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Title:
The eclipsing millisecond pulsar PSR J1740-5340 and its red straggler companion
Authors:
Orosz, J. A.; van Kerkwijk, M. H.
Affiliation:
AA(Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands), AB(Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands)
Publication:
Astronomy and Astrophysics, v.397, p.237-247 (2003) (A&A Homepage)
Publication Date:
01/2003
Origin:
A&A
A&A Keywords:
binaries: close, pulsars: individual: PSR J1740-5340, globular clusters: individual: NGC 6397
DOI:
10.1051/0004-6361:20021468
Bibliographic Code:
2003A&A...397..237O

Abstract

We present a high-resolution echelle spectrum taken with the Very Large Telescope and analyse archival Hubble Space Telescope photometry of the recently identified companion of the eclipsing millisecond radio pulsar PSR J1740-5340 in the globular cluster NGC 6397. From the spectrum, we show that the companion is metal poor, as expected for a member of NGC 6397. Using synthetic photometry, and assuming a true distance modulus of 12.13+/-0.15 mag and a colour excess of EB-V=0.179, we derive a radius of 1.60+/-0.17 Rsun and an effective temperature of 5410+/-50K, implying a luminosity of 2.0+/-0.4 Lsun (1sigma errors). These properties make it similar to the so-called ``sub-subgiants'' in the old open cluster M 67 and ``red stragglers'' in 47 Tuc, which have luminosities comparable to those of turn-off stars, but cooler temperatures and larger radii. The light curve of the companion is well described by ellipsoidal variations, and despite the incomplete ( ~ 60%) phase coverage, we are able to derive good constraints on a number of the system parameters. In particular, for the inclination we find a 2-sigma lower limit of i>48deg (i>46o at 3sigma ). Assuming a pulsar mass of 1.2<MMSP<2.4 Msun, this implies a companion mass in the range 0.14<Mcomp<0.38 Msun. Combined with the photometric constraint, we find a best fit for i =~ 50o and Mcomp =~ 0.3 Msun. We infer a Roche lobe filling factor by radius of ~ 97%. Surprisingly, we find no evidence whatsoever for irradiation of the companion, despite the high inferred rotational energy loss of the pulsar (dot {E} =~ 1.4x 1035 erg s-1). We discuss possible reasons, but find most lacking. We hypothesise that the system is a triple, and that the acceleration due to a third body in a wide orbit around the binary led to an overestimate of the intrinsic spin-down rate and hence the spin-down luminosity. This can be tested by further timing observations. We also discuss two other puzzles, viz., the system's location far outside the cluster core and the companion's large radius and luminosity. We suggest that the system was formed in a binary-binary encounter in the core, due to which the system acquired a substantial velocity, and the companion - which must have been a somewhat evolved turn-off star - lost much of its envelope. We suggest other ``red stragglers'' and ``sub-subgiants'' might have formed by similarly drastic encounters. Based on observations made with ESO Telescopes at the Paranal Observatories under programme 267.D-5716 and observations made with the NASA/ESA Hubble Space Telescope, obtained from the ST-ECF data archive.
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