The role of General Relativity in the evolution of Low Mass X-ray Binaries

We study the evolution of Low Mass X-ray Binaries (LMXBs) and of millisecond binary radio pulsars (MSPs), with numerical simulations that keep into account the evolution of the companion, of the binary system and of the neutron star. According to general relativity, when energy is released, the system loses gravitational mass. Moreover, the neutron star can collapse to a black hole if its mass exceeds a critical limit, that depends on the equation of state. These facts have some interesting consequences: 1) In a MSP the mass-energy is lost with a specific angular momentum that is smaller than the one of the system, resulting in a positive contribution to the orbital period derivative. If this contribution is dominant and can be measured, we can extract information about the moment of inertia of the neutron star, since the energy loss rate depends on it. Such a measurement can therefore help to put constraints on the equation of state of ultradense matter. 2) In LMXBs below the bifurcation period (\sim 18 h), the neutron star survives the period gap only if its mass is smaller than the maximum non-rotating mass when the companion becomes fully convective and accretion pauses. Therefore short period (P < 2h) millisecond X-ray pulsar like SAX J1808.4-3658 can be formed only if either a large part of the accreting matter has been ejected from the system, or the equation of state of ultradense matter is very stiff. 3) In Low Mass X-ray binaries above the bifurcation period, the mass-energy loss lowers the mass transfer rate. As side effect, the inner core of the companion star becomes 1% bigger than in a system with a non-collapsed primary. Due to this difference, the final orbital period of the system becomes 20% larger than what is obtained if the mass-energy loss effect is not taken into account.Comment: 7 pages, 3 figures, accepted by the MNRA

Resolving the Fe xxv Triplet with Chandra in Cen X-3

We present the results of a 45 ks Chandra observation of the high-mass X-ray binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse post-egress phases). Here we concentrate on the study of discrete features in the energy spectrum at energies between 6 and 7 keV, i.e. on the iron K$_\alpha$ line region, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We clearly see a K$_\alpha$ neutral iron line at $\sim 6.40$ keV and were able to distinguish the three lines of the \ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the K$_\alpha$ neutral iron line is 13 eV, an order of magnitude lower than previous measures. We discuss the possibility that the small equivalent width is due to a decrease of the solid angle subtended by the reflector.Comment: 11 pages, 2 figures, To appear in the Astrophysical Journal Letter

To accrete or not to accrete: the dilemma of the recycling scenario

We study the evolution of a low-mass X-ray binary by coupling a binary stellar evolution code with a general relativistic code that describes the behaviour of the neutron star. We find that non-conservative mass transfer scenarios are required to prevent the formation of submillisecond pulsars and/or the collapse to a black hole. We discuss the sweeping effects of an active magneto-dipole rotator on the transferred matter as a promising mechanism to obtain highly non-conservative evolutions.Comment: 7 pages, including 2 figures. To appear in proceedings of Aspen Center for Physics Conference on ``Binary Radio Pulsars'' Eds. F. Rasio and I. Stair

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

Radio-ejection and bump-related orbital period gap of millisecond binary pulsars

The monotonic increase of the radius of low mass stars during their ascent on the red giant branch halts when they suffer a temporary contraction. This occurs when the hydrogen burning shell reaches the discontinuity in hydrogen content left from the maximum increase in the convective extension, at the time of the first dredge up, and produces a well known "bump" in the luminosity function of the red giants of globular clusters. If the giant is the mass losing component in a binary in which mass transfer occurs on the nuclear evolution time scale, this event produces a temporary stop in the mass transfer, which we will name "bump related" detachment. If the accreting companion is a neutron star, in which the previous mass transfer has spun up the pulsar down to millisecond periods, the subsequent mass transfer phase may be altered by the presence of the energetic pulsar. In fact, the onset of a radio--ejection phase produces loss of mass and angular momentum from the sytem. We show that this sequence of events may be at the basis of the shortage of systems with periods between ~ 20 and 60 days in the distribution of binaries containing millisecond pulsars. We predict that systems which can be discovered at periods into the gap should have preferentially either magnetic moments smaller than ~ 2 x10^{26}Gcm^3, or larger than ~ 4x10^{26}Gcm^3. We further show that this period gap should not be present in population II

Spin Down of Rotating Compact Magnetized Strange Stars in General Relativity

We find that in general relativity slow down of the pulsar rotation due to the magnetodipolar radiation is more faster for the strange star with comparison to that for the neutron star of the same mass. Comparison with astrophysical observations on pulsars spindown data may provide an evidence for the strange star existence and, thus, serve as a test for distinguishing it from the neutron star.Comment: 6 pages; Accepted for publication in Astrophysics and Space Scienc

The Different Fates of a Low-Mass X-ray Binary. I: Conservative Mass Transfer

We study the evolution of a low mass x-ray binary coupling a binary stellar evolution code with a general relativistic code that describes the behavior of the neutron star. We assume the neutron star to be low--magnetized (B~10^8 G). In the systems investigated in this paper, our computations show that during the binary evolution the companion transfers as much as 1 solar mass to the neutron star, with an accretion rate of 10^-9 solar masses/yr. This is sufficient to keep the inner rim of the accretion disc in contact with the neutron star surface, thus preventing the onset of a propeller phase capable of ejecting a significant fraction of the matter transferred by the companion. We find that, for neutron stars governed by equations of state from soft up to moderately stiff, an accretion induced collapse to a black hole is almost unavoidable. The collapse to a black hole can occur either during the accretion phase or after the end of the mass transfer when the neutron star is left in a supramassive sequence. In this last case the collapse is driven by energy losses of the fast spinning radio pulsar. For extremely supramassive neutron stars these energy losses cause a spin up. As a consequence the pulsar will have a much shorter lifetime than that of a canonical radio pulsar. This behavior depends on the equation of state for ultra-dense matter and therefore could be used to constrain the internal structure of the neutron star. If the r-modes of the neutron star are excited during the accretion process, the gravitational waves emisson limits the maximum spin attainable by a NS to roughly 2 ms. In this case the collapse during the accretion phase is even more common since the maximum mass achievable before the collapse to a black hole during accretion is smaller due to the limited spin frequency.Comment: 11 pages, 5 figures. Accepted for publication in MNRA

Average hard X-ray emission from NS LMXBs: Observational evidence of different spectral states in NS LMXBs

We studied and compared the long-term average hard X-ray (>20keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright ("GX") bulge sources while two are weaker ones in the 2-10keV range (H1750-440 and H1608-55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H1750-440 up to 150keV. The Z and bright "GX" Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5-3keV. Six sources show a hard tail in their average spectrum: CygX-2 (Z), GX340+0 (Z), GX17+2 (Z), GX5-1 (Z), ScoX-1 (Z) and GX13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX13+1. Using radio data from the literature we find, in all Z sources and bright "GX" Atolls, a systematic positive correlation between the X-ray hard tail (40-100keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.Comment: 12 pages, 7 figures, A&A in press; updated Fig 4 with an additional spectral state, typos corrected and added further comments for completenes