The mass of the black hole in GRS 1915+105: new constraints from IR spectroscopy

GRS 1915+105 has the largest mass function of any Galactic black hole system, although the error is relatively large. Here we present spectroscopic analysis of medium-resolution IR VLT archival data of GRS 1915+105 in the K-band. We find an updated ephemeris, and report on attempts to improve the mass function by a refinement of the radial velocity estimate. We show that the spectra are significantly affected by the presence of phase-dependent CO bandhead emission, possibly originating from the accretion disc: we discuss the impact this has on efforts to better constrain the black hole mass. We report on a possible way to measure the radial velocity utilising apparent H-band atomic absorption features and also discuss the general uncertainty of the system parameters of this well-studied objectComment: 7 pages, 7 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journa

Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455

We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Halpha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km/s, placing a lower limit of 0.05 Msun on the secondary mass. For a 1.4 Msun primary, this implies that the orbital inclination is low, < 20 degrees. Utilizing the observed relationship between the secondary mass and orbital period in short period cataclysmic variables, we estimate the secondary mass to be ~0.085 Msun, which implies an upper limit of ~2.4 Msun for the primary mass.Comment: 8 pages, 6 figures; Accepted for publication in MNRAS. Minor revisions to match final published versio

Optical spectroscopy and Doppler tomography of Cygnus X-2

We present phase resolved optical spectroscopy and Doppler tomography of V1341 Cygni, the optical counterpart to the neutron star low mass X-ray binary Cygnus X-2. We derive a radial velocity curve for the secondary star, finding a projected radial velocity semi-amplitude of K2 = 79 +/- 3 km/s, leading to a mass function of 0.51 +/- 0.06 Msun, ~30% lower than the previous estimate. We tentatively attribute the lower value of K2 (compared to that obtained by other authors) to variations in the X-ray irradiation of the secondary star at different epochs of observations. The limited phase coverage and/or longer timebase of previous observations may also contribute to the difference in K2. Our value for the mass function implies a primary mass of 1.5 +/- 0.3 Msun, somewhat lower than previous dynamical estimates, but consistent with the value found by analysis of type-I X-ray bursts from this system. Our Doppler tomography of the broad He II 4686 line reveals that most of the emission from this line is produced on the irradiated face of the donor star, with little emission from the accretion disc. In contrast, the Doppler tomogram of the N III 4640.64 Bowen blend line shows bright emission from near the gas stream/accretion disc impact region, with fainter emission from the gas stream and secondary star. This is the first LMXB for which the Bowen blend is dominated by emission from the gas stream/accretion disc impact region, without comparable emission from the secondary star. This has implications for the interpretation of Bowen blend Doppler tomograms of other LMXBs for which the ephemeris may not be accurately known.Comment: 11 pages, 10 figures, 4 tables; Accepted for publication in MNRA

Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N iiiΛ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 ± 15 km s −1 ; applying a ‘ K -correction’, we find the velocity of the secondary star projected on to the line of sight to be 370 ± 40 km s −1 . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044 +0.005 −0.004 , and the mass function for the pulsar to be 0.44 +0.16 −0.13  M ⊙ . Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of 1.4 M ⊙ . Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74893/1/j.1365-2966.2009.14562.x.pd

Study of measured pulsar masses and their possible conclusions

We study the statistics of 61 measured masses of neutron stars (NSs) in binary pulsar systems, including 18 double NS (DNS) systems, 26 radio pulsars (10 in our Galaxy) with white dwarf (WD) companions, 3 NSs with main-sequence companions, 13 NSs in X-ray binaries, and one undetermined system. We derive a mean value of M = 1.46 +/- 0.30 solar masses. When the 46 NSs with measured spin periods are divided into two groups at 20 milliseconds, i.e., the millisecond pulsar (MSP) group and others, we find that their mass averages are, respectively, M=1.57 +/- 0.35 solar masses and M=1.37+/- 0.23 solar masses. In the framework of the pulsar recycling hypothesis, this suggests that an accretion of approximately 0.2 solar mass is sufficient to spin up a neutron star and place it in the millisecond pulsar group. An empirical relation between the accreting mass and MSP spin period is \Delta M=0.43 (solar mass)(P/1 ms)^{-2/3}. UNlike the standard recycling process, if a MSP is formed by the accretion induced collapse (AIC) of a white dwarf with a mass less than Chandrasekha limit, e.g. 1.35 solar mass, then the binary MSPs involved in AICs is not be higher than 20%, which imposes a constraint on the AIC origin of MSPs.Comment: 6 pages, 5 figures, in press, Astronomy and Astrophysics 2011, 527, 8

Accreting Millisecond X-Ray Pulsars

Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the past fifteen years of discoveries in the field. We summarize the observations of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength observations that have been carried out since the discovery of the first AMXP in 1998. We review accretion torque theory, the pulse formation process, and how AMXP observations have changed our view on the interaction of plasma and magnetic fields in strong gravity. We also explain how the AMXPs have deepened our understanding of the thermonuclear burst process, in particular the phenomenon of burst oscillations. We conclude with a discussion of the open problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer; [revision with literature updated, several typos removed, 1 new AMXP added

The double-peaked 2008 outburst of the accreting milli-second X-ray pulsar, IGR J00291+5934

Context. In August 2008, the accreting milli-second X-ray pulsar (AMXP), IGR J00291+5934, underwent an outburst lasting ∼100 days, the first since its discovery in 2004. Aims. We present data from the 2008 double-peaked outburst of IGR J00291+5934 from Faulkes Telescope North, the Isaac Newton Telescope, the Keck Telescope, PAIRITEL, theWesterbork Synthesis Radio Telescope and the Swift,XMM-Newton and RXTE X-ray missions. We study the outburst’s evolution at various wavelengths, allowing us to probe accretion physics in this AMXP. Methods. We study the light curve morphology, presenting the first radio-X-ray spectral energy Distributions (SEDs) for this source and the most detailed UV-IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mechanisms responsible for the SEDs. We analyse short-timescale optical variability, and compare a medium resolution optical spectrum with those from 2004. Results. The outburst morphology is unusual for an AMXP, comprising two peaks, the second containing a “plateau” of ∼10 days at maximum brightness within 30 days of the initial activity. This has implications on duty cycles of short-period X-ray transients. The X-ray spectrum can be fitted by a single, hard power-law. We detect optical variability of ∼0.05 mag, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated disc, and a transient near-infrared (NIR) excess. This excess is consistent with a simple model of an optically thick synchrotron jet (as seen in other outbursting AMXPs), however we discuss other potential origins. The optical spectrum shows a double-peaked Hα profile, a diagnostic of an accretion disc, but we do not clearly see other lines (e.g. He I, II) that were reported in 2004. Conclusions. Optical/IR observations of AMXPs appear to be excellent for studying the evolution of both the outer accretion disc and the inner jet, and may eventually provide us with tight constraints to model disc-jet coupling in accreting neutron stars

Doppler and modulation tomography of XTE J1118+480 in quiescence

We present Doppler and modulation tomography of the X-ray nova XTE J1118+480 with data obtained during quiescence using the 10-m Keck II telescope. The hotspot where the gas stream hits the accretion disc is seen in H alpha, H beta, He i lambda 5876 and Ca ii lambda 8662, thus verifying the presence of continued mass transfer within the system. The disc is clearly seen in H alpha and Ca ii lambda 8662. We image the mass-donor star in narrow absorption lines of Na i lambda lambda 5890, 5896, 8183, 8195 and Ca ii lambda 8662, implying an origin from the secondary itself rather than the interstellar medium. We also detect deviations in the centroid of the double peak of H alpha akin to those found by Zurita et al. suggesting disc eccentricity