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Register a new userThe masses of PSR J1911-5958A and its white dwarf companion
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Authors
C.G. Bassa
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We present spectroscopic and photometric observations of the optical counterpart to PSR J1911-5958A, a millisecond pulsar located towards the globular cluster NGC 6752. We measure radial velocities from the spectra and determine the systemic radial velocity of the binary and the radial-velocity amplitude of the white-dwarf orbit. Combined with the pulsar orbit obtained from radio timing, we infer a mass ratio of Mpsr/Mwd=7.36+-0.25. The spectrum of the counterpart is that of a hydrogen atmosphere, showing Balmer absorption lines upto H12, and we identify the counterpart as a helium-core white dwarf of spectral type DA5. Comparison of the spectra with hydrogen atmosphere models yield a temperature Teff=10090+-150 K and a surface gravity log g=6.44+-0.20 cm s^-2. Using mass-radius relations appropriate for low-mass helium-core white dwarfs, we infer the white-dwarf mass Mwd=0.18+-0.02 Msun and radius Rwd=0.043+-0.009 Rsun. Combined with the mass ratio, this constrains the pulsar mass to Mpsr=1.40^+0.16_-0.10 Msun. If we instead use the white-dwarf spectrum and the distance of NGC 6752 to determine the white-dwarf radius, we find Rwd=0.058+-0.004 Rsun. For the observed temperature, the mass-radius relations predict a white-dwarf mass of Mwd=0.175+-0.010 Msun, constraining the pulsar mass to Mpsr=1.34+-0.08 Msun. We find that the white-dwarf radius determined from the spectrum and the systemic radial velocity of the binary are only marginally consistent with the values that are expected if PSR J1911-5958A is associated with NGC 6752. We discuss possible causes to explain this inconsistency, but conclude that our observations do not conclusively confirm nor disprove the assocation of the pulsar binary with the globular cluster.
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We have used phase-resolved high-resolution images and low resolution spectra taken at the ESO Very Large Telescope, to study the properties of the low-mass Helium White Dwarf companion to the millisecond pulsar psr (hereafter COM J1911$-$5958A), in the halo of the Galactic Globular Cluster NGC 6752. The radial velocity curve confirms that com is orbiting the pulsar and allows to derive a systemic velocity of the binary system nicely in agreement with that of NGC 6752. This strongly indicates that the system is a member of the cluster, despite its very offset position ($sim 74$ core radii) with respect to the core. Constraints on the orbital inclination ($gapp 70^circ$) and pulsar mass ($1.2-1.5 {rm M_odot}$) are derived from the mass ratio $M_{PSR}/M_{COM}= 7.49pm0.64$ and photometric properties of COM J1911$-$5958A. The light curve in B-band shows two phases of unequal brightening ($Delta$mag$sim 0.3$ and 0.2, respectively) located close to quadratures and superimposed on an almost steady baseline emission: this feature is quite surprising and needs to be further investigated.
We report on the identification of the optical counterpart of the binary millisecond pulsar PSR J1911-5958A, located in the outskirts of the globular cluster NGC 6752. At the position of the pulsar we find an object with V=22.08, B-V=0.38, U-B=-0.49. The object is blue with respect to the cluster main sequence by 0.8 magnitudes in B-V. We argue that the object is the white dwarf companion of the pulsar. Comparison with white dwarf cooling models shows that this magnitude and colors are consistent with a low-mass white dwarf at the distance of NGC 6752. If associated with NGC 6752, the white dwarf is relatively young, <2 Gyr, which sets constraints on the formation of the binary and its ejection from the core of the globular cluster.
We report on Keck optical BVRI images and spectroscopy of the companion of the binary millisecond pulsar PSR J0218+4232. A faint bluish (V=24.2, B-V=0.25) counterpart is observed at the pulsar location. Spectra of this counterpart reveal Balmer lines which confirm that the companion is a Helium-core white dwarf. We find that the white dwarf has a temperature of Teff=8060+-150 K. Unfortunately, the spectra are of insufficient quality to put a strong constraint on the surface gravity, although the best fit is for low log g and hence low mass (~0.2 Msun), as expected. We compare predicted white dwarf cooling ages with the characteristic age of the pulsar and find similar values for white dwarf masses of 0.19 to 0.3 Msun. These masses would imply a distance of 2.5 to 4 kpc to the system. The spectroscopic observations also enable us to estimate the mass ratio between the white dwarf and the pulsar. We find q=7.5+-2.4, which is consistent with the current knowledge of white dwarf companions to millisecond pulsars.
Splaver and coworkers have measured the masses of the white dwarf and the neutron star components of the PSR J1713+0747 binary system pair by Shapiro Delay. We attempt to find the original configuration of this system performing a set of binary evolution calculations to simultaneously account for the masses of both stars and the orbital period. We considered initial masses of 1.5 and 1.4 msun for the normal (donor) and the neutron star, respectively. We assumed two metallicity values (Z = 0.010 and 0.020), and an initial orbital period near 3 days. We assume that the neutron star is only able to retain lesssim 0.10 of the matter transferred by the donor star. Calculations were performed employing our binary hydro code that handles the mass transfer rate in a fully implicit way together with state-of-the-art physical ingredients, diffusion and a non-grey atmospheres. We compare the structure of the resulting white dwarfs with the characteristic age of PSR J1713+0747 finding a nice agreement with observations by Lundgren et al. especially for the case of a donor star with Z= 0.010. This result indicates that the evolution of this kind of binary system is well understood. The models predict that, due to diffusion, the atmosphere of the white dwarf is an almost hydrogen-pure one. We find that such structures are unable to account for the colours measured by Lundgren et al. within their error bars. Thus, some discrepances in the white dwarf emergent radiation remain to be explained.
Binaries harbouring millisecond pulsars enable a unique path to determine neutron star masses: radio pulsations reveal the motion of the neutron star, while that of the companion can be characterised through studies in the optical range. PSR J1012+5307 is a millisecond pulsar in a 14.5-h orbit with a helium-core white dwarf companion. In this work we present the analysis of an optical spectroscopic campaign, where the companion star absorption features reveal one of the lightest known white dwarfs. We determine a white dwarf radial velocity semi-amplitude of K_2 = 218.9 +- 2.2 km/s, which combined with that of the pulsar derived from the precise radio timing, yields a mass ratio of q=10.44+- 0.11. We also attempt to infer the white dwarf mass from observational constraints using new binary evolution models for extremely low-mass white dwarfs, but find that they cannot reproduce all observed parameters simultaneously. In particular, we cannot reconcile the radius predicted from binary evolution with the measurement from the photometric analysis (R_WD=0.047+-0.003 Rsun). Our limited understanding of extremely low-mass white dwarf evolution, which results from binary interaction, therefore comes as the main factor limiting the precision with which we can measure the mass of the white dwarf in this system. Our conservative white dwarf mass estimate of M_WD = 0.165 +- 0.015 Msun, along with the mass ratio enables us to infer a pulsar mass of M_NS = 1.72 +- 0.16 Msun. This value is clearly above the canonical 1.4 Msun, therefore adding PSR J1012+5307 to the growing list of massive millisecond pulsars.
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