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

We perform a timing analysis on RXTE data of the accreting millisecond pulsar XTE J1751-305 observed during the April 2002 outburst. After having corrected for Doppler effects on the pulse phases due to the orbital motion of the source, we performed a timing analysis on the phase delays, which gives, for the first time for this source, an estimate of the average spin frequency derivative = (3.7 +/- 1.0)E-13 Hz/s. We discuss the torque resulting from the spin-up of the neutron star deriving a dynamical estimate of the mass accretion rate and comparing it with the one obtained from X-ray flux. Constraints on the distance to the source are discussed, leading to a lower limit of \sim 6.7 kpc.Comment: 7 pages, 3 figures, Accepted for publication by MNRA

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=15388.7229(2)s, and of the projected semimajor axis, a sini/c= 390.633(9) lt-ms. 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 (nu=314.35610879(1) Hz) and the first estimate of the spin frequency derivative of this source while accreting (nu^dot=(-6.7 +/- 0.7) 10^(-14) Hz/s). 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.Comment: 7 pages, 4 figures, Accepted for publication by MNRA

Timing of the accreting millisecond pulsar IGR~J17511--3057

{Timing analysis of Accretion-powered Millisecond Pulsars (AMPs) is a powerful tool to probe the physics of compact objects. The recently discovered \newigrj is the 12 discovered out of the 13 AMPs known. The Rossi XTE satellite provided an extensive coverage of the 25 days-long observation of the source outburst.} {Our goal is to investigate the complex interaction between the neutron star magnetic field and the accretion disk, determining the angular momentum exchange between them. The presence of a millisecond coherent flux modulation allows us to investigate such interaction from the study of pulse arrival times. In order to separate the neutron star proper spin frequency variations from other effects, a precise set of orbital ephemeris is mandatory.} {Using timing techniques, we analysed the pulse phase delays fitting differential corrections to the orbital parameters. To remove the effects of pulse phase fluctuations we applied the timing technique already successfully applied to the case of an another AMP, XTE J1807-294.} {We report a precise set of orbital ephemeris. We demonstrate that the companion star is a main sequence star. We find pulse phase delays fluctuations on the first harmonic with a characteristic amplitude of about 0.05, similar to what also observed in the case of 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 show a puzzling behaviour, while the second harmonic phase delays show a clear spin-up. Also the third harmonic shows a spin-up, although not highly significant (3$\sigma$ c.l.). The presence of a fourth harmonic is also reported. In the hypothesis that the second harmonic is a good tracer of the spin frequency of the neutron star, we find a mean spin frequency derivative for this source of \np{1.65(18)}{-13} Hz s$^{-1}$.} (continue ...)Comment: 9 pages, 12 figures, A&A accepted on 23/10/201

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

We performed a timing analysis of the 2003 outburst of the accreting X-ray millisecond pulsar XTE J1807-294 observed by RXTE. 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 to what appears as 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 law. As in previous cases, we have therefore analyzed separately 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. In the hypothesis that the second harmonic pulse phase delays are a good tracer of the spin frequency evolution we give for the first time a estimation of the spin frequency derivative in this source. The source shows a clear spin-up of $\dot \nu = 2.5(7) \times 10^{-14}$ Hz sec$^{-1}$ (1 $\sigma$ confidence level). The largest source of uncertainty in the value of the spin-up rate is given by the uncertainties on the source position in the sky. We discuss this systematics on the spin frequency and its derivative.Comment: 17 pages, 4 figures, Accepted by Ap

Temporal Analysis of the Millisecond X-ray Pulsar SAX J1808.4-3658 During the 2000 Outburst

We report a temporal analysis of the millisecond X-ray Pulsar SAX J1808.4-3658 during the 2000 outburst, observed with RXTE. The observed maximum luminosity was approximately a factor of ten lower than in the other outbursts exhibited by the source, and this low flux level forced us to use a technique based on the χ2 obtained with an epoch folding search to discriminate between different possible orbital solutions, in order to correct the data for the orbital motion. In the subsequent searches for periodicities we clearly detected the 401Hz pulsation in at least two observations, but in the faintest the pulsed fraction varied from 20 % ca. to the absence of signs of coherent pulsation at all, while the measured flux remained at an almost constat level. This erratic behaviour is discussed in the context of the centrifugal inhibition of accretion

The near-IR counterpart of IGR J17480-2446 in Terzan 5

Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray binaries. Terzan 5 has the richest population among globular clusters of X- and radio-pulsars and low-mass X-ray binaries. The detection and study of optical/IR counterparts of low-mass X-ray binaries is fundamental to characterizing both the low-mass donor in the binary system and investigating the mechanisms of the formation and evolution of this class of objects. We aim at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446 discovered in Terzan 5. Adaptive optics (AO) systems represent the only possibility for studying the very dense environment of GC cores from the ground. We carried out observations of the core of Terzan 5 in the near-IR bands with the ESO-VLT NAOS-CONICA instrument. We present the discovery of the likely counterpart in the Ks band and discuss its properties both in outburst and in quiescence. Archival HST observations are used to extend our discussion to the optical bands. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increase from quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of the double stellar generation population of Terzan 5 by using recent stellar evolution models.Comment: 7 pages, 4 figure

Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934

We present here a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of December 2004. 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 find a clear parabolic trend of the pulse phase delays showing that the pulsar is spinning up 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.Comment: Proceedings of the Frascati Workshop 2005: Multifrequency Behaviour of High Energy Cosmic Sources, Vulcano, May 23-28. 7 pages including 1 figur

Spin down of an Accreting Millisecond Pulsar, the case of XTE J1814-338

We report about a timing analysis performed on the data gathered by RXTE of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst. The first full orbital solution of this binary system is given. Moreover the evolution of the phase of the pulsed emission reveals that the rotating compact object is spinning down at a rate \u3bd\u2d9 = (-6.7 +/- 0.7) 7 10-14 Hz/s, while accreting. This behavior is considered as a result of the braking effect due to the interaction between the magnetosphere and the inner parts of the accretion disc, in the case of an accretion rate low enough to allow the expansion of the magnetospheric radius to the corotation limit. In this context we derive an estimate of the superficial magnetic field, BS ~= 8 7 108 G, which lies in the plausible range for the accreting millisecond pulsars to be the progenitors of the radio millisecond pulsar. We also report about a modulation of the phase delays around the mean timing solution, which we interpret as a signature of movements of the accreting hotspot, resulting from a variable accretion rate

A relativistically broadened iron line from an Accreting Millisecond Pulsar

The capabilities of XMM-Newton have been fully exploited to detect a broadened iron K\u3b1 emission line from the 2.5 ms Accreting Millisecond Pulsar, SAX J1808.4-3658. The energy of the transition is compatible with fluorescence from neutral/lowly ionized iron. The observed large width (FWHM more than 1 keV) can be explained through Doppler and relativistic broadening from the inner rings of an accretion disc close to the NS. From a fit of the line shape with a diskline model we obtain an estimate of the inner disc radius of 18.0-5.6+7.6km for a 1.4 M 99 neutron star. The disc is therefore truncated inside the corotation radius (31 km for SAX J1808.4-3658), in agreement with the observation of coherent pulsations. From our estimate of the inner disc radius, we infer that the magnetic field of the neutron star is in the range 1-5 7108G. \ua9 2010 American Institute of Physics

Migraine and cluster headache show impaired neurosteroids patterns

Background: Perturbation of neuronal excitability contributes to migraine. Neurosteroids modulate the activity of γ-aminobutyric acid A and N-methyl-d-aspartate receptors, and might be involved in the pathogenesis of migraine. Here, we measured plasma levels of four neurosteroids, i.e., allopregnanolone, epiallopregnanolone, dehydroepiandrosterone and deydroepiandrosterone sulfate, in patients affected by episodic migraine, chronic migraine, or cluster headache. Methods: Nineteen female patients affected by episodic migraine, 51 female patients affected by chronic migraine, and 18 male patients affected by cluster headache were recruited to the study. Sex- and age-matched healthy control subjects (31 females and 16 males) were also recruited. Patients were clinically characterized by using validated questionnaires. Plasma neurosteroid levels were measured by liquid chromatography-tandem mass spectrometry. Results: We found disease-specific changes in neurosteroid levels in our study groups. For example, allopregnanolone levels were significantly increased in episodic migraine and chronic migraine patients than in control subjects, whereas they were reduced in patients affected by cluster headache. Dehydroepiandrosterone and dehydroepiandrosterone sulfate levels were reduced in patients affected by chronic migraine, but did not change in patients affected by cluster headache. Conclusion: We have shown for the first time that large and disease-specific changes in circulating neurosteroid levels are associated with chronic headache disorders, raising the interesting possibility that fluctuations of neurosteroids at their site of action might shape the natural course of migraine and cluster headache. Whether the observed changes in neurosteroids are genetically determined or rather result from exposure to environmental or intrinsic stressors is unknown. This might also be matter for further investigation because stress is a known triggering factor for headache attacks in both migraineurs and cluster headache patients