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Post Info TOPIC: SWIFT J1749.4-2807
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RE: SWIFT J1749.4-2807
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Title: Chandra X-ray and Gemini near-infrared observations of the eclipsing msec pulsar SWIFT J1749.4-2807 in quiescence
Authors: Peter G. Jonker, Manuel A.P. Torres, Danny Steeghs, Deepto Chakrabarty

We report on Chandra X-ray and Gemini-North near-infrared K-band observations of the eclipsing accretion-powered millisecond X-ray pulsar SWIFT J1749.4-2807 in quiescence. Using the Chandra observation we derive a source position of Right Ascencion: 17:49:31.73 and Declination:-28:08:05.08. The position is accurate to 0.6" (90 per cent confidence). We find one source at a magnitude K=18.44±0.03 with a position fully consistent with the accurate Chandra X-ray localisation and a second source at K=19.2±0.1 that falls close to the edge of the error circle in the deep K-band images. The presence of a few weaker sources as suggested by previous H-band observations presented in the literature cannot the ruled out. There is marginal evidence that the brighter of the these two sources is variable. Follow-up spectroscopy of this potential counterpart will show if this source is the true counterpart to SWIFT J1749.4-2807. If so, baring the presence of complicating effects such as heating of the mass-donor star, it would allow for the mass of the neutron star to be measured through the measurement of periodic radial velocity variations.

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Blobrana


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Eclipsing Pulsar Promises Clues to Crushed Matter

Astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) have found the first fast X-ray pulsar to be eclipsed by its companion star. Further studies of this unique stellar system will shed light on some of the most compressed matter in the universe and test a key prediction of Einstein's relativity theory.
The pulsar is a rapidly spinning neutron star -- the crushed core of a massive star that long ago exploded as a supernova. Neutron stars pack more than the sun's mass into a ball nearly 60,000 times smaller. With estimated sizes between 10 and 15 miles across, a neutron star would just span Manhattan or the District of Columbia.
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Blobrana


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Title: SWIFT J1749.4-2807: A neutron or quark star?
Authors: Junwei Yu, Renxin Xu

We investigate an unique accreting millisecond pulsar with X-ray eclipses, SWIFT J1749.4-2807 (hereafter J1749), and try to limit the binary system by various methods including that of the Roche lobe, the mass-radius relations of both a main sequence (MS) and a white dwarf (WD) companion stars, as well as the measured mass function of the pulsar. The calculations are based on the assumption that the radius of the companion star has reached its Roche radius (or at 90\%), but the pulsar's mass has not been assumed to be a certain value. Our results are as follows. The companion star should be a MS. For the case that the radius equals to its Roche one, we have a companion star with mass M\simeq 0.51 solar mass and radius R_{c}\simeq 0.52 solar radaii, and the inclination angle is i\simeq 76.5°; for the case that the radius reaches 90% of its Roche one, we have M\simeq 0.43 solar mass, R_{c}\simeq 0.44 solar radaii and i\simeq 75.7°. We also obtain the mass of J1749, M_{p}\simeq 1 solar mass, and conclude that the pulsar could be a quark star if the ratio of the critical frequency of rotation-mode instability to the Keplerian one is higher than ~ 0.3. The relatively low pulsar mass (about ~ solar mass) may also challenge the conventional recycling scenario for the origin and evolution of millisecond pulsars. The results presented in this paper are expected to be tested by future observations.

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