3,238 research outputs found
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
X-Ray and Optical Flux Anomalies in the Quadruply Lensed QSO 1RXS J1131-1231
Optical and X-ray observations of the quadruply imaged quasar 1RXS J1131-1231
show flux ratio anomalies among the images factors of ~2 in the optical and
\~3-9 in X-rays. Temporal variability of the quasar seems an unlikely
explanation for the discrepancies between the X-ray and optical flux ratio
anomalies. The negative parity of the most affected image and the decreasing
trend of the anomalies with wavelength suggest microlensing as a possible
explanation; this would imply that the source of optical radiation in RXS J1131
is ~10^4 R_g in size for a black hole mass of ~10^8 M_sun. We also present
evidence for different X-ray spectral hardness ratios among the four images.Comment: 5 pages, 1 color figure, replaced with version accepted to Ap
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