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Home , Be star, Binary pulsar, Minimum mass, Neutron star, OB star, Pulsar

Reports from Observers

Measuring the of

Lex Kaper1 Arjen van der Meer1 Marten van Kerkwijk2 Ed van den Heuvel1 Visualisation: B. Pounds 1 Astronomical Institute “Anton Panne- koek” and Centre for High , University of Amsterdam, the Netherlands 2 Department of and Astro- , 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 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 . As bosons do distribution provides important informa- not contribute to the fermi pressure1, their binary hosting a massive OB-type and a compact X-ray source, a 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 at mass will be low (~ 1.5 MA); for a high- 1807 with a mass of 2.1 ± 0.2 MA (Nice et supranuclear . er mass, the object would collapse into al. 2005). These results favour a stiff EOS. a . 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 3.2 MA, a limit set by general re- ing one massive neutron star (M ≥ 2 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 2.5 MA. Neutron stars also 10 the density of an atomic nucle- is thus important for our understand- have a 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 ) 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 , as X-ray The most accurate masses have been . 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 2004). 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 (EOS), (Thorsett and Chakrabarty 1999). An ex- full available mass range depends on the i.e. the relation between ception is the X-ray X-1 with formation mechanism, i.e. the . and density in the neutron star interior a mass of 1.86 ± 0.16 MA (Barziv et al. (Lattimer and Prakash 2004). So far, 2001). This important result was obtained We focus here on the initially most mas- the physical properties of matter under following a nine- spectroscopic sive binary systems2, 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 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 (fermions) exert 2 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- . from the equatorial disc around a .

The Messenger 126 – December 2006 27 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 () supernova a bound of about 1.4 MA. However, most spectro- Due to a phase of , 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 determinations were carried out more the system before the primary supernova, PSR 1913+16 (or a neutron star – than 20 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 less than about a year 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 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 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, 28). 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 required for the secondary to evolve in the and the Magellanic of massive binaries. They are composed into a supergiant when the 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. 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 high (LX ~ 10 erg s ) powered by 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 accretion rate. The latter sys- flow. About a dozen HMXBs are known to overflow its , 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 (GX301-2, Kaper et al. 35 tively tight (Porb several days) with pact object, turning it into a very strong 2006) 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 2 the 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 2004, McClintock of interstellar (e.g. Negueruela et al. system can proceed in different ways, and Remillard 2005). The neutron stars 2005); these are not included in the table. depending on the orbital separation. In occupy a relatively narrow mass range

the relatively wide Be/X-ray binaries the near 1.4 MA. The most accurate neutron-

Name Spectral Type MOB (MA) MX (MA) Porb (d) Ppulse (d) 2S0114+650 B1ia 16 ± 5 1.7 ± 0.5 11.6 860 SMC X-1 B0 ib 15.7 ± 1.5 1.06 ± 0.11 3.89 0.71 LMC X-4 O7 iii–iv 14.5 ± 1.0 1.25 ± 0.11 1.40 13.5 Vela X-1 B0.5 ib 23.8 ± 2.4 1.86 ± 0.16 8.96 283 Cen X-3 O6.5 ii–iii 20.2 ± 1.8 1.34 ± 0.16 2.09 4.84 GX301-2 B1.5 ia+ 40 ± 10 1.9 ± 0.6 41.5 696 Table 1: High-mass X-ray binaries with OB-super- 4U1538-52 B0 iab 16 ± 5 1.1 ± 0.4 3.73 529 giant companion in the Milky Way and the Magellan- ic Clouds (ordered according to ). 4U1700-37 O6.5 iaf+ 58 ± 11 2.4 ± 0.3 3.41 The name corresponds to the X-ray source, the 4U1907+09 early B i ~ 27 ~ 1.4 8.38 438 spectral type to the OB-supergiant. For the systems LMC X-3 B3 ve ~ 6 6–9 1.70 hosting an X-ray pulsar the masses of both binary LMC X-1 O7-9 iii ~ 20 4–10 4.22 components can be measured (given an estimate of the inclination of the system). The last five systems LS5039 O6.5 v((f)) 20–35 1.4 ± 0.4 4.43 most likely contain a black-hole candidate; for the SS433 A3-7 i 11 ± 3 2.9 ± 0.7 13.08 Galactic sources among them relativistic jets have Cyg X-1 O9.7 iab 18 ± 6 10 ± 5 5.60 been detected.

28 The Messenger 126 – December 2006 Figure 2: Neutron-star and black-hole masses show a wider spread, including two systems with a

obtained from the literature (Stairs 2004, McClintock neutron-star mass near 2 MA. Such a high neutron- and Remillard 2005, and references therein). The star mass would rule out a soft equation of state. neutron stars, especially the binary radio pulsars (at The black-hole candidates (at the top) are significant- the bottom), occupy a relatively narrow mass range ly more massive, indicative of a different formation

near 1.35 MA. The X-ray pulsars (to the middle) mechanism. e.g., The Messenger 109, 37, and this is- 0422+32 sue, page 16) the hypothesis would be 0620−003 1009−45 that neutron stars are formed in ‘ordinary’ 1118+480 supernovae, while black holes originate in 1124−684 1543−475 gamma-ray bursts. 1550−564 1655−40 BH binaries 1705−250 1819.3−2525 VLT/UVES observations 1859+226 1915+350 2000+251 If the HMXB hosts an X-ray pulsar, its 2023+338 orbit can be very accurately determined LMC_X-3 through pulse-timing analysis. When LMC_X-1 Cyg_X-1 the radial-velocity curve of the OB-super- SMC X-1 giant is also obtained, the mass of the LMC X-4 neutron star and that of the massive star Cen X-3 4U 1700−37 can be derived with precision, given an Vela X-1 NS X-ray binaries estimate of the system inclination. In sys- 4U 1538−52 Her X-1 tems showing an X-ray eclipse the in- Cyg X-2 clination must be larger than i ~ 65˚. For XTE J2123−058 Roche-lobe overflow systems a valid 2A 1822−371 1518+49 assumption is that the OB-supergiant is 1518+49 in corotation with the orbit, which pro- 1534+12 1534+12 vides a strong constraint on the system 1913+16 NS-NS binaries inclination. 1913+16 2127+11C 2127+11C Van der Meer et al. (2006) have analysed J0737−3039A the radial-velocity curves of the three J0737−3039B known OB-supergiant systems with an B2303+46 J1012+5307 (eclipsing) X-ray pulsar undergoing J1713+0747 Roche-lobe overflow. These systems are B1802−07 B1855+09 Cen X-3, the first detected binary X-ray J0621+1002 NS-WD binaries pulsar (Giacconi et al. 1971) in the Milky J0751+1807 Way; LMC X-4 in the Large Magellanic J0437−4715 J1141−6545 Cloud; and SMC X-1 in the Small Magel- J1045−4509 lanic Cloud. The OB-supergiant coun- J1804−2718 J2019+2425 terparts to these X-ray pulsars have V magnitudes in the range 13–14. Other 0 2 4 6 8 10 12 14 properties of these systems are listed Mass (M ) � in Table 1. High spectral resolution (R ~ 40 000) and high signal-to-noise spectra star masses have been derived for the ways. If, for example, black holes are the of these systems have been obtained binary radio pulsars (NS-NS binaries), result of ‘failed’ supernovae in which with VLT/UVES, covering 12 epochs

with an average mass of 1.35 ± 0.04 MA the stellar mantle is not blown away, but evenly spread over the (circular) orbit of (Thorsett and Chakrabarty 1999). The accretes onto the compact remnant, the system. X-ray pulsars (NS-X-ray binaries) show a one would expect a significant difference somewhat larger mass range, extending in mass between neutron stars and black To obtain a radial-velocity measurement,

both below and above 1.35 MA. Besides holes. However, if the mass of the (proto) often the complete spectra are cross Vela X-1, also a high mass is claimed neutron star is increased by the fall back correlated with a template . This

for the system 4U1700-37 (2.4 ± 0.3 MA, of material which was located outside approach has many advantages when Clark et al. 2002), although the X-ray the collapsing degenerate Fe core of the using spectra with relatively low spectral source is, contrary to Vela X-1, not an exploding star, one would predict that resolution and poor signal to noise. In X-ray pulsar (and perhaps a low-mass neutron stars would occupy a range our case the spectra are of such high black hole). in mass, up to the maximum neutron-star quality (Figure 3) that the radial-velocity mass allowed by the equation of state. amplitude can be determined for each The estimated masses of black-hole can- Certainly in the binary radio pulsars such line separately (Figure 4). The advantage didates are substantially larger (8.4 ± a mass distribution is not observed. of such a strategy is that it is possible

2.0 MA) than those measured for neutron With the recent evidence that a black hole to assess the influence of possible distor- stars. This suggests that neutron stars may be formed during the collapse of tions due to e.g. X-ray heating and grav- and black holes are formed in different a massive star in a gamma-ray burst (see, ity darkening, as in these systems the

The Messenger 126 – December 2006 29 Reports from Observers Kaper L. et al., Measuring the Masses of Neutron Stars

Figure 3: VLT/UVES spectrum of the B-super- giant companion to the X-ray pulsar SMC X-1. The hydrogen and lines characteristic of early- type star spectra are indicated. The spectral lines move back and forth due to the orbital of the star (from Van der Meer et al. 2006).

220 1 ux fl 0.8 210 5 5 7 5 2 7 37 94 .8 .1 .2 .9 .0 .2

0.6 4. 1. 97 03 11 87 05 Normalised

36 200 35 3634 372 37

: : : 37 I I I 4: 3: 373 7: 0.4 6: 37 5: ) He He H1 H1 H1 H1 He H1 −1

3500 3550 3600 3 650 3700 3750 s 190 (Å) m (k

0 180

1 K ux fl 0.8 170 6 7 .6 .4 0 3 5 4 2 8 5 7 .1 .6 .9 .6 .3 .0

0.6 .0 3933 3968 50 70 : : 26.5 19 II II 70

Normalised 160 3797 38 39 37 37 3835 3889 39 Ca Ca : : I I : : 0: 2: : 0.4 1: i.s. i.s. He H9 H1 H1 He H1 H8 H7

3750 3800 3850 3900 3950 4000 150 Wavelength (Å) 5

1 s 0 re ux fl

d 0.8 −5 6 3 6 1 4 0 3 76 .8 .2 .2 0.6 .1 .7 3. 99 09 26 16 20.8 01 Normalise 0 0.5 1 1.5 41 4088.8 41 41 40 414 40 41 : : : : : : : I I I II I : IV 0.4 IV δ Si Si He He He He He H ∅

4000 4050 4100 4150 4200 4250 Wavelength (Å) Figure 4: Radial-velocity curve of the B-supergiant companion to SMC X-1 based on the measurement of a single line (hydrogen Balmer 2–10). The result- 1 ing radial-velocity amplitude of the B supergiant is ux

fl –1

d 0.8 20.2 ± 1.2 km s . Combined with the mass function 0

3 of the X-ray pulsar and the system inclination of 6 .5 .9

0.6 .4 87 71 67 ± 5 degrees a neutron star mass of 1.06 ± 0.1 MA Normalise 44 43 4340 : : I I 0.4 : is obtained. This makes SMC X-1 the lowest mass γ H He He neutron star known (from Van der Meer et al. 2006). 4250 4300 4350 4400 4 450 4500 Wavelength (Å)

OB-star is irradiated by a powerful X-ray (M ≤ 14 MA). A reconstruction of the evo- near 1.3 MA are produced by progenitors source and is filling its Roche lobe. For lutionary history of this binary system in the range 14 to 19 MA. The most mas- more details on the applied methods we shows that this is possible (Van der Meer sive neutron stars, like Vela X-1, come refer to Van der Meer et al. (2006). et al. 2006). Initially, the system would from stars with an initial mass higher than

have contained a 13 MA and a 9 MA star. 19 MA. The apparent lack of low-mass The resulting masses are 1.06 ± 0.11 MA The nuclear burning rate of the 13 MA black holes (M ~ 3 MA) suggests that for SMC X-1, 1.25 ± 0.10 MA for LMC X-4, star is fastest, so that it will have reached black holes are formed by a different and 1.34 ± 0.16 MA for Cen X-3 (at a the end of nuclear hydrogen burning mechanism than neutron stars, e.g. a 1s confidence level), i.e. an improvement first. When it starts to become a super- gamma-ray burst rather than a supernova. in accuracy by at least a factor of two. giant, it starts overflowing its Roche lobe,

Whereas some HMXBs have shown so that the companion 9 MA star will Acknowledgement to host a neutron star with a mass higher receive mass and becomes a 16 MA star than 1.4 M , as is the case for Vela X-1 (i.e. the current mass of the B super- A This research has been supported by the Dutch and possibly 4U1700-37, the mass of giant companion to SMC X-1, neglecting Research School in Astronomy (NOVA). SMC X-1 is low, just above the minimum the mass that the supergiant has lost neutron-star mass of ~ 1 MA, and signifi- due to its ). The current short cantly different from the mass of Vela X-1. of the system (3.89 d) in- References We thus conclude that the neutron stars dicates that during this process of mass Barziv O. et al. 2001, A&A 377, 925 in HMXBs have different masses, i.e. they transfer the system must have lost an- Blaauw A. 1961, Bull. Astr. Inst. Neth. 15, 265 do not all have the same canonical mass. gular momentum (otherwise the orbit Clark J. S. et al. 2002, A&A 392, 909 would have become much wider), but not Giacconi R. et al. 1971, ApJ 167, L67 Kaper L., Van der Meer A. and Najarro P. 2006, The three studied systems do not host a much mass. A&A 457, 595 massive neutron star. Following the Lattimer J. M. and Prakash M. 2004, 304, hypothesis that the mass of the compact Our current hypothesis is that the lowest- 536 remnant depends on the mass of the mass neutron star (e.g. SMC X-1) is McClintock J. E. and Remillard R. E. 2006, in “Compact Stellar X-Ray Sources”, progenitor star, one would conclude that the end product of a massive star in the Cambridge University Press, astro-ph/0306213 the progenitors of the neutron stars in range of about 8 to 14 MA (stars with a Nice D. et al. 2005, ApJ 634, 1242 Stairs I. H. 2004, Science 304, 547 these systems had a relatively low mass, mass less than 8 MA do not explode as a especially the progenitor of SMC X-1 supernova). Neutron stars with a mass Thorsett S. E. and Chakrabarty D. 1999, ApJ 512, 288 Van der Meer A. et al. 2006, A&A, submitted

30 The Messenger 126 – December 2006 The turbulent region around the ring- shaped DEM L 299 in the . Colour com- posite based on images obtained with the Wide-Field-Imager (WFI) at the ESO/MPG 2.2-m telescope at the La Silla . (From ESO Press Photo 34a/04)