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Sco X-1, the brightest low mass X-ray binary, is likely to be a source for gravitational wave emission. In one mechanism, emission of a gravitational wave arrests the increase in spin frequency due to the accretion torque in a low mass X-ray binary. Since the gravitational waveform is unknown, a detection method assuming no apriori knowledge of the signal is preferable. In this paper, we propose to search for a gravitational wave from Sco X-1 using a {{it source tracking}} method based on a coherent network analysis. In the method, we combine data from several interferometric gravitational wave detectors taking into account of the direction to Sco X-1, and reconstruct two polarization waveforms at the location of Sco X-1 in the sky as Sco X-1 is moving. The source tracking method opens up the possibility of searching for a wide variety of signals. We perform Monte Carlo simulations and show results for bursts, modeled, short duration periodic sources using a simple excess power and a matched filter method on the reconstructed signals.
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We study hard X-ray emission of the brightest accreting neutron star Sco X-1 with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of deadtime corrected exposure on this source. We show that hard X-ray tail in time average spectrum of Sco X-1 has a power law shape without cutoff up to energies ~200-300 keV. An absence of the high energy cutoff does not agree with the predictions of a model, in which the tail is formed as a result of Comptonization of soft seed photons on bulk motion of matter near the compact object. The amplitude of the tail varies with time with factor more than ten with the faintest tail at the top of the so-called flaring branch of its color-color diagram. We show that the minimal amplitude of the power law tail is recorded when the component, corresponding to the innermost part of optically thick accretion disk, disappears from the emission spectrum. Therefore we show that the presence of the hard X-ray tail may be related with the existence of the inner part of the optically thick disk. We estimate cooling time for these energetic electrons and show that they can not be thermal. We propose that the hard X-ray tail emission originates as a Compton upscattering of soft seed photons on electrons, which might have initial non-thermal distribution.
Rapidly-rotating neutron stars are the only candidates for persistent high-frequency gravitational wave emission, for which a targeted search can be performed based on the spin period measured from electromagnetic (e.g. radio and X-ray) observations. The principal factor determining the sensitivity of such searches is the measurement precision of the physical parameters of the system. Neutron stars in X-ray binaries present additional computational demands for searches due to the uncertainty in the binary parameters. We present the results of a pilot study with the goal of improving the measurement precision of binary orbital parameters for candidate gravitational wave sources. We observed the optical counterpart of Sco X-1 in 2011 June with the William Herschel Telescope, and also made use of Very Large Telescope observations in 2011, to provide an additional epoch of radial-velocity measurements to earlier measurements in 1999. From a circular orbit fit to the combined dataset, we obtained an improvement of a factor of two in the orbital period precision, and a factor of 2.5 in the epoch of inferior conjunction $T_0$. While the new orbital period is consistent with the previous value of Gottllieb et al. (1975), the new $T_0$ (and the amplitude of variation of the Bowen line velocities) exhibited a significant shift, which we attribute to variations in the emission geometry with epoch. We propagate the uncertainties on these parameters through to the expected Advanced LIGO & VIRGO detector network observation epochs, and quantify the improvement obtained with additional optical observations.
We report the detection of a non-thermal hard X-ray component from Sco X-1 based upon the analysis of 20-220 keV spectra obtained with the HEXTE experiment on-board the RXTE satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in 5 of 16 observations analyzed. Using PCA data we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power law index hardens with increasing mass accretion rate, as indicated from the position on the Z diagram. We find that the derived non-thermal luminosities are at order of 10% of that derived for the brightest of the atoll sources.
We present simultaneous X-ray (RXTE) and optical (ULTRACAM) narrow band (Bowen blend/HeII and nearby continuum) observations of Sco X-1 at 2-10 Hz time resolution. We find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ~0.5 when Sco X-1 is at the top of the Flaring Branch. Evidence of echoes is also seen at the bottom of the Flaring Branch but with time-lags of 5-10s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.
Sco X-1 is a low-mass X-ray binary (LMXB) that has one of the most precisely determined set of binary parameters such as the mass accretion rate, companions mass ratio and the orbital period. For this system, as well as for a large fraction of other well-studied LMXBs, the observationally-inferred mass accretion rate is known to strongly exceed the theoretically expected mass transfer rate. We suggest that this discrepancy can be solved by applying a modified magnetic braking prescription, which accounts for increased wind mass loss in evolved stars compared to main sequence stars. Using our mass transfer framework based on {tt MESA}, we explore a large range of binaries at the onset of the mass transfer. We identify the subset of binaries for which the mass transfer tracks cross the Sco X-1 values for the mass ratio and the orbital period. We confirm that no solution can be found for which the standard magnetic braking can provide the observed accretion rates, while wind-boosted magnetic braking can provide the observed accretion rates for many progenitor binaries that evolve to the observed orbital period and mass ratio.
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