arXiv subjects
Publications and source records attributed to Andrea Possenti.
We have discovered 12 new millisecond pulsars in 6 globular clusters in which no pulsars were previously known, in the first two years of a search at 1.4 GHz in progress at the Parkes radio telescope. Here we briefly describe the motivation, the new hardware and software systems adopted for this survey, and we present the results obtained thus far.
We report the possible optical identification of the companion to the eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397. A bright variable star with an anomalous red colour and optical variability which nicely correlates to the orbital period of the pulsar has been found close to the pulsar position. If confirmed, the optical light curve, reminiscent of tidal distorsions similar to those observed in detached and contact binaries, support the idea that this is the first case of a Roche lobe filling companion to a millisecond pulsar.
We used a set of archived HST/WFPC2 images to probe the stellar population in the core of the nearby Galactic Globular Cluster (GGC) 47 Tuc. From the ultraviolet (UV) Color Magnitude Diagrams (CMDs) obtained for ~4,000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (i) 43 blue stragglers stars (BSSs) including 20 new candidates; (ii) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; (iii) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The WD candidates identified here define a clean pattern in the CMD, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster -- if the existence of such a large population is confirmed, we have finally found the long-searched population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by CHANDRA and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions together with the huge stellar crowding in the cluster center prevent statistically reliable identifications based only on positional coincidences.
We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. In the case of Cen X-4 strong neutrino emission from the inner core is necessary to explain the observation: this may indicate that the neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the possibility of using the quiescent emission of Soft X-Ray Transients as a tool for probing the core superfluidity in relation to the mass of the neutron star.
The thermal, spin and magnetic evolution of neutron stars in the old low mass binaries is first explored. Recycled to very short periods via accretion torques, the neutron stars lose their magnetism progressively. If accretion proceeds undisturbed for 100 Myrs these stars can rotate close to break up with periods far below the minimum observed of 1.558 ms. We investigate their histories using population synthesis models to show that a tail should exist in the period distribution below 1.558 ms. The search of these ultrafastly spinning neutron stars as pulsars can help discriminating among the various equations of state for nuclear matter, and can shed light into the physics of binary evolution. The evolution of isolated neutron stars in the Galaxy is explored beyond the pulsar phase. Moving through the tenuous interstellar medium, these old solitary neutron stars lose their rotational energy. Whether also their magnetism fades is still a mystery. A population synthesis model has revealed that only a tiny fraction of them is able to accrete from the interstellar medium, shining in the X-rays. There is the hope that these solitary stars will eventually appear as faint sources in the Chandra sky survey. This might give insight on the long term evolution of the magnetic field in isolated objects.
We explore the thermal evolution of a neutron star undergoing episodes of intense accretion, separated by long periods of quiescence. By using an exact cooling code we follow in detail the flow of heat in the star due to the time-dependent accretion-induced heating from pycno-nuclear reactions in the stellar crust, to the surface photon emission, and the neutrino cooling. These models allow us to study the neutron stars of the Soft X-Ray Transients. In agreement with Brown, Bildsten and Rutledge (1998) we conclude that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522, and of the recently discovered SAX J1808.4, can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. However, we show that in the case of Cen X-4, despite its long recurrence time, strong neutrino emission from the neutron star inner core is necessary to understand the observed low ratio of quiescent to outburst luminosity. This result implies that the neutron star in Cen X-4 is heavier than the one in the other systems and the pairing critical temperature in its center must be low enough (well below 10^9 K) to avoid a strong suppression of the neutrino emission.
We investigate the statistical evolution of magnetic neutron stars recycled in Low Mass Binary (LMB) systems, simulating synthetic populations. Irrespective to the details of the physical models, we find to be significant the fraction of neutron stars spinning close to their mass shedding limit relative to the millisecond pulsar population. The simulated neutron stars show a tail in their period distribution at periods shorter than 1.558 ms, the minimum detected so far. Crustal magnetic field decay models predict also the existence of massive rapidly spinning neutron stars with very low magnetic moment.
We investigate the statistical evolution of magnetic neutron stars, recycled in binary systems, simulating synthetic populations. To bracket uncertainties, we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear matter and explore the hypothesis that the magnetic field is confined in the stellar crust. We follow the magneto-rotational evolution within a simple recycling scenario, including the possibility of magnetospheric propeller. We find the presence of a tail in the period distribution of the synthetic populations at periods shorter than 1.558 ms, the minimum detected so far. For the soft EoS the recycling gives rise to a spin distribution which is increasing monotonically toward short periods and a clear ``barrier'' forms at the minimum period for the onset of mass shedding. For the stiff EoS the distribution is flatter displaying a broad maximum about 2-4 ms. The estimated fraction of neutron stars spinning close to their shedding limit over the millisecond pulsar population is found to be significant. Crustal magnetic field decay models predict the existence of massive (M>1.4 M_sun) rapidly spinning neutron stars with very low magnetic moment.
Fast spinning neutron stars, recycled in low mass binaries, may have accreted a substantial amount of mass. The available relativistic measurements of neutron star masses, all clustering around 1.4 M_sun, however refer mostly to slowly rotating neutron stars which accreted a tiny amount of mass during evolution in a massive binary system. We develop a semi-analytical model for studying the evolution of the spin period P of a magnetic neutron star as a function of the baryonic mass load M_{ac}; evolution is followed down to submillisecond periods and the magnetic field is allowed to decay significantly before the end of recycling. We use different equations of state and include rotational deformation effects, the presence of a strong gravitational field and of a magnetosphere. For the non-magnetic case, comparison with numerical relativistic codes shows the accuracy of our description. The minimum accreted mass requested to spin-up a magnetized 1.35M_sun-neutron star at a few millisecond is 0.05 M_sun, while this value doubles for an unmagnetized neutron star. Below 1 millisecond the request is of at least 0.25 M_sun. There may exist a yet undetected population of massive submillisecond neutron stars. The discovery of a submillisecond neutron star would imply a lower limit for its mass of about 1.7M_sun.