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On The Hunt Of The X-Ray Binary SAX J18027-2016

15/08/2011

One of the outstanding problems in physics is the behavior of matter at extreme densities. This behavior is modelled using quantum-chromodynamics calculations, but these cannot yet determine reliably the densities at which, e.g. meson condensation and the hadron to quark-gluon phase transition occur. At densities slightly above nuclear and high temperature, models can be tested with heavy-nuclei collision experiments (Lattimer & Prakash 2010). For higher densities and low temperatures, only comparison with neutron-star parameters is possible (Lattimer & Prakash 2007).

The different models lead to different equations of state (EoS), which predict different mass-radius relations for neutron stars. Unfortunately, most attempts at observational tests have been frustrated by susceptibility to systematic errors and modeling uncertainties (Weissenborn et al. 2011). The most robust tests have involved measurements of extrema. For instance, the fastest measured spin period, 1.4ms (Ter 5ad; Hessels et al. 2006), excludes the stiffest EoS, for which neutron stars would be too large to spin so fast. This poses the question: just how big (or, rather, how massive) can a neutron star be?

With the recent mass determination of 1.97±0.04 for the neutron star (NS) PSR J1614-2230 by Demorest et al. (2010) and the 2.40±0.12 for the black widow Pulsar by van Kerkwijk et al (2011), the understanding of neutron star mass limits, and subsequently the precise nature of the EoS*, has been thrown into contention. PSR J1614-2230 has been the subject of a blog post here previously (PSR J1614-2230: A Neutron Star Born Massive?). Before this discovery, the theoretical constraints of a neutron star’s mass was between 1.4 and 1.6 (Kaplan & Nelson 1986). In the case of PSR J1614-2230 it is believed that the over-massive NS in this system was created by the accretion of matter whilst being spun-up to become a millisecond pulsar (MSP), but how sure can we be of this scenario?

N.B. Over 100 equations of state for neutron stars have been provided! (see Kaper et al. 2006).

It is reasonable, since it fits with the observed MSP population: older, magnetically weaker stars that have spin periods less than that of radio pulsars; meaning they must have been “spun-up” by some mechanism. This mechanism is the accretion of matter.The transfer of angular momentum from this accretion event can theoretically increase the rotation rate of the pulsar to hundreds of times a second. However, there has been recent evidence that the standard evolutionary model fails to explain the evolution of all millisecond pulsars, especially young millisecond pulsars with relatively high magnetic fields, e.g. PSR B1937+21 (Kızıltan & Thorsett 2009).

Another possible means of high mass NS production may occur where massive progenitors have yielded a high pre-SN core mass. High-mass X-ray binary systems (HMXBs) are potential testing grounds for this idea as in the majority of cases the progenitor of the NS will be more massive than the donor star. One such candidate for this analysis is eclipsing high mass X-ray binary pulsar IGR J18027-2016 with its supergiant donor.

IGR J18027-2016 was first detected by INTEGRAL in 2003* (Revnivtsev et al. 2004) and was spatially associated with the X-ray pulsar SAX J18027.7-2017 which was discovered serendipitously during observations of the LMXB GX9+1 and found to have a pulse period of 139.6 s (Augello et al. 2003). From timing analysis IGR J18027-2016 was found to have an orbital period of 4.5696±0.0009 days (Hill et al. 2005). Using a donor mass-radius relation together with approximations of the Roche-lobe radius Hill et al. (2005) proposed that the donor is a class 09 – B1 supergiant, with Torrejón et al. (2010) propising a spectral class of B1 Ib and Mason et al. (2011) providing an agreement of B0 – B1 I.

N.B. It was in September 2003 that the discovery of the serendipitous source SAX J1802.7-2017 in archival BeppoSAX data was announced, with Augello et al. (2003) identifying this source as an X-ray pulsar with a pulse period of 139.612 s based on a ~4 day observation in September of 2001.

Construction of an orbital solution from near-Infra Red radial velocity measurements and measurement of the orbital parameters of the system, provided Mason et al. (2011) with a dynamically determined compact object of 1.4±0.2 – 1.6±0.3 for IGR J18027-2016*. The mass and radius of the supergiant donor, calculated as M ~ 18 – 22 and R ~ 17 – 20 respectively, is in agreement with that suggested from X-ray observations by Hill et al. (2005).

N.B. These lower and upper limits were obtained under the assumption that the system is viewed edge-on (β = 0.89 with i = 90◦) for the lower limit and the donor fills its Roche lobe (β = 1 with i = 73.1◦) for the upper limit respectively.

What can these results tell us about the maximum mass of a neutron star and which EoS is correct? Well, a typical neutron star has a mass between 1.35 and about 2.0 solar masses (Kızıltan, Kottas & Thorsett 2010), with a corresponding radius of about 12 km if the Akmal-Pandharipande-Ravenhall equation of state (APR EoS) is used (Akmal, Pandharipande & Ravenhall 1998; Pethick et al. 2000). A radius for the compact object of 12km is very difficult to resolve, however, as shown, the mass is within the APR EoS.

So, determination of the mass of HMXBs such as SAX J18027-2016 it is possible to begin placing constraints on the mass of known NSs, and therefore provide a greater understanding of the maximum mass of a neutron star. In time, further studies will allow researchers to rule out the EoSs that do not tie theory to observation. Providing the physicists with limits to further boost the understanding of dense matter.

Journal References:

  • Hill, A. B. et al. (2005) The 1-50 keV Spectral & Timing Analysis Of IGR J18027-2016: An Eclipsing, High-Mass X-Ray BinaryAstronomy & Astrophysics439 (1) pp.255-263.
  • Mason, A.B. et al. (2010) Preliminary Determinations Of The Masses Of The Neutron Star & Mass Donor In The High-Mass X-Ray Binary System EXO 1722-363Astronomy & Astrophysics509 Article I.D.: 79.
  • Torrejón, J. M. et al. (2010) Near-IR Survey Of High-Mass X-Ray Binary CandidatesAstronomy & Astrophysics510 Article I.D.: 61.
  • Mason, A. B. et al. (2011) The Masses Of The Neutron & Donor Star In The High-Mass X-Ray Binary IGR J18027-2016Astronomy & Astrophysics532Article I.D.: 124.

Suggested Further Reading: 

  • Akmal, A.; Pandharipande, V. R.; Ravenhall, D. G. (1998) Equation Of State Of Nucleon Matter & Neutron Star StructurePhysical Review C (Nuclear Physics), 58 (3) pp.1804-1828.
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