Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?

In this Paper we report on phase resolved I-band optical spectroscopic and photometric observations of CirX-1 obtained with the Very Large Telescope. The spectra are dominated by Paschen absorption lines at nearly all orbital phases except near phase zero (coinciding with the X-ray dip) when the absorption lines are filled-in by broad Paschen emission lines. The radial velocity curve of the absorption lines corresponds to an eccentric orbit (e=0.45) whose period and time of periastron passage are consistent with the period and phase predicted by the most recent X-ray dip ephemeris. We found that the I-band magnitude decreases from 17.6 to ~16.8 near phase 0.9-1.0, this brightening coincides in phase with the X-ray dip. Even though it is likely that the absorption line spectrum is associated with the companion star of CirX-1, we cannot exclude the possibility that the spectrum originates in the accretion disc. However, if the spectrum belongs to the companion star, it must be a supergiant of spectral type B5-A0. If we assume that the compact object does not move through the companion star at periastron, the companion star mass is constrained to ~<10 Msun for a 1.4 Msun neutron star, whereas the inclination has to be ~> 13.7 degrees. Alternatively, the measured absorption lines and their radial velocity curve can be associated with the accretion disc surrounding a 1.4 Msun neutron star and its motion around the centre of mass. An absorption line spectrum from an accretion disc is typically found when our line-of-sight passes through the accretion disc rim implying a high inclination. However, from radio observations it was found that the angle between the line-of-sight and the jet axis is smaller than 5 degrees implying that the jet ploughs through the accretion disc in this scenario.Comment: 8 pages, 4 figures, 3 tables, accepted by MNRA

Enabling pulsar and fast transient searches using coherent dedispersion

We present an implementation of the coherent dedispersion algorithm capable of dedispersing high-time-resolution radio observations to many different dispersion measures (DMs). This approach allows the removal of the dispersive effects of the interstellar medium and enables searches for pulsed emission from pulsars and other millisecond-duration transients at low observing frequencies and/or high DMs where time broadening of the signal due to dispersive smearing would otherwise severely reduce the sensitivity. The implementation, called 'cdmt', for Coherent Dispersion Measure Trials, exploits the parallel processing capability of general-purpose graphics processing units to accelerate the computations. We describe the coherent dedispersion algorithm and detail how cdmt implements the algorithm to efficiently compute many coherent DM trials. We present the concept of a semi-coherent dedispersion search, where coherently dedispersed trials at coarsely separated DMs are subsequently incoherently dedispersed at finer steps in DM. The software is used in an ongoing LOFAR pilot survey to test the feasibility of performing semi-coherent dedispersion searches for millisecond pulsars at 135MHz. This pilot survey has led to the discovery of a radio millisecond pulsar -- the first at these low frequencies. This is the first time that such a broad and comprehensive search in DM-space has been done using coherent dedispersion, and we argue that future low-frequency pulsar searches using this approach are both scientifically compelling and feasible. Finally, we compare the performance of cdmt with other available alternatives.Comment: 8 pages, 7 figures, submitted to Astronomy and Computin

Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision

We have measured variations in scattering time scales in the Crab Pulsar over a 30-year period, using observations made at 610 MHz with the 42-ft telescope at Jodrell Bank Observatory. Over more recent years, where regular Lovell Telescope observations at frequencies around 1400 MHz were available, we have also determined the dispersion measure variations, after disentangling the scattering delay from the dispersive delay. We demonstrate a relationship between scattering and dispersion measure variations, with a correlation coefficient of $0.56\pm0.01$. The short time scales over which these quantities vary, the size of the variations, and the close correlation between scattering and dispersion measure all suggest that the effects are due to discrete structures within the Crab Nebula, with size scales of $\sim6$ AU (corresponding to an angular size of $\sim2$ mas at an assumed distance of 2200 pc). We mitigate the effects of scattering on the observed pulse shape by using the measured scattering information to modify the template used for generating the pulse arrival times, thus improving the precision to which the pulsar can be timed. We test this on timing data taken during periods of high scattering, and obtain a factor of two improvement in the root mean square of the timing residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA

Constraining the neutron star equation of state using quiescent low-mass X-ray binaries

Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can provide important constraints on the equation of state of neutron stars. The mass and radius of the neutron star can potentially be determined from fitting a neutron star atmosphere model to the observed X-ray spectrum. For a radius measurement it is of critical importance that the distance to the source is well constrained since the fractional uncertainty in the radius is at least as large as the fractional uncertainty in the distance. Uncertainties in modelling the neutron star atmosphere remain. At this stage it is not yet clear if the soft thermal component in the spectra of many quiescent X-ray binaries is variable on timescales too short to be accommodated by the cooling neutron star scenario. This can be tested with a long XMM-Newton observation of the neutron star X-ray transient CenX-4 in quiescence. With such an observation one can use the Reflection Grating Spectrometer spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X-ray spectral fitting of the EPIC pn and MOS spectra and allows one to investigate whether the variability observed in the quiescent X-ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N_H. This will test whether the soft thermal component can indeed be due to the hot thermal glow of the neutron star. Irrespective of the outcome of such a study, the observed cooling in quiescence in sources for which the crust is significantly out of thermal equilibrium with the core due to a prolonged outburst, such as KS 1731-260, seem excellent candidates for mass and radius determinations through modelling the observed X-rays with a neutron star atmosphere model.Comment: 7 pages, 3 figures, proceedings "40 years of pulsars" conferenc

On the reconstruction of a magnetosphere of pulsars nearby the light cylinder surface

A mechanism of generation of a toroidal component of large scale magnetic field, leading to the reconstruction of the pulsar magnetospheres is presented. In order to understand twisting of magnetic field lines, we investigate kinematics of a plasma stream rotating in the pulsar magnetosphere. Studying an exact set of equations describing the behavior of relativistic plasma flows, the increment of the curvature drift instability is derived, and estimated for $1s$ pulsars. It is shown that a new parametric mechanism is very efficient and can explain rotation energy pumping in the pulsar magnetospheres.Comment: 6 pages, 2 figure

Celestial position of the companion of PSR J1740-5340

We present optical astrometry of archival ground and space based imaging of the companion to PSR J1740-5340. The optical position of the companion is significantly offset from the timing position of the pulsar. We briefly investigate the effects of this inconsistency on other timing parameters and compare our position with an improved position of PSR J1740-5340 from recent, preliminary, timing results.Comment: 3 pages, 1 figure, accepted for publication in A&

Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars

Recent observations by XMM-Newton detected rotational pulsations in the total brightness and spectrum of several neutron stars. To properly interpret the data, accurate modeling of neutron star emission is necessary. Detailed analysis of the shape and strength of the rotational variations allows a measurement of the surface composition and magnetic field, as well as constrains the nuclear equation of state. We discuss our models of the spectra and light curves of two of the most observed neutron stars, RX J1856.5-3754 and 1E 1207.4-5209, and discuss some implications of our results and the direction of future work.Comment: 5 pages, 6 figures; Proceedings of "40 Years of Pulsars", eds. C. Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, submitte

The neutron star soft X-ray transient 1H1905+000 in quiescence

In this Paper we report on our analysis of a ~25 ksec. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope at Paranal. The X-ray counterpart of 1H1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2x10^-4 counts s^-1. For different spectral models this yields an upper limit on the luminosity of 1.8x10^31 erg s^-1 (for an upper limit on the distance of 10 kpc.) This luminosity limit makes 1H1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole SXTs in quiescence. This low luminosity for a neutron star SXT challanges the hypothesis presented in the literature that black hole SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a black hole event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 years after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time averaged mass accretion rate is lower than 2x10^-12 M_sun yr^-1, core cooling has to proceed via enhanced neutrino emission processes. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of M_I>7.8 assuming the source is at 10 kpc. This is in line with 1H1905+000 being an ultra-compact X-ray binary, as has been proposed based on the low outburst V-band absolute magnitude.Comment: 10 pages, 5 figures, accepted for publication in MNRA

The quasi-persistent neutron star soft X-ray transient 1M 1716-315 in quiescence

We report on our analysis of a 20 ksec Chandra X-ray observation of the quasi-persistent neutron star soft X-ray transient (SXT) 1M1716-315 in quiescence. Only one source was detected in the HEAO-I error region. Its luminosity is 1.6E32-1.3E33 erg s-1. In this the range is dominated by the uncertainty in the source distance. The source spectrum is well described by an absorbed soft spectrum, e.g. a neutron star atmosphere or black body model. No optical or near-infrared counterpart is present at the location of the X-ray source, down to a magnitude limit of I> 23.5 and K_s> 19.5. The positional evidence, the soft X-ray spectrum together with the optical and near-infrared non-detections provide strong evidence that this source is the quiescent neutron star SXT. The source is 10-100 times too bright in X-rays in order to be explained by stellar coronal X-ray emission. Together with the interstellar extinction measured in outburst and estimates for the source distance, the reported optical and near-infrared limit give an upper limit on the absolute magnitude of the counterpart of I>8.6 and K_s>5.1. This implies that the system is either an ultra-compact X-ray binary having P_orb<1 hr or the companion star is an M-dwarf. We reconstructed the long term X-ray lightcurve of the source. 1M1716-315 has been active for more than 12 years before returning to quiescence, the reported Chandra observation started 16.9+-4.1 years after the outburst ended.Comment: 7 pages, 4 figures, accepted to MNRA