We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar, in deep quiescence with XMM-Newton and (near simultaneously) Gemini-South. The X-ray spectrum of 1808 is similar to that ...observed in quiescence in 2001 and 2006, describable by an absorbed power law with photon index 1.74 ± 0.11 and unabsorbed X-ray luminosity LX = 7.9 ± 0.7 X 1031 ergs s-1, for NH = 1.3 X 1021 cm-2. Fitting all the quiescent XMM-Newton X-ray spectra with a power law, we constrain any thermally emitting neutron star (NS) with a hydrogen atmosphere to have a temperature less than 30 eV and L NS (0.01-10 keV) <6.2 X 1030 ergs s-1. A thermal plasma model also gives an acceptable fit to the continuum. Adding an NS component to the plasma model produces less stringent constraints on the NS; a temperature of 36+4 -8 eV and L NS (0.01-10 keV) = 1.3+0.6 -0.8 X 1031 ergs s-1. In the framework of the current theory of NS heating and cooling, the constraints on the thermal luminosity of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these NSs. We compile data from the literature on the mass transfer rates and quiescent thermal flux of the largest possible sample of transient NS low-mass X-ray binaries. We identify a thermal component in the quiescent spectrum of the accreting millisecond pulsar IGR J00291+5934, which is consistent with the standard cooling model. The contrast between the cooling rates of IGR J00291+5934 and 1808 suggests that 1808 may have a significantly larger mass. This can be interpreted as arising from differences in the binary evolution history or initial NS mass in these otherwise similar systems.
We calculate the full stellar structural evolution of donors in AM CVnAM Canum Venaticorum (AM CVn) systems formed through the white dwarf (WD) channel coupled to the binary's evolution. Contrary to ...assumptions made in prior modelling, these donors are not fully convective over much of the AM CVn phase and do not evolve adiabatically under mass loss indefinitely. Instead, we identify three distinct phases of evolution: a mass-transfer turn-on phase (during which Porb continues to decrease even after contact, the donor contracts, and the mass-transfer rate accelerates to its maximum), a phase in which the donor expands adiabatically in response to mass loss, and a cooling phase beginning at Porb≈ 45–55 min during which the donor contracts. The physics that determines the behaviour in the first and third phases, both of which are new outcomes of this study, are discussed in some detail. We find the overall duration of the turn-on phase to be between ∼104 and ∼106 yr, significantly longer than prior estimates. We predict the donor's luminosity, L, and effective temperature, Teff. During the adiabatic expansion phase (ignoring irradiation effects), L≈ 10−6–10−4 L⊙ and Teff≈ 1000–1800 K. However, the flux generated in the accretion flow dominates the donor's intrinsic light at all times. The impact of irradiation on the donor extends the phase of adiabatic expansion to longer Porb, slows the contraction during the cooling phase, and alters the donor's observational characteristics. Irradiated donors during the adiabatic phase can attain surface luminosities up to ≈10−2 L⊙. We argue that the turn-on and cooling phases both will leave significant imprints on the AM CVn population's Porb-distribution. Finally, we show that the eclipsing AM CVn system SDSS J0926+3624 provides evidence that WD channel systems with non-zero entropy donors contribute to the AM CVn population, and we discuss the observational signature of the donor in this system.
We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical ...and ultraviolet emission. We estimate the WD's effective temperature, T sub(eff), using the optical continuum for a number of observed binaries, and we show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time-averaged accretion rate, < >, decreases monotonically with increasing P sub(orb), covering 6 orders of magnitude. If the short-period (P sub(orb) < 10 minutes) systems accrete at a rate consistent with gravitational radiation via direct impact, we predict their inpulsed optical/UV light to be that of the T sub(eff) > 50,000 K accreting WD. At longer P sub(orb) we calculate the T sub(eff) and absolute visual magnitude, M sub(V), that the accreting WD will have during low accretion states, and we find that the WD naturally crosses the pulsational instability strip. Discovery and study of pulsations could allow for the measurement of the accumulated helium mass on the accreting WD, as well as its rotation rate. Accretion heats the WD core, but for P sub(orb) > 40 minutes, the WD's T sub(eff) is set by its cooling as < > plummets. For the two long-period AM CVn binaries with measured parallaxes, GP Com and CE 315, we show that the optical broadband colors and intensity are those expected from a pure helium atmosphere WD. This confirms that the WD brightness sets the minimum light in wide AM CVn binaries, allowing for meaningful constraints on their population density from deep optical searches, both in the field and in globular clusters.
Galactic interacting double white dwarfs (DWDs) are guaranteed gravitational wave (GW) sources for the Laser Interferometer Space Antenna GW detector, with more than 10 super(4) binaries expected to ...be detected over the mission's lifetime. Part of this population is expected to be eccentric, and here we investigate the potential for constraining the white dwarf (WD) properties through apsidal precession in these binaries. We analyze the tidal, rotational, and general relativistic contributions to apsidal precession by using detailed He WD models, where the evolution of the star's interior is followed throughout the cooling phase. In agreement with previous studies of zero-temperature WDs, we find that apsidal precession in eccentric DWDs can lead to a detectable shift in the emitted GW signal when binaries with cool (old) components are considered. This shift increases significantly for hot (young) WDs. We find that apsidal motion in hot (cool) DWDs is dominated by tides at orbital frequencies above > ~ 10 super(-4) Hz (10 super(-3) Hz). The analysis of apsidal precession in these sources while ignoring the tidal component would lead to an extreme bias in the mass determination, and could lead us to misidentify WDs as neutron stars or black holes. We use the detailed WD models to show that for older, cold WDs, there is a unique relationship that ties the radius and apsidal precession constant to the WD masses, therefore allowing tides to be used as a tool to constrain the source masses.
We apply the Deloye & Bildsten isentropic models for donors in ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact, interacting binaries. The mass-radius relations of these ...systems' donors in the mass range of interest (M sub(2) < 0.1 M sub( )) are not single-valued, but parameterized by the donor's specific entropy. This produces a range in the relationships between system observables, such as orbital period P sub(orb) and mass transfer rate M. For a reasonable range in donor specific entropy, M can range over several orders of magnitude at fixed P sub(orb). We determine the unique relation between M and M sub(2) in the AM CVn systems with known donor-to-accretor mass ratios q = M sub(2)/M sub(1). We use structural arguments, as well as each system's photometric behavior, to place limits on M and M sub(2) in each. Most systems allow a factor of about 3 variation in M, although V803 Cen, if the current estimates of its q are accurate, is an exception and must have M sub(2) - 0.02 M sub( )and M - 10 super(-10) M sub( )yr super(-1). Our donor models also constrain each donor's core temperature (T sub(c)) range and correlate T sub(c) with M sub(2). We examine how variations in donor specific entropy across the white dwarf family of AM CVn systems affects this population's current Galactic distribution. Allowing for donors that are not fully degenerate produces a shift in systems toward longer P sub(orb) and higher M, increasing the parameter space in which these systems can be found. This shift increases the fraction of systems whose P sub(orb) is long enough that their gravity wave (GW) signal is obscured by the background of detached double white dwarf binaries that dominate the GW spectrum below a frequency of -2 mHz.
We observed the accreting millisecond pulsar SAX J1808.4−3658 with Gemini-South in g′ and i′ bands, nearly simultaneous with XMM–Newton observations. A clear periodic flux modulation on the system's ...orbital period is present, consistent with the varying aspect of the donor star's heated face. We model the contributions of a disc and donor star to these optical bands. To produce the observed modulation amplitudes, we conclude that the donor must be irradiated by an external flux two orders of magnitude greater than provided by the measured X-ray luminosity. A possible explanation for this irradiation is that the radio pulsar mechanism becomes active during the quiescent state as suggested by Burderi et al., with relativistic particles heating the donor's day side face. Our modelling constrains the binary inclination to be 36°–67°. We obtain estimates for the pulsar mass of >2.2 M⊙ (although this limit is sensitive to the source's distance), consistent with the accelerated neutron star cooling in this system indicated by X-ray observations. We also estimate the donor mass to be 0.07–0.11 M⊙, providing further indications that the system underwent non-standard binary evolution to reach its current state.
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift ...Burst Alert Telescope hard X-ray transient monitor. The pulsar was subsequently observed by both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an orbital period of 54.7 minutes. The minimum companion mass is between 0.0067 and 0.0086 M unk, depending on the mass of the neutron star, and the upper limit on the mass is 0.030 M unk. (95% confidence level). Such a low mass is inconsistent with brown dwarf models, and comparison with white dwarf models suggests that the companion is a He-dominated donor whose thermal cooling has been at least modestly slowed by irradiation from the accretion flux. No X-ray bursts, dips, eclipses, or quasi-periodic oscillations were detected. The current outburst lasted approximately 13 days, and no earlier outbursts were found in archival data.
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