INTERACTING BINARIES: Accretion, Evolution, and Outcomes
797(2005); http://dx.doi.org/10.1063/1.2130209View Description Hide Description
A review of the basic principles of population synthesis for binary stars is presented. We discuss the break‐up of low and intermediate mass close binaries over different evolutionary scenarios and, as an example, briefly consider results of the population synthesis for SN Ia.
797(2005); http://dx.doi.org/10.1063/1.2130210View Description Hide Description
Our deep Chandra exposures of 47Tuc and moderate exposures of NGC 6397 reveal a wealth of new phenomena for interacting X‐ray binaries (IXBs) in globular clusters. In this (late) Review, updated since the conference, I summarize recent and ongoing analysis of the millisecond pulsars, the compact binaries containing white dwarfs and neutron stars, and the chromospherically active binaries in both globular clusters. Spectral variability analysis enables new insights into source properties and evolutionary history. These binary populations, now so “easily” visible, are large enough that their properties and spatial distributions reveal new hints of compact object formation and binary interactions with their parent cluster. Neutron stars appear overabundant, relative to white dwarfs, in 47Tuc vs. NGC 6397. The IXBs containing neutron stars (i.e., MSPs and qLMXBs), as the most massive and ancient compact binary sample, may trace the protocluster disk in 47Tuc, whereas compact binaries may have been ejected preferentially along the cluster rotation equator during the recent core collapse in NGC 6397.
797(2005); http://dx.doi.org/10.1063/1.2130211View Description Hide Description
A large number of X‐ray sources in globular clusters of galaxies other than the Milky Way has been found with Chandra. We discuss three issues relating to these sources. The X‐ray luminosity function (XLF) of the sources in globular clusters of M31 is marginally compatible with the XLF of globular clusters of the Milky Way. The individual XLFs of a dozen elliptical galaxies, after correction for incompleteness, are compatible with one another and show no break; however, the XLF found by adding the individual XLFs of elliptical galaxies has a break at Lx ≃ 5 × 1038 erg s−1. For the moment there is no evidence for a difference between the XLFs of sources inside and outside globular clusters of elliptical galaxies. It is not (yet?) possible to decide which fraction of low‐mass X‐ray binaries in elliptical galaxies outside globular clusters have formed inside globular clusters.
797(2005); http://dx.doi.org/10.1063/1.2130212View Description Hide Description
Observations with BeppoSAX, RXTE and Chandra suggest that many of the bright X‐ray sources in globular clusters have ultrashort binary periods. This is remarkable as such systems are not easily formed. With accurate optical astrometry of HST images, the large numbers of low‐luminosity X‐ray sources discovered with Chandra can be classified as quiescent low‐mass X‐ray binaries, pulsars, cataclysmic variables, and magnetically active binaries. The number of cataclysmic variables is found to scale with the number of close stellar encounters.
797(2005); http://dx.doi.org/10.1063/1.2130213View Description Hide Description
Chandra observations of globular clusters provide insight into the formation, evolution, and X‐ray emission mechanisms of X‐ray binary populations. Our recent (2002) deep observations of 47 Tuc allow detailed study of its populations of quiescent LMXBs, CVs, MSPs, and active binaries (ABs). First results include the confirmation of a magnetic CV in a globular cluster, the identification of 31 additional chromospherically active binaries, and the identification of three additional likely quiescent LMXBs containing neutron stars. Comparison of the X‐ray properties of the known MSPs in 47 Tuc with the properties of the sources of uncertain nature indicates that relatively few X‐ray sources are MSPs, probably only ∼30 and not more than 60. Considering the ∼30 implied MSPs and 5 (candidate) quiescent LMXBs, and their canonical lifetimes of 10 and 1 Gyr respectively, the relative birthrates of MSPs and LMXBs in 47 Tuc are comparable.
797(2005); http://dx.doi.org/10.1063/1.2130214View Description Hide Description
I summarize some aspects of the research at OAR on the neutron star population in Globular clusters. In particular I discuss the evolutionary stage of the interacting millisecond pulsar binary PSR J1740‐5340 in the cluster NGC 6397 and point out that the donor component can not be a recently acquired low mass star. The precise mass derived for this star helps to constrain the loss of angular momentum during the radio‐ejection phase which this system is traversing. I then discuss the problem of the large number of neutron stars present in globular clusters and show that it might be due to an atypical initial mass function, which is necessary to understand the chemical anomalies shown by the low mass stars evolving today.
797(2005); http://dx.doi.org/10.1063/1.2130215View Description Hide Description
The population of compact binaries in dense stellar systems is affected strongly by frequent dynamical interactions between stars and their interplay with the stellar evolution. In this contribution, we consider these effects on binaries with a white dwarf accretor, in particular cataclysmic variables and AM CVns. We examine which processes can successfully lead to the creation of such X‐ray binaries. Using numerical simulations, we identify predominant formation channels and predict the expected numbers of detectable systems. We discuss also why the distribution of cataclysmic variables has a weaker dependence upon the cluster density than the distribution of quiescent low‐mass X‐ray binaries and why dwarf nova outbursts may not occur among globular cluster cataclysmic variables.
797(2005); http://dx.doi.org/10.1063/1.2130216View Description Hide Description
We present a photometric survey aimed at the identification of variable stars in the galactic globular cluster ω Cen. Our photometric catalogue is based on ∼ 400 exposures of the central region of the cluster(Fov ∼ 15′ × 15′) collected with the Danish Telescope. The catalogue has an accuracy of about ∼ 0.02 mag at B ∼ 18 mag. We present here preliminary results concerning the possible identification of two variable stars and an eclipsing binary candidate in ω Cen.
797(2005); http://dx.doi.org/10.1063/1.2130217View Description Hide Description
Despite considerable evidence verifying that millisecond pulsars are spun up through sustained accretion in low‐mass X‐ray binaries (LMXBs), it has proven surprisingly difficult to actually detect millisecond X‐ray pulsars in LMXBs. There are only 5 accretion‐powered millisecond X‐ray pulsars known among more than 80 LMXBs containing neutron stars, but there are another 11 “nuclear‐powered” millisecond pulsars which reveal their spin only during brief, thermonuclear X‐ray bursts. In addition, 2 of the accretion‐powered pulsars also exhibit X‐ray burst oscillations, and their unusual properties, along with the absence of persistent pulsations in most LMXBs, suggest that the magnetic fields in many LMXBs may be hidden by accreted material. Interestingly, the nuclear‐powered pulsars offer a statistically unbiased probe of the spin distribution of recycled pulsars and show that this distribution cuts off sharply above 730 Hz, well below the breakup spin rate for most neutron star equations of state. This indicates that some mechanism acts to halt or balance spin‐up due to accretion and that submillisecond pulsars must be very rare (and are possibly nonexistent). It is unclear what provides the necessary angular momentum sink, although gravitational radiation is an attractive possibility.
797(2005); http://dx.doi.org/10.1063/1.2130218View Description Hide Description
Millisecond radio pulsars are neutron stars that have been spun‐up by the transfer of angular momentum during the low‐mass X‐ray binary phase. The transition from an accretion‐powered to a rotation‐powered pulsar takes place on evolutionary timescales at the end of the accretion process, however it may also occur sporadically in systems undergoing transient X‐ray activity. We have obtained the first optical spectrum of the low mass transient X‐ray pulsar SAX J1808.4‐3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X‐ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ∼ 100 larger than directly observed, must be present in the system. This most likely derives from a rotation‐powered neutron star that resumes activity during quiescence.
797(2005); http://dx.doi.org/10.1063/1.2130219View Description Hide Description
We analyze the power spectrum of SAX J1808.4‐3658, the first accreting millisecond binary pulsar discovered, to look for a broadening in the wings of the harmonic line, in analogy to what had been previously found in some high mass X‐ray binaries. We indeed detect a broadening at the base of the 401 Hz peak, which is consistent with the convolution of the low frequency noise present in the power spectrum with the harmonic line. We interpret this as the result of a coupling between a fraction of the aperiodic and periodic variability, suggesting that at least part of the noise originates in a region close to the neutron star surface at the magnetic poles.
797(2005); http://dx.doi.org/10.1063/1.2130220View Description Hide Description
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.
797(2005); http://dx.doi.org/10.1063/1.2130221View Description Hide Description
We are carring out a programme which exploits the advantages offered by the current generation of astronomical instruments (from the ground and from space) in a coordinate effort to understand the formation mechanism and evolution of Millisecond Pulsar (MSP) binary systems in Globular Clusters. In this framework I present a flower of the most recent results obtained by our group.
797(2005); http://dx.doi.org/10.1063/1.2130222View Description Hide Description
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.
797(2005); http://dx.doi.org/10.1063/1.2130223View Description Hide Description
When a radio pulsar brakes down due to magnetodipole emission, its gravitational mass decreases accordingly. If the pulsar is hosted in a binary system, this mass loss will increase the orbital period of the system. We show that this relativistic effect can be indeed observable if the neutron star is fast and magnetized enough and that, if observed, it will help to put tight constraints to the equation of state of ultradense matter. Moreover, in Low Mass X‐ray Binaries that evolve towards short periods, the neutron star lights up as a radio pulsar during the “period gap”. As the effect we consider contrasts the orbital period decay, the system spends a longer time in this phase. As a consequence, the neutron star can survive this phase only if it is non‐supramassive. Since in such binaries ∼ 0.8M ⊙ can be accreted onto the neutron star, short period (P ⩽ 2 h) millisecond X‐ray pulsars 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.
797(2005); http://dx.doi.org/10.1063/1.2130224View Description Hide Description
I discuss the implication of the temporal structure of GRBs to the nature of their inner engine. I argue that the temporal strucutre shows that GRBs must involve internal shocks (or another kind of internal interaction within a relativistic outflow). To produce these internal shocks GRB inner engines must vary on a time scale of a fraction of a second and, on the other hand, they should be active for the whole duration of the burst, namely for several dozen of seconds. This implies that from the point of view of the central engine GRBs are a “quasi steady state” phenomenon. Accretion onto a newly formed black hole is the most likely mechanism that can satisfy these conditions and can power GRBs. I discuss the implication of accretion models of massive disks around black holes to GRB modelling.
797(2005); http://dx.doi.org/10.1063/1.2130225View Description Hide Description
We report on a new model which is able to explain how a gamma‐ray burst (GRB) can take place days or years after a supernova explosion. We show that above a threshold value of the gravitational mass a pure hadronic star (“neutron star”) is metastable to the conversion into a quark star (hybrid star or strange star), i.e. a star made at least in part of deconfined quark matter. The stellar conversion process can be delayed if finite size effects at the interface between hadronic and deconfined quark matter phases are taken into account. A huge amount of energy, on the order of 1052 – 1053 ergs, is released during the conversion process and can produce a powerful gamma‐ray burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay inferred in GRB 990705 and in GRB 011211.
797(2005); http://dx.doi.org/10.1063/1.2130226View Description Hide Description
The evolution of accretion disks in the so‐called hypercritical regime, where the main source of cooling is neutrino emission, is relevant for the study of gamma ray burst (GRB) central engines. For short bursts, which may arise from compact binary merger remnant disks, no external agent feeds the disk, and when the initial mass supply is exhausted no further energy release is possible. For long bursts, possibly arising from massive rotating core collapse, the infalling envelope may supply the disk with matter and allow the energy release to continue for some time. We give here a general overview of the conditions in such disks, and present detailed calculations of their structure and evolution (for the particular case of disks arising from mergers), taking into account the effects of neutrino opacities.
797(2005); http://dx.doi.org/10.1063/1.2130227View Description Hide Description
Polarimetry of Gamma‐Ray Burst (GRB) afterglows in the last few years has been considered one of the most effective tool to probe the geometry, energetic, dynamics and the environment of GRBs. We report some of the most recent results and discuss their implications and future perspectives.