binaries 1 INTRODUCTIONAccreting millisecond pulsars (AMSP in the following) arethe long sought connection between low mass X-ray binaries(LMXBs) and millisecond radio pulsars. In fact, although itwas hypothesised soon after their discovery that fast spin-ning radio pulsars were “recycled” by an accretion phasein a LMXB system, during which the neutron star (NS) isspun–up (see for a review Bhattacharya & van den Heuvel1991), evidence has been elusive since SAX J1808.4-3658,the first accretion-driven millisecond X-ray pulsar, was dis-covered (Wijnands & van der Klis 1998). SAX J1808.4-3658,with a spin period of 2.5 ms, exhibiting both X-ray burstsand coherent pulsations, proved to be the missing link be-tween the two classes of sources. Since then, six more mil-lisecond X-ray pulsars were discovered (see Wijnands 2006for an observational review).All of these sources are transients with usually low dutycycles. Except for the case of HETE J1900.1-2455 which re-mained active for more than a year after its discovery inJune 2005 (Galloway et al. 2007), the outbursts of AMSPlast for no more than a couple of months, with recurrencetimes usually larger than 2 yr (Galloway 2006). Althoughthe sample is still small, monitoring of future outbursts ex-hibited by the known sources is extremely important for ourunderstanding of LMXBs and their evolution.

Timing of the accreting millisecond pulsar IGR J17511-3057

Context. Timing analysis of accretion-powered millisecond pulsars (AMPs) is a powerful tool for probing the physics of compact objects. The recently discovered IGR J17511-3057 was the twelfth discovered of the 13 AMPs known. The Rossi XTE satellite provided an extensive coverage of the 25 days-long observation of the source outburst. Aims. Our goal is to investigate the complex interaction between the neutron star magnetic field and the accretion disk by determining the angular momentum exchange between them. The presence of a millisecond coherent flux modulation allows us to investigate this interaction from the study of pulse arrival times. To separate the neutron star proper spin frequency variations from other effects, a precise set of orbital ephemeris is mandatory. Methods. Using timing techniques, we analysed the pulse phase delays by fitting differential corrections to the orbital parameters. To remove the effects of pulse phase fluctuations, we applied the timing technique that had been already successfully applied to the case of an another AMP, XTE J1807-294. Results. We report a precise set of orbital ephemeris. We demonstrate that the companion star is a main-sequence star. We find pulse phase delay fluctuations on the first harmonic with a characteristic amplitude of about 0.05, similar to those also observed for the AMP XTE J1814-338. For the second time, an AMP shows a third harmonic detected during the entire outburst. The first harmonic phase delays exhibit a puzzling behaviour, while the second harmonic phase delays clearly spin-up. The third harmonic also shows a spin-up, although not highly significant (3σ c.l.). The presence of a fourth harmonic is also reported. If we assume that the second harmonic is a good tracer of the spin frequency of the neutron star, we infer a mean spin frequency derivative for this source of 1.65(18) × 10 −13 Hz s −1 . Conclusions. To interpret the pulse phase delays of the four harmonics, we apply the disk threading model, but obtain different and incompatible u M estimates for each harmonic. In particular, the phase delays of the first harmonic are heavily affected by phase noise, and consequently, on the basis of these data, it is not possible to derive a reliable estimate of u M. The second harmonic gives a u M consistent with the flux assuming that the source is at a distance of 6.3 kpc. The third harmonic gives a lower u M value, with respect to the first and second harmonic, and this would reduce the distance estimate to 3.6 kpc.