Natale R. Robba

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

Context. 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. Aims. The detection and study of optical/IR counterparts of low-mass X-ray binaries is fundamental to characterizing both the lowmass 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. Methods. 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. Results. 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. Conclusions. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increasefrom quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of th e double stellar generation population of Terzan 5 by using recent stellar evolution models.

High Resolution and Broad Band Spectra of Low Mass X-ray Binaries: a Comparison between Black Holes and Neutron Stars

A common question about compact objects in high energy astrophysics is whether it is possible to distinguish black hole from neutron star systems with some other property that is not the mass of the compact object. Up to now a few characteristics have been found which are typical of neutron stars (like quasi periodic oscillations at kHz frequencies or type-I X-ray bursts), but in many respects black hole and neutron star systems show very similar behaviors. We present here a spectral study of low mass X-ray binaries containing neutron stars and show that these systems have spectral characteristics that are very similar to what is found for black hole systems. This implies that it is unlikely we can distinguish between black holes and neutron stars from their X-ray spectra, except for the fact that black hole systems show sometimes a more extreme behavior with respect to neutron star systems.

Hard X-ray emission from low mass X-ray binaries

In this paper we review our current knowledge of the hard X-ray emission properties of old accreting neutron stars in low mass X-ray binaries, with particular attention to recent results obtained for the brightest sources of this class, the so-called Z sources. While less luminous low mass X-ray binaries often show quite hard spectra, sometimes extending up to energies g 100 keV, the spectra of Z sources are always very soft, dominated by thermal components with characteristic temperatures ∼ 3-6 keV. However, recent broad band observations revealed the presence of a weak hard (power-law) component that is sometimes present in the spectra of these sources. These observations have strengthened the analogies between the spectral behavior of low mass X-ray binaries hosting neutron stars and binary systems containing black hole candidates. The physical parameters regulating the presence of this hard component are unknown yet. The first parameter may be the mass accretion rate, as indicated by the general anticorrelation between the fraction luminosity in hard X-rays and mass accretion rate apparent over different sources spanning a large range of luminosities as well as individual sources undergoing state changes. However, a second, yet unknown, parameter is probably needed to explain all the phenomenology. The broad high energy coverage and good sensitivity of the INTEGRAL mission can represent an important step forward in the understanding of the origin and properties of high energy components in accreting X-ray binaries.

A relativistically smeared line profile in the spectrum of the bright Z-source GX 340+0

We present preliminary results of a 50 ks long XMM-Newton obs ervation of the bright Z-source GX 340+0. We study the temporal and spectral variability of t he source, performing a time resolved analysis. In the energy spectra, a broad asymmetri c emission line in the Fe K α energy band is always present. Its shape is compatible with a relati vistically smeared profile arising from reflection on a hot accretion disk extending close to the cent ral accreting neutron star. Despite a significant change in the continuum emission and luminosity , the line profile remains substantially unchanged. The line is produced by recombination of highly i onized iron (Fe XXV), the reflecting disk has an inner radius close to 10 gravitational radii, whi le the fit requires a high value for the outer disk radius. The inclination of the source is well cons trained at 35 deg, while the emissivity index is -2.33.

Accretion and Magneto‐Dipole Emission in Fast‐Rotating Neutron Stars: New Spin‐Equilibrium Lines

Although most of the proposed equations of state predict minimum spin periods well below one millisecond if more than about 0.3 solar masses are accreted onto a low magnetized neutron star through a Keplerian accretion disc, the spin periods of the recently discovered accreting millisecond pulsars all cluster in the quite narrow range between 1.7 and 5.4 ms; these spin periods are uncomfortably higher than the theoretical predictions. We propose and discuss here the possibility that emission due to a magneto‐dipole rotator is relevant even during the accretion phase in fast‐spinning neutron stars; this mechanism is able to explain the quite long spin periods observed in both low mass X‐ray binaries and millisecond radio pulsars.

BeppoSAX observation of the X-ray pulsar 4U 1538-52

We report preliminary results of the temporal and spectral analysis performed on the X-ray pulsar 4U1538-52 out of eclipse observed by BeppoSAX. We obtain a new estimate of the pulse period of the neutron star P=528.24±0.01 s (corrected for the orbital motion of the X-ray source): the source is still in the spin-up state, as since 1988. The broad band (0.12–100 keV) spectral analysys shows the presence of an absorption feature at ∼21 keV is present, interpreted as due to cyclotron resonant scattering. Another absorption feature at ∼51 keV seems also to be present (at 99% confidence level). If confirmed this might be interpreted as a cyclotron line, but its energy is not compatible with being double than the energy of the first line.

Relativistic smearing detected in the spectrum of Cyg X-1 in hard state

We report on an observation of the black hole candidate Cyg X-1 performed by the Narrow Field Instruments on board BeppoSAX satellite. During the BeppoSAX observation Cyg X-1 was in its usual low (hard) state with a 0.1–200 keV luminosity of ∼2.6×1037 erg/s, adopting a distance of 2.5 kpc. The broad band spectrum (0.1–30 keV) is well fitted by an absorbed power-law partially reflected by cold matter and a soft excess. The reprocessed component is relativistically smeared and the inferred inner radius of the disc is 20±10 gravitational radii. This inner radius is much smaller than the prediction of the so called Advection Dominated Accretion Flow models: Rin>200 Rg.

Neutron star spin evolution: A semi-analytical approach

We report on 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 Macc. We have taken into account different equations of state and included rotational deformation effects. The presence of a strong gravitational field has also been considered in the context of general relativity. A comparison with numerical fully relativistic codes shows that our description is accurate within 5%.