1,687 research outputs found
A Search for Supernova-Remnant Masers Toward Unidentified EGRET Sources
Supernova remnants expanding into adjacent molecular clouds are believed to
be sites of cosmic ray acceleration and sources of energetic gamma-rays. Under
certain environmental conditions, such interactions also give rise to unusual
OH masers in which the 1720 MHz satellite line dominates over the more common
1665/7 MHz emission. Motivated by the apparent coincidence of a handful of
EGRET sources with OH(1720 MHz) maser-producing supernova remnants, we have
carried out a search using the Very Large Array for new OH(1720 MHz) masers
within the error regions of 11 unidentified EGRET sources at low Galactic
latitude. While a previously known maser associated with an HII region was
serendipitously detected, initial results indicate that no new masers were
found down to a limiting flux of, typically, 50 mJy. We discuss the
implications of this result on the nature of the unidentified Galactic EGRET
sources.Comment: 5 pages, 1 figure. To appear in Proceedings, GAMMA2001 (Baltimore,
MD, April 4-6, 2001), eds. N. Gehrels, C. Shrader, and S. Rit
Orbital Variability in the Eclipsing Pulsar Binary PSR B1957+20
We have conducted timing observations of the eclipsing millisecond binary
pulsar PSR~B1957+20, extending the span of data on this pulsar to more than
five years. During this time the orbital period of the system has varied by
roughly , changing quadratically with time
and displaying an orbital period second derivative s. The previous measurement of a large negative
orbital period derivative reflected only the short-term behavior of the system
during the early observations; the orbital period derivative is now positive
and increasing rapidly. If, as we suspect, the PSR~B1957+20 system is
undergoing quasi-cyclic orbital period variations similar to those found in
other close binaries such as Algol and RS CVn, then the
companion to PSR~B1957+20 is most likely non-degenerate, convective, and
magnetically active.Comment: 9 pages, 3 figures, LaTeX, submitted ApJL 13 Dec. 1993, arz-00
Measuring Neutron Star Radii via Pulse Profile Modeling with NICER
The Neutron-star Interior Composition Explorer (NICER) is an X-ray
astrophysics payload that will be placed on the International Space Station.
Its primary science goal is to measure with high accuracy the pulse profiles
that arise from the non-uniform thermal surface emission of rotation-powered
pulsars. Modeling general relativistic effects on the profiles will lead to
measuring the radii of these neutron stars and to constraining their equation
of state. Achieving this goal will depend, among other things, on accurate
knowledge of the source, sky, and instrument backgrounds. We use here simple
analytic estimates to quantify the level at which these backgrounds need to be
known in order for the upcoming measurements to provide significant constraints
on the properties of neutron stars. We show that, even in the
minimal-information scenario, knowledge of the background at a few percent
level for a background-to-source countrate ratio of 0.2 allows for a
measurement of the neutron star compactness to better than 10% uncertainty for
most of the parameter space. These constraints improve further when more
realistic assumptions are made about the neutron star emission and spin, and
when additional information about the source itself, such as its mass or
distance, are incorporated.Comment: Submitted to Ap
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