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Constraints On The Compact Object In The Eclipsing HMXB XMMU J013236.7+303228


Lurking within Messier 33 (a.k.a the Triangulum Galaxy or NGC 0598) lies the high-mass X-ray binary (HMXB) XMMU J013236.7+303228. As eluded to in previous posts, most stars are actually a member of a douplet: a binary system. Some binary systems are strong in X rays, and in which the normal stellar component is a massive star: usually an O or B star, perhaps sometimes a Be star or a blue supergiant. The compact, X-ray emitting component has been theoretically established to the result of a neutron star, or possibly a black hole, where a considerable fraction of the stellar wind of the massive normal star is captured by the compact object emitting photons in the X-ray energy band as it falls onto the compact object. XMMU J013236.7+303228 is one such HMXB system where a neutron star is the cause of the observed X-ray emission from the system (Pietsch et al. 2004).

There are many differing factors which effect potential neutron star masses, such as the initial mass of the progenitor’s stellar core, the details of the explosion (in particular mass accretion as the explosion develops), subsequent mass accretion from a binary companion and the pressure-density relation, or equation of state (EOS), of the neutron star matter. Such factors, differing as much as they are numerous, are extremely interesting in stellar evolutionary studies.

On the low-mass end, producing a neutron star requires the progenitor’s core to exceed the Chandrasekhar mass, which depends on the uncertain electron fraction. Theoretical models place this minimum mass in the range ≳ 0.9 – 1.3 (Timmes et al. 1996). The largest possible neutron star mass depends on the unknown physics determining the EOS – for example whether kaon condensates or strange matter can from in the interior (e.g. Lattimer & Prakash 2005). The highest-mass neutron star to-date, the millisecond pulsar (MSP) PSR J1614–2230, “weighs” in at 1.97 ± 0.04 (Demorest et al. 2010), which already rules out the presence of exotic hadronic matter at the nuclear saturation density (Demorest et al. 2010; Lattimer et al. 2010).

If one wishes to test the predictions made by a supernova model or a binary evolution model it is necessary to determine neutron star masses at the extreme end of the high-mass spectrum in a variety of systems with different evolutionary schemes and progenitor masses. One such scheme, where a neutron star accompanied by either a high-mass star or another neutron star is believed to have accreted little to no matter over its evolutionary lifetime.

Recently, Bhalerao et al. (2012) presented optical spectroscopic measurements of the eclipsing High Mass X-ray Binary XMMUJ013236.7+303228 in M33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital period they determined physical as well as orbital parameters for the donor star: a B1.5IV sub-giant with effective temperature is T = 22000 − 23000K. From the luminosity, temperature and known distance to M33 (~800 kpc) Bhalerao et al. (2012) derived a radius of .

From the radial–velocity measurements, Bhalerao et al. (2012) determined a velocity semi-amplitude of , whilst from the the physical properties of the B-star determined from the optical spectrum, they were also estimate the star’s mass to be . Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B-star mass the neutron star mass to be ; which is a result consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 found for many millisecond pulsars.

Bhalerao et al. (2012) attempted to use as an additional constraint that the B star radius, inferred from temperature, flux and distance, should equate the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but from trying to apply the technique to known systems the masses are consistently overestimated. Attempting to account for that in our uncertainties, the most accurate derivation for the mass of the neutron star was  and for the B-type companion star . Bhalerao et al. (2012) conclude that precise constraints require detailed modeling of the shape of the Roche surface.


  • Pietsch, W.; Misanovic, Z.; Haberl, F.; Hatzidimitriou, D.; Ehle, M.; Trinchieri, G. (2004) XMM-Newton Survey Of The Local Group Galaxy M 33. Astronomy & Astrophysics, 426: pp.11-24.
  • van Kerkwijk, M. H.; Breton, R. P.; Kulkarni, S. R. (2011) Evidence For A Massive Neutron Star From A Radial-Velocity Study Of The Companion To The Black-Widow Pulsar PSR B1957+20. The Astrophysical Journal, 728 (2): Article I.D.# 95.
  • Bhalerao, V.B.; van Kerkwijk, M.H.; Harrison, F.A. (2012) Constraints On The Compact Object Mass In The Eclipsing High-Mass X-Ray Binary XMMU J013236.7+303228 In M 33. The Astrophysical Journal, 757 (1): Article I.D.# 10.

Suggested Further Reading:

  • Liu, Q. Z.; van Paradijs, J.; van den Heuvel, E. P. J. (2000) A Catalogue Of High-Mass X-Ray Binaries (HMXBs). Astronomy & Astrophysics Supplement, 147: pp.25-49.
  • Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe. Cambridge University Press, 2nd Edition. Cambridge, United Kingdom


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