7 research outputs found
Search for young galactic supernova remnants
A sample of 9 small-diameter radio sources has been selected from the Molonglo Galactic Plane Survey (MGPS) and observed with the Australia Telescope Compact Array (ATCA) in the radio recombination line (RRL) at 5 GHz, in a search for young Galactic SNRs. Since the RRL emission is an unambiguous indicator of a thermal source, this method has been used to eliminate HII regions from the selected sample. In addition, the IRAS and MSX infrared data and spectral index measurements have been combined with the RRL studies to distinguish thermal and non-thermal sources in the selected sample. One source (G282.8-1.2) is identified here as a possible new young Galactic supernova remnant, based on its relatively weak infrared emission, steep radio spectrum and possible x-ray emission. However, the ATCA data are inconclusive and further studies are needed to confirm this result. Radio recombination line emission (H107 alpha) has been detected in 3 of the selected sources, eliminating them from the sample of SNR candidates. In addition, the parameters of the RRL emission from the identified HII regions have been used to estimate their properties. The RRL data are inconclusive for the remaining low brightness, extended sources in the sample. However, some of these sources are likely to be thermal HII regions according to the infrared and spectral index data. The selected method for distinguishing thermal and non-thermal Galactic radio sources seems promising. The selected ATCA configuration was appropriate for imaging relatively bright, compact sources, but a slightly modified observing technique is needed to successfully image low surface brightness, extended sources
Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?
Recent theoretical and observational studies have shown that ashes from
thermonuclear burning may be ejected during radius-expansion bursts, giving
rise to photoionisation edges in the X-ray spectra. We report a search for such
features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.
We analysed the spectra from four radius-expansion bursts detected in 2006
July, and two in 2002 March, but found no evidence for discrete features. We
estimate upper limits for the equivalent widths of edges of a few hundred eV,
which for the moderate temperatures observed during the bursts, are comparable
with the predictions. During the 2006 July observation 4U 1728-34 exhibited
weak, unusually frequent bursts (separated by <2 hr in some cases), with
profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence
times as short as those measured are insufficient to exhaust the accreted
hydrogen at solar composition, suggesting that the source accretes hydrogen
deficient fuel, for example from an evolved donor. The detection for the first
time of a 10.77 min periodic signal in the persistent intensity, perhaps
arising from orbital modulation, supports this explanation, and suggests that
this system is an ultracompact binary similar to 4U 1820-30.Comment: 9 pages, 6 figures, accepted by Ap
Chandra observation of the TeV source HESS J1834-087
Chandra ACIS observed the field of the extended TeV source HESS J1834-087 for
47 ks. A previous XMM-Newton EPIC observation of the same field revealed a
point-like source (XMMU J183435.3-084443) and an offset region of faint
extended emission. In the low-resolution, binned EPIC images the two appear to
be connected. However, the high-resolution Chandra ACIS images do not support
the alleged connection. Instead, in these images XMMU J183435.3-084443 is
resolved into a point source, CXOU~J183434.9--084443 (L=2.5 x 10e33 ergs/s, for
a distance of 4 kpc; photon index 1.1), and a compact (~20'') nebula with an
isotropic morphology and a softer spectrum (L=4.2 x10e33 ergs/s, photon index
2.7). The nature of the nebula is uncertain. We discuss a dust scattering halo
and a pulsar-wind nebula as possible interpretations. Based on our analysis of
the X-ray data, we re-evaluate the previously suggested interpretations of HESS
J1834-087 and discuss a possible connection to the Fermi LAT source 1FGL
J1834.3-0842c. We also obtained an upper limit of 3 x 10e-14 ergs/s cm^2 on the
unabsorbed flux of the SGR J1833--0832 (in quiescence), which happened to be in
the ACIS field of view.Comment: 11 pages, 7 figures, submitted to Ap
Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34
We analysed thermonuclear (type-I) X-ray bursts observed from the low-mass
X-ray binary 4U1728-34 by RXTE, Chandra and INTEGRAL. We compared the variation
in burst energy and recurrence times as a function of accretion rate with the
predictions of a numerical ignition model including a treatment of the heating
and cooling in the crust. We found that the measured burst ignition column
depths are significantly below the theoretically predicted values, regardless
of the assumed thermal structure of the neutron star interior. While it is
possible that the accretion rate measured by Chandra is underestimated, due to
additional persistent spectral components outside the sensitivity band, the
required correction factor is typically 3.6 and as high as 6, which is
implausible. Furthermore, such underestimation is even more unlikely for RXTE
and INTEGRAL, which have much broader bandpasses. Possible explanations for the
observed discrepancy include shear-triggered mixing of the accreted helium to
larger column depths, resulting in earlier ignition, or the fractional covering
of the accreted fuel on the neutron star surface.Comment: 12 pages, 5 figures, accepted for publication in Ap