The maximum mass of a neutron star (NS) is poorly defined. Theoretical
attempts to define this mass have thus far been unsuccessful. Observational
results currently provide the only means of narrowing this mass range down.
Eclipsing X-ray binary (XRB) pulsar systems are the only interacting binaries
in which the mass of the NS may be measured directly. Only 10 such systems are
known to exist, 6 of which have yielded NS masses in the range 1.06 - 1.86
M$_{\odot}$.We present the first orbital solutions of two further eclipsing
systems, OAO 1657-415 and EXO 1722-363, whose donor stars have only recently
been identified. Using observations obtained using the VLT/ISAAC NIR
spectrograph, our initial work was concerned with providing an accurate
spectral classification of the two counterpart stars, leading to a consistent
explanation of the mechanism for spin period evolution of OAO 1657-415.
Calculating radial velocities allowed orbital solutions for both systems to be
computed. These are the first accurate determinations of the NS and counterpart
masses in XRB pulsar systems to be made employing NIR spectroscopy.Comment: 5 pages, 3 figures, contribution to the proceedings of "The
  multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
  Coll., 12-16 July 201

Clark, J. S.

Mason, A. B.

Negueruela, I.

Norton, A. J.

Roche, P.

English

arXiv

Open Research Online (Open)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsHigh mass X-ray binaries in the NIRorbital solutions oftwo highly obscured systemsJournal ItemHow to cite:Mason, A. B.; Norton, A. J.; Clark, J. S.; Roche, P. and Negueruela, I. (2011). High mass X-ray binaries inthe NIRorbital solutions of two highly obscured systems. Bulletin de la Societe Royales des Sciences de Lie`ge, 80, pp.699–703.For guidance on citations see FAQs.c© Not knownVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.srsl-ulg.net/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukHigh Mass X-ray Binaries in the NIR: Orbitalsolutions of two highly obscured systems. ∗A.B. Mason1†,A.J. Norton1,J.S. Clark1,P. Roche1,I. Negueruela21 The Open University, Milton Keynes, UK2 Universidad de Alicante, Alicante, SpainAbstract: The maximum mass of a neutron star (NS) is poorly defined. Theoretical attempts to define thismass have thus far been unsuccessful. Observational results currently provide the only means of narrowing thismass range down. Eclipsing X-ray binary (XRB) pulsar systems are the only interacting binaries in which themass of the NS may be measured directly. Only 10 such systems are known to exist, 6 of which have yieldedNS masses in the range 1.06 - 1.86 M.We present the first orbital solutions of two further eclipsing systems,OAO 1657-415 and EXO 1722-363, whose donor stars have only recently been identified. Using observationsobtained using the VLT/ISAAC NIR spectrograph, our initial work was concerned with providing an accuratespectral classification of the two counterpart stars, leading to a consistent explanation of the mechanism for spinperiod evolution of OAO 1657-415. Calculating radial velocities allowed orbital solutions for both systems tobe computed. These are the first accurate determinations of the NS and counterpart masses in XRB pulsarsystems to be made employing NIR spectroscopy.1 IntroductionDespite extensive and ongoing theoretical work on the NS equation of state (EOS), the precise natureof the fundamental physical properties of NS matter is still poorly defined. Observational work canassist in reducing the number of contending theories by eliminating those that place unrealistic con-straints on the mass range of observed NS. NS masses may only be determined from binary systems,within this paper we consider a specific class of these objects, those containing an eclipsing X-raybinary pulsar. At present only 10 such systems are known within our Galaxy. Prior to this work6 of the NS in these systems have had mass determinations, with the donor star in each observableoptically. Within this paper we discuss the first mass estimates found for NS employing near-infrared(NIR) spectroscopy. We have studied two eclipsing X-ray pulsar systems containing a High Massdonor, OAO 1657-415 and EXO 1722-363. Initially an accurate spectral classification for each of thedonor stars was conducted utilising observations made using the VLT/ISAAC NIR spectrograph andcurrent NIR spectral atlases. Using multi-epoch NIR spectra of each system we were able to deter-mine the radial velocity of the donor star in each of the two systems thus enabling the construction ofan orbital solution. This solution was then employed in calculating the mass estimate of each NS and∗Based on observations carried out at the European Southern Observatory under programmes 081.D-0073(A), 077.B-0872(A) and 081.D-0073(B).†a.mason@open.ac.uk“The multi-wavelength view of hot, massive stars”; 39th Lie`ge Int. Astroph. Coll., 12-16 July 2010arXiv:1010.6001v1  [astro-ph.HE]  28 Oct 2010the corresponding high mass donor, placing constraints upon the system inclination and separation ofthe binary system.2 Spectral classification2.1 Spectral classification of EXO 1722-363EXO 1722-363 (alternatively designated IGR J17252-3616) was discovered in 1984 by EXOSATGalactic plane observations (Warwick et al. 1988). XMM-Newton observations narrowed downthe source position location to 4′′. This allowed the identification of an IR counterpart 2MASSJ17251139-3616575 (with magnitude J = 14.2, H = 11.8 and Ks = 10.7 (Zurita Heras et al. 2006)).Examining Fig.1 we can see that all of absorption lines in this spectrum are narrow, indicative of theobject being a supergiant. EXO 1722-363 shows the singlet He I 2.058 µm line in emission, this linebeing highly sensitive to wind and temperature properties. The N III 2.115 µm emission line is a com-mon feature in B0-B1 supergiants. The absence of strong Brγ 2.1655 µm emission features impliesthat EXO 1722-363 does not exhibit a strong stellar wind. From a qualitative comparison of spectrafrom Hanson et al. 2005, we identify EXO 1722-363 as being of spectral type B0-B1 Ia (Masonet al. 2009). By comparison with evolutionary rotational massive star models (Meynet & Maeder,2000) we find an initial progenitor mass for EXO 1722-363 in the range 30M - 40M. Followingthe method for determining spectroscopic distance as detailed in Bibby et al. 2008, we determined adistance for EXO 1722-363 of 8.0+2.5−2.0 kpc which is comparable within errors to the distance deducedin Thompson et al, 2007. Comparing our calculated distance with model fluxes derived from spectralfits to EXO 1722-363 (Corbet et al, 2005), we found that EXO 1722-363 has an intrinsic X-ray fluxvariability (in the range 2-60 keV) such that Fmin = 0.78 × 10−10 erg cm−2 s−1 and Fmax = 12.2 ×10−10 erg cm−2 s−1. We derive X-ray luminosities for EXO 1722-363 such that LXmin = 3.4 × 1035erg s−1 and LXmax = 1.6 × 1037 erg s−1. We find this luminosity range entirely consistent with EXO1722-363 being the donor within an SGXRB system.2.2 Spectral classification of OAO 1657-415OAO 1657-415 was first detected over 30 years ago by the Copernicus X-ray satellite. From anexamination of the orbital parameters of the X-ray pulsar it was determined that OAO 1657-415 wasa high-mass system, indicating the mass of the donor lay between 14-18 M with a correspondingradius range of 25-32R. Determination of these stellar parameters led to a suggested classificationof B0-6 I (Chakrabarty et al, 1993).The correct identification of the donor in this system requiredthe precise location of OAO 1657-415 to be accurately made. This was achieved by the ChandraX-Ray Observatory narrowing the X-ray location error radius down to 0.5′′. Optical imaging of thisposition did not detect any donor candidates down to a magnitude of V>23. Near infrared imagingwas employed to overcome significant levels of interstellar reddening, resulting in the identificationof a donor located within the Chandra error radius. A corresponding IR counterpart was located inthe 2MASS catalogue, 2MASS J17004888-4139214 (with magnitudes J = 14.1, H = 11.7 and Ks =10.4) with AV = 20.4 ± 1.3, located at a distance of 6.4 ± 1.5 kpc (Chakrabarty et al, 2002). NIRKs band spectroscopy of the donor obtained in 2008 (Mason et al, 2009) led to a re-evaluation ofthe spectral classification. Close examination revealed that OAO 1657-415 shared a similar spectralmorphology with that of Ofpe/WNL stars. These are stars in transition between the OB main sequenceand hydrogen depleted Wolf-Rayet stars, whose evolution follows from a wide range of progenitormasses. The spectrum of the mass donor in OAO1657-415 is presented in Fig. 1, and is dominated byHe I 2.058 µm and Brγ emission, the former stronger than the latter. We find a poor correspondenceFigure 1: Left Figure : Comparison of EXO 1722-363 and template O9-B3 Ia spectra from Hansonet al., 2005. Right Figure: Topmost shows a spectrum of OAO1657-415 compared with a Ofpe/WNLspectrum of IRS16NW from Martins et al., 2007.with the spectra of B0-6 supergiants (Hanson et al, 1996, Hanson et al, 2005) - as suggested forthe mass donor by Chakrabarty et al, 2002 on the basis of a combination of photometric and X-raydata. However, comparison with the spectra of massive transitional objects presented by Morris etal, 1996 is more encouraging. In particular OAO 1657-415 shows pronounced similarities to the hotOfpe/WNL stars. Consequently we may not a priori determine a unique distance to OAO 1657-415on the basis of this classification. We thus find inevitably unconstructive limits of 4.4 kpc < d <12 kpc. In turn this results in 1.5 × 1036 erg−1 < LX < 1037 erg s−1, also entirely consistentwith observed luminosities of SGXRBs. Adopting the distance derived by Audley et al, 2006 leadsto log(L/L) ∼ 5.7. For such a luminosity, comparison to the evolutionary tracks for massive stars(Meynet & Maeder, 2000) imply an initial mass of ∼40 M.3 OAO 1657-415 : A mechanism for spin-period evolutionWe now turn to the implications of the Ofpe/WNL classification for the X-ray properties of OAO1657-415. The anomalous position of OAO 1657-415 within the Corbet diagram (Figure 2) (Corbetet al, 1986), is then naturally explained in terms of the properties of its stellar wind. Compared tonormal OB supergiants (Crowther et al, 2006), Ofpe/WNL stars typically demonstrate systematicallylower wind velocities and higher mass loss rates (Martins et al. 2007). This combination of windproperties permits a higher accretion rate and hence transfer of angular momentum to the NS, in turnleading to a smaller (instantaneous) equilibrium spin period with respect to normal OB supergiants(Pspin ∝ M˙−3/7v12/7∞ from Eqn. 12 of Waters et al, 1989, where Pspin, M˙ and v∞ are the spin periodof the NS and the mass loss rate and terminal velocity of the mass donor wind respectively).Figure 2: Corbet diagram marking position of OAO 1657-415 and other HMXBs. OAO 1657-415is marked by an X, EXO 1722-363 by a filled diamond. SGXRB Roche-Lobe Overflow systems(Squares), Be/X binaries (Triangles), SGXRB Wind-fed systems (Diamonds) and anomalous systems(+)4 Orbital solution for EXO 1722-363The orbital solution we have calculated was obtained from archival ESO VLT data. Using a smallsubset of the available archive data, (11 spectra taken at different epochs spanning a wide range oforbital phase, from a set of 104 in total) we were able to measure radial velocities and construct theorbital solution shown in Fig. 3 (left). These spectra were centered on 2.1µm, having an integrationtime of 700s, and were obtained using the SW MRes mode with a 0.6′′ slit. This resulted in high S/Nspectra at a resolution R ≈ 4200. The resulting NS mass that we have determined from our orbitalsolution for EXO 1722-363 is consistent with the canonical mass of 1.4 M measured in most othereclipsing HMXBs, except for that in Vela X-1, (Quaintrell et al., 2003). The NS mass range we havedetermined stems from a lower and upper limit obtained using the following constraints - Lower : thesystem is viewed edge on (i.e. i = 90◦), Upper : the donor star fills its Roche lobe. Utilising this orbitalsolution we find a NS mass range of 1.5 - 1.6 M (Mason et al. 2010). In a similar way the measuredmass and radius of the supergiant donor, M ∼ 13− 15 M and R ∼ 25− 28 R is determined, andthis lends support to the B0-1 Ia spectral classification that we previously found (Mason et al., 2009).5 Orbital solution for OAO 1657-415As the mass donor in OAO 1657-415 is faint (H ∼ 11.7) we employed the NIR spectrometer ISAACon the VLT to obtain high resolution (R ∼ 3000) and S/N spectra in the H band. Observations wereconducted between 2008 May 13th and 2008 September 25th in the SW MRes mode with a 0.8′′ slit.Cross-correlation was performed using the standard IRAF routine fxcor. 12 high quality spectra wereobtained that covered a wide range of orbital phase, sufficient to determine a dynamical mass solutionfor OAO 1657-415 (Fig. 3). Utilising this orbital solution we find a NS mass range of ≈ 1.4 - 1.7 Mwith a corresponding mass range for the counterpart star of ≈ 14 - 17 M. For a more precise massdetermination please refer to Mason et al, 2011.Figure 3: Left : Radial velocity data for the donor in EXO 1722–363. Right : Radial velocity datafor the donor in OAO 1657-415. In both cases the solid line is the best fitting sinusoid with three freeparameters, the dashed line is that with a fixed zero phase in line with the published ephemeris. In thecase of EXO 1722–363 the orbital phase is based upon the ephemeris of Thompson et al, 2007. ForOAO 1657-415 the orbital phase is based upon the ephemeris of Bildsten et al, 1997.AcknowledgementsABM acknowledges support from an STFC studentship. JSC acknowledges support from an RCUKfellowship. This research is partially supported by grants AYA2008-06166-C03-03 and Consolider-GTC CSD-2006-00070 from the Spanish Ministerio de Ciencia e Innovacio´n (MICINN).Based onobservations carried out at the European Southern Observatory, Chile through programmes 081.D-0073(A and B) and 077.B-0872(A) .ReferencesAudley, M. 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High mass X-ray binaries in the NIRorbital solutions of two highly obscured systems

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High mass X-ray binaries in the NIRorbital solutions of two highly obscured systems

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