1,113 research outputs found
Variable Radio Sources in the Galactic Plane
Using three epochs of VLA observations of the Galactic Plane in the first
quadrant taken ~15 years apart, we have conducted a search for a population of
variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm.
We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting
for various selection effects and for the extragalactic variable population of
active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg
which vary by more than 50% on a time scale of years (or shorter). We show that
these sources are much more highly variable than extragalactic objects; more
than 50% show variability by a factor >2 compared to <10% for extragalactic
objects in the same flux density range. We also show that the fraction of
variable sources increases toward the Galactic center (another indication that
this is a Galactic population), and that the spectral indices of many of these
sources are flat or inverted. A small number of the variables are coincident
with mid-IR sources and two are coincident with X-ray emitters, but most have
no known counterparts at other wavelengths. Intriguingly, one lies at the
center of a supernova remnant, while another appears to be a very compact
planetary nebula; several are likely to represent activity associated with star
formation regions. We discuss the possible source classes which could
contribute to the variable cohort and followup observations which could clarify
the nature of these sources.Comment: 11 pages, 7 figures; to be published in the Astronomical Journal;
data available on MAGPIS website at http://third.ucllnl.org/gps
A Shell of Thermal X-ray Emission Associated with the Young Crab-like Remnant 3C58
Deep X-ray imaging spectroscopy of the bright pulsar wind nebula 3C58
confirms the existence of an embedded thermal X-ray shell surrounding the
pulsar PSR J0205+6449. Radially resolved spectra obtained with the XMM-Newton
telescope are well-characterized by a power-law model with the addition of a
soft thermal emission component in varying proportions. These fits reproduce
the well-studied increase in the spectral index with radius attributed to
synchrotron burn-off of high energy electrons. Most interestingly, a radially
resolved thermal component is shown to map out a shell-like structure ~6' in
diameter. The presence of a strong emission line corresponding to the Ne IX
He-like transition requires an overabundance of ~3 x [Ne/Ne(sun)] in the
Raymond-Smith plasma model. The best-fit temperature kT ~ 0.23 keV is
essentially independent of radius for the derived column density of N_H = (4.2
+/- 0.1)E21 per cm squared. Our result suggests that thermal shells can be
obscured in the early evolution of a supernova remnant by non-thermal pulsar
wind nebulae emission; the luminosity of the 3C58 shell is more than an order
of magnitude below the upper limit on a similar shell in the Crab Nebula. We
find the shell centroid to be offset from the pulsar location. If this neutron
star has a velocity similar to that of the Crab pulsar, we derive an age of
3700 yr and a velocity vector aligned with the long axis of the PWN. The shell
parameters and pulsar offset add to the accumulating evidence that 3C58 is not
the remnant of the supernova of CE 1181.Comment: 7 pages, 8 figures, 2 tables, Latex emulateapj style. To appear in
the Astrophysical Journa
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