429 research outputs found
The longest observation of a low intensity state from a Supergiant Fast X-ray Transient: Suzaku observes IGRJ08408-4503
We report here on the longest deep X-ray observation of a SFXT outside
outburst, with an average luminosity level of 1E33 erg/s (assuming 3 kpc
distance). This observation was performed with Suzaku in December 2009 and was
targeted on IGRJ08408-4503, with a net exposure with the X-ray imaging
spectrometer (XIS, 0.4-10 keV) and the hard X-ray detector (HXD, 15-100 keV) of
67.4 ks and 64.7 ks, respectively, spanning about three days. The source was
caught in a low intensity state characterized by an initially average X-ray
luminosity level of 4E32 erg/s (0.5-10 keV) during the first 120 ks, followed
by two long flares (about 45 ks each) peaking at a flux a factor of about 3
higher than the initial pre-flare emission. Both XIS spectra (initial emission
and the two subsequent long flares) can be fitted with a double component
spectrum, with a soft thermal plasma model together with a power law,
differently absorbed. The spectral characteristics suggest that the source is
accreting matter even at this very low intensity level. From the HXD
observation we place an upper limit of 6E33 erg/s (15-40 keV; 3 kpc distance)
to the hard X-ray emission, which is the most stringent constrain to the hard
X-ray emission during a low intensity state in a SFXT, to date. The timescale
observed for the two low intensity long flares is indicative of an orbital
separation of the order of 1E13 cm in IGRJ08408-4503.Comment: Accepted for publication in MNRAS. Accepted 2010 July 6. Received
2010 July 6; in original form 2010 June 9. The paper contains 5 figures and 3
table
XMMU J174716.1-281048: a "quasi-persistent" very faint X-ray transient?
The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with
XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its
spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray
observations of the source field performed in 2000 and 2001 did not detect the
source, indicative of a quiescent emission at least two orders of magnitude
fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc)
indicates that the source is a member of the ``very faint X-ray transients''
class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray
burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray
transient. This source was soon found to be positionally coincident, within the
uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the
X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I
burst nature. We also re-analysed XMM-Newton and Chandra archival observations
of the source field. We discuss the implications of these new findings,
particularly related to the source distance as well as the source
classification.Comment: 4 pages, 8 figures, accepted for publication in A&A Letter
The Very Massive Star Content of the Nuclear Star Clusters in NGC 5253
The blue compact dwarf galaxy NGC 5253 hosts a very young starburst
containing twin nuclear star clusters, separated by a projected distance of 5
pc. One cluster (#5) coincides with the peak of the H-alpha emission and the
other (#11) with a massive ultracompact H II region. A recent analysis of these
clusters shows that they have a photometric age of 1+/-1 Myr, in apparent
contradiction with the age of 3-5 Myr inferred from the presence of Wolf-Rayet
features in the cluster #5 spectrum. We examine Hubble Space Telescope
ultraviolet and Very Large Telescope optical spectroscopy of #5 and show that
the stellar features arise from very massive stars (VMS), with masses greater
than 100 Msun, at an age of 1-2 Myr. We further show that the very high
ionizing flux from the nuclear clusters can only be explained if VMS are
present. We investigate the origin of the observed nitrogen enrichment in the
circum-cluster ionized gas and find that the excess N can be produced by
massive rotating stars within the first 1 Myr. We find similarities between the
NGC 5253 cluster spectrum and those of metal poor, high redshift galaxies. We
discuss the presence of VMS in young, star-forming galaxies at high redshift;
these should be detected in rest frame UV spectra to be obtained with the James
Webb Space Telescope. We emphasize that population synthesis models with upper
mass cut-offs greater than 100 Msun are crucial for future studies of young
massive star clusters at all redshifts.Comment: 11 pages, 7 figures, accepted for publication in Astrophysical
Journa
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Spectral properties of the soft excess pulsar RX J0059.2-7138 during its 2013 outburst
We report on an X-ray observation of the Be X-ray Binary Pulsar RX
J0059.2-7138, performed by XMM-Newton in March 2014. The 19 ks long observation
was carried out about three months after the discovery of the latest outburst
from this Small Magellanic Cloud transient, when the source luminosity was Lx ~
10 erg/s. A spin period of P=2.762383(5) s was derived, corresponding to
an average spin-up of
s from the only previous period measurement, obtained more than 20
years earlier. The time-averaged continuum spectrum (0.2-12 keV) consisted of a
hard power-law (photon index ~0.44) with an exponential cut-off at a
phase-dependent energy (20-50 keV) plus a significant soft excess below about
0.5 keV. In addition, several features were observed in the spectrum: an
emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9-1 keV
likely due to a blend of Fe L-shell lines, and narrow emission and absorption
lines consistent with transitions in highly ionized oxygen, nitrogen and iron
visible in the high resolution RGS data (0.4-2.1 keV). Given the different
ionization stages of the narrow line components, indicative of photoionization
from the luminous X-ray pulsar, we argue that the soft excess in RX
J0059.2-7138 is produced by reprocessing of the pulsar emission in the inner
regions of the accretion disc.Comment: Accepted for publication in Mon. Not. R. Astron. Soc. 9 pages, 5
figure
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