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Measuring the Masses of Neutron Stars

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Measuring the Masses of Neutron Stars Reports from Observers Measuring the Masses of Neutron Stars Lex Kaper1 Arjen van der Meer1 Marten van Kerkwijk Ed van den Heuvel1 Visualisation: B. Pounds 1 Astronomical Institute “Anton Panne- koek” and Centre for High Energy Astrophysics, University of Amsterdam, the Netherlands Department of Astronomy and Astro- physics, University of Toronto, Canada Until a few years ago the common un- derstanding was that neutron stars, the compact remnants of massive stars, have a canonical mass of about 1.4 MA. Recent observations with VLT/UVES support the view that the neutron stars in high-mass X-ray binaries display a relatively large spread in mass, ranging from the theoretical lower mass limit Figure 1: Artist’s impression of a high-mass X-ray of 1 MA up to over 2 MA. Such a mass ter of such a high density. As bosons do distribution provides important informa- not contribute to the fermi pressure1, their binary hosting a massive OB-type star and a compact X-ray source, a neutron star or a black tion on the formation mechanism of presence will tend to ‘soften’ the EOS. hole. neutron stars (i.e. the supernovae), and For a soft EOS, the maximum neutron star on the (unknown) behaviour of matter at mass will be low (~ 1.5 MA); for a high- 1807 with a mass of .1 ± 0. MA (Nice et supranuclear densities. er mass, the object would collapse into al. 005). These results favour a stiff EOS. a black hole. More than 100 candidate equations of state are proposed, but only A neutron star cannot be more massive The compact remnants of massive stars one EOS can be the correct one. Find- than 3. MA, a limit set by general re- ing one massive neutron star (M ≥ MA) lativity. It is likely that the maximum neu- A neutron star is the compact remnant would rule out the soft equations of state. tron-star mass is determined by the of a massive star (M ≥ 8 MA) with a cen- stiffness of the EOS, and is expected to tral density that can be as high as 5 to The measurement of neutron-star masses be about .5 MA. Neutron stars also 10 times the density of an atomic nucle- is thus important for our understand- have a minimum mass limit. The minimum us. Neutron stars can be detected as ing of the EOS of matter at supranuclear stable neutron-star mass is about 0.1 MA, radio sources (radio pulsars) or, when densities. In practice, this can only be although a more realistic minimum stems they accrete matter coming from a com- done for neutron stars in binary systems. from a neutron star’s origin in a super- panion star in a binary system, as X-ray The most accurate masses have been nova. Lepton-rich proto neutron stars are sources. derived for the binary radio pulsars. Until unbound if their masses are less than recently, all of these were consistent about 1 MA (Lattimer and Prakash 004). The global structure of a neutron star with a small mass range near 1.35 MA Whether or not neutron stars occupy the depends on the equation of state (EOS), (Thorsett and Chakrabarty 1999). An ex- full available mass range depends on the i.e. the relation between pressure ception is the X-ray pulsar Vela X-1 with formation mechanism, i.e. the supernova. and density in the neutron star interior a mass of 1.86 ± 0.16 MA (Barziv et al. (Lattimer and Prakash 004). So far, 001). This important result was obtained We focus here on the initially most mas- the physical properties of matter under following a nine-month spectroscopic sive binary systems, which consist of these extreme conditions can only be monitoring campaign with the ESO Coudé a massive OB-supergiant and a neutron studied on the basis of theoretical mod- Auxiliary Telescope and the CES spec- star (or a black hole). The main motiva- els. It is not yet possible to produce the trograph, covering 36 orbits of the binary tion to concentrate on these systems is required extremely high density in ac- system. Over the past year a few more that they are the most likely hosts of celerator experiments. Given an EOS, a massive neutron stars were discovered, massive neutron stars. About a dozen of mass-radius relation for the neutron e.g. the millisecond radio pulsar J0751+ these systems are known; five of them star and a corresponding maximum neu- tron-star mass can be derived. The 1 The repelling force that neutrons (fermions) exert About 80 % of the HMXBs are Be/X-ray binaries, ‘stiffness’ of the EOS depends e.g. on on each other so that a neutron star can sustain where the (transient) X-ray source accretes material how many bosons are present in mat- gravity. from the equatorial disc around a Be star. The Messenger 126 – December 006 7 Reports from Observers Kaper L. et al., Measuring the Masses of Neutron Stars contain an eclipsing X-ray pulsar. The The compact companion is the remnant spiral-in likely results in the removal of masses of all but one (Vela X-1) are con- of the initially most massive star in the the envelope of the Be companion, and sistent (within their errors) with a value system that exploded as a supernova. after the (second) supernova a bound of about 1.4 MA. However, most spectro- Due to a phase of mass transfer, the sec- (or disrupted) double neutron star re- scopic observations used for these mass ondary became the most massive star in mains, like the Hulse-Taylor binary pulsar determinations were carried out more the system before the primary supernova, PSR 1913+16 (or a neutron star – white than 0 years ago, before the advent of so that the system remained bound. A dwarf system, if the mass of the Be com- sensitive CCD detectors and 8-m-class consequence, however, is that HMXBs panion is less than 8 MA). In HMXBs with telescopes, which allow high-resolution are runaways due to the kick velocity an orbital period less than about a year spectroscopy of the optical companions. exerted by the supernova (Blaauw 1961). the compact object will enter the core of The uncertainties in the earlier radial When they run through space with super- the OB companion which will become velocity measurements are too large to sonic velocity, the interaction of the OB- a Thorne-Zytkow object, a red supergiant measure a significant spread in mass supergiant wind with the interstellar me- with a high mass loss rate. These ob- among these neutron stars, if present. dium can result in the formation of a bow jects have been predicted on evolutionary shock (see also The Messenger 89, 8). grounds, but have so far not been recog- nised as such. High-mass X-ray binaries The HMXB phase is relatively short for OB-supergiant systems, of the order of Table 1 lists the basic properties of the High-mass X-ray binaries (HMXBs) repre- 10 000 years. This corresponds to the HMXBs with OB-supergiant compan- sent an important phase in the evolution time required for the secondary to evolve ions in the Milky Way and the Magellanic of massive binaries. They are composed into a supergiant when the hydrogen in Clouds. Most sources contain an X-ray of a massive OB-type star and a com- the nucleus has been exhausted. As soon pulsar; the pulse period (i.e. rotation pe- pact object, either a neutron star or a as the secondary has become an OB- riod of the neutron star) is short and the 38 –1 black hole (Figure 1). The X-ray source is supergiant it develops a strong stellar X-ray luminosity high (LX ~ 10 erg s ) powered by accretion of material origi- wind. The accretion of wind material turns in systems undergoing Roche-lobe over- nating from the OB star, transported by the compact object into an observable flow due to the higher mass- and angular- the OB-star wind or by Roche-lobe over- X-ray source. Once the supergiant starts momentum accretion rate. The latter sys- flow. About a dozen HMXBs are known to overflow its Roche lobe, the mass tems also have circular orbits, while to host a massive OB-supergiant com- transfer rate increases further so that an the wind-fed systems have eccentricities panion (about 10 to over 40 MA), in a rela- accretion disc is formed around the com- up to e = 0.45 (GX301-, Kaper et al. 35 tively tight orbit (Porb several days) with pact object, turning it into a very strong 006) and an X-ray luminosity LX ~ 10 – an X-ray pulsar or black-hole companion X-ray source accreting at the Eddington 1036 erg s–1. (Table 1). Some of these X-ray binaries limit. Soon after, the increasing mass include a dense accretion disc and pro- transfer rate causes the system to enter duce relativistic jets. Recently, several a phase of common-envelope evolution Neutron stars versus black holes new sources have been discovered with swamping the X-ray source and finally the ESA gamma- and hard-X-ray observ- causing the compact object to spiral into In Figure the mass distribution of neu- atory INTEGRAL that show the character- the OB-supergiant. tron stars and black holes is shown, istics of a HMXB with an OB-supergiant based on measurements collected from companion hidden by large amounts From this stage on the evolution of the the literature (Stairs 004, McClintock of interstellar dust (e.g.
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