Masses of Neutron Stars in HMXBs
High mass X-ray binaries consist of a compact object (black hole or a neutron star) with an OB star as the companion. We are measuring radial velocities of the OB stars with optical and IR spectroscopy, to determine the mass function of the neutron star. Combining this with X-ray timing information, we can calculate the mass ratios of the components of the binary, and place constraints on the mass of the neutron star.
Note: If you want to know more details about this work, look at my thesis.
MOTIVATION
Neutron stars are the compact remnants of high mass stars. The following three questions are central to our study of neutron stars:- What is the relation between the mass of the neutron star and the mass of the progenitor star (the initial-final mass mapping)?
- Are there exotic states of matter in the interiors of neutron stars?
- What are the paths by which neutron stars are formed?
Theories of formation of neutron stars have not yet matured fully. Model calculations by Timmes et al. (1996) predict that type II (core-collapse) supernovae should form a bimodal distribution peaked at 1.28 and 1.73 M⊙, while Ib supernovae will produce supernovae with a small range around 1.32 M⊙.
The global structure of a neutron star depends on the equation of state (EOS) of matter under extreme conditions, i.e. the relation between pressure and density in the neutron star. Given an EOS, the maximum mass of a neutron star can be calculated. Our current under- standing of nuclear physics predicts a "stiff" EOS, where a given density can support higher pressure. In contrast, if the EOS is soft then kaon condensates or strange mat- ter can form in the interior. The radius will be smaller and an upper limit of > 1.55 M⊙ is expected (van der Meer et al. 2007).
HIGH MASS X-RAY BINARIES (HMXBS)
HMXBs are binary systems containing a neutron star and massive (≈ 20M⊙) OB companions, with orbital periods ranging from days to months. HMXBs probe a unique path in the evolution of neutron stars. Within the few available measurements, two systems seem to have masses around 2 – 2.5 M⊙. This suggests that there might be other HMXBs with heavy neutron stars. However, both the large dispersion and small number statistics preclude a firm conclusion.OB stars are typically found within 50-60pc of the galactic plane, obscured by several magnitudes of extinction. Hence, relatively few HMXBs are known, and even fewer have optical / infrared counterparts. Fortunately, two advances: the discovery of new HMXBs made pos- sible by the INTEGRAL and Swift missions and high spectral resolution infrared spectroscopy (made possible by Nirspec on Keck) offer an opportunity to make great progress.
METHODOLOGY
We undertook an effort to measure masses of HMXB neutron stars from radial velocity measurements obtained from optical and near-IR spectroscopy. We started with a list of HMXBs discovered by INTEGRAL and sources in Liu et al. (2006), and selected 19 candidates for study. We were allocated six nights at Nirspec (Keck), two at LRIS (Keck) and six at Doublespec (Palomar) for this project. For most of our objects, we obtained >5 spectra at different phases, well-spaced over the orbit.We favored systems with constraints on orbital inclination (either eclipsing systems or those with phase resolved x-ray spectra) so that masses can be determined. Where no constraint on inclination exists we will place lower limits on the NS masses – which is interesting given the suggestion of some HMXBs with NS masses > 1.4 M⊙. Detecting a heavy neutron star can challenge several theories of the EOS for neutron stars. With mass measurements for 15–20 systems, we will expand the current sample size by a factor of few. This will enable us to make infer statistics of NS masses in HMXBs.
CURRENT STATUS
You can find the results of all this work in my PhD thesis. I have acquired soem more data from SALT on other binaries, stay tuned for updates!Image credit http://www.spacetelescope.org/images/cygx1_illustration_orig/ .