320 research outputs found
The 2006 Outburst of the Magnetar CXOU J164710.2-455216
We report on data obtained with the Chandra, XMM-Newton, Suzaku and Swift
X-ray observatories, following the 2006 outburst of the Anomalous X-ray Pulsar
CXO J164710.2-455216. We find no evidence for the very large glitch and rapid
exponential decay as was reported previously for this source. We set a 3 sigma
upper limit on any fractional frequency increase at the time of the outburst of
Delta nu/nu < 1.5 x 10^{-5}. Our timing analysis, based on the longest time
baseline yet, yields a spin-down rate for the pulsar that implies a surface
dipolar magnetic field of ~9 x 10^{13} G, although this could be biased high by
possible recovery from an undetected glitch. We also present an analysis of the
source flux and spectral evolution, and find no evidence for long-term spectral
relaxation post-outburst as was previously reported.Comment: Submitted to Ap
The X-ray Binary GRS 1741.9-2853 in Outburst and Quiescence
We report Chandra and XMM-Newton observations of the transient neutron star
low-mass X-ray binary GRS 1741.9-2853. Chandra detected the source in outburst
on 2000 October 26 at an X-ray luminosity of ~10^{36} erg/s (2--8 keV; 8 kpc),
and in quiescence on 2001 July 18 at ~10^{32} erg/s. The latter observation is
the first detection of GRS 1741.9-2853 in quiescence. We obtain an accurate
position for the source of 17h 45m 2.33s, -28o 54' 49.7" (J2000), with an
uncertainty of 0.7". GRS 1741.9-2853 was not detected significantly in three
other Chandra observations, nor in three XMM-Newton observations, indicating
that the luminosity of the source in quiescence varies by at least a factor of
5 between (< 0.9 - 5.0) \times 10^{32} erg/s (2--8 keV). A weak X-ray burst
with a peak luminosity of 5 \times 10^{36} erg/s above the persistent level was
observed with Chandra during the outburst on 2000 October 26. The energy of
this burst, 10^{38} erg, is unexpectedly low, and may suggest that the accreted
material is confined to the polar caps of the neutron star. A search of the
literature reveals that GRS 1741.9-2853 was observed in outburst with ASCA in
Fall 1996 as well, when the BeppoSAX WFC detected the three previous X-ray
bursts from this source. The lack of X-ray bursts from GRS 1741.9-2853 at other
epochs suggests that it produces bursts only during transient outbursts when
the accretion rate onto the surface of the neutron star is about 10^{-10}
M_sun/yr. A similar situation may hold for other low-luminosity bursters
recently identified from WFC data.Comment: Submitted to ApJ. 9 pages, including 5 figure
A Search for New Galactic Magnetars in Archival Chandra and XMM-Newton Observations
We present constraints on the number of Galactic magnetars, which we have
established by searching for sources with periodic variability in 506 archival
Chandra observations and 441 archival XMM-Newton observations of the Galactic
plane (|b|<5 degree). Our search revealed four sources with periodic
variability on time scales of 200-5000 s, all of which are probably accreting
white dwarfs. We identify 7 of 12 known Galactic magnetars, but find no new
examples with periods between 5 and 20 s. We convert this non-detection into
limits on the total number of Galactic magnetars by computing the fraction of
the young Galactic stellar population that was included in our survey. We find
that easily-detectable magnetars, modeled after persistent anomalous X-ray
pulsars, could have been identified in 5% of the Galactic spiral arms by mass.
If we assume there are 3 previously-known examples within our random survey,
then there are 59 (+92,-32) in the Galaxy. Transient magnetars in quiescence
could have been identified throughout 0.4% of the spiral arms, and the lack of
new examples implies that <540 exist in the Galaxy (90% confidence). Similar
constraints are found by considering the detectability of transient magnetars
in outburst by current and past X-ray missions. For assumed lifetimes of 1e4
yr, we find that the birth rate of magnetars could range between 0.003 and 0.06
per year. Therefore, the birth rate of magnetars is at least 10% of that for
normal radio pulsars. The magnetar birth rate could exceed that of radio
pulsars, unless the lifetimes of transient magnetars are >1e5 yr. Obtaining
better constraints will require wide-field X-ray or radio searches for
transient X-ray pulsars similar to XTE J1810--197, AX J1845.0--0250, CXOU
J164710.2--455216, and 1E 1547.0-5408.Comment: 16 pages, 10 figures, one with a bit of color. submitted to Ap
RXTE Monitoring of the Anomalous X-ray Pulsar 1E 1048.1-5937
We report on long-term monitoring of the anomalous X-ray pulsar 1E
1048.1-5937 using the Rossi X-ray Timing Explorer. This pulsar's timing
behavior is different from that of other AXPs. In particular, the pulsar shows
significant deviations from simple spin-down such that phase-coherent timing
has not been possible over time spans longer than a few months. We show that in
spite of the rotational irregularities, the pulsar exhibits neither pulse
profile changes nor large pulsed flux variations. We discuss the implications
of our results for AXP models. We suggest that 1E 1048.1-5937 may be a
transition object between the soft gamma-ray repeater and AXP populations, and
the AXP most likely to one day undergo an outburst.Comment: 6 pages, to appear in Proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, AIP pres
The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center
We examine the X-ray spectra and variability of the sample of X-ray sources
with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the
Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We
find that the spectra of the point sources near the Galactic center are very
hard between 2--8 keV, even after accounting for absorption. When modeled as
power laws the median photon index is Gamma=0.7, while when modeled as thermal
plasma we can only obtain lower limits to the temperature of kT>8 keV. The
combined spectra of the point sources is similarly hard, with a photon index of
Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and
H-like Fe, both in the average spectra and in the brightest individual sources.
The line ratios of the highly-ionized Fe in the average spectra are consistent
with emission from a plasma in thermal equilibrium. This line emission is
observed whether average spectra are examined as a function of the count rate
from the source, or as a function of the hardness ratios of individual sources.
This suggests that the hardness of the spectra may in fact to due local
absorption that partially-covers the X-ray emitting regions in the Galactic
center systems. We suggest that most of these sources are intermediate polars,
which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely
exhibit either flares on short time scales or changes in their mean X-ray flux
on long time scales, and (3) are the most numerous hard X-ray sources with
comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004,
v613 issue. An electronic version of table 2 is on
http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data
files for each source are available on
http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm
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