618 research outputs found
INTEGRAL study of temporal properties of bright flares in Supergiant Fast X-ray Transients
We have characterized the typical temporal behaviour of the bright X-ray
flares detected from the three Supergiant Fast X-ray Transients showing the
most extreme transient behaviour (XTEJ1739-302, IGRJ17544-2619,
SAXJ1818.6-1703). We focus here on the cumulative distributions of the
waiting-time (time interval between two consecutive X-ray flares), and the
duration of the hard X-ray activity (duration of the brightest phase of an SFXT
outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting
the cumulative distribution of waiting-times, it is possible to identify the
typical timescale that clearly separates different outbursts, each composed by
several single flares at ks timescale. This allowed us to measure the duration
of the brightest phase of the outbursts from these three targets, finding that
they show heavy-tailed cumulative distributions. We observe a correlation
between the total energy emitted during SFXT outbursts and the time interval
covered by the outbursts (defined as the elapsed time between the first and the
last flare belonging to the same outburst as observed by INTEGRAL). We show
that temporal properties of flares and outbursts of the sources, which share
common properties regardless different orbital parameters, can be interpreted
in the model of magnetized stellar winds with fractal structure from the
OB-supergiant stars.Comment: 10 pages, 8 figures, 1 table. Accepted for publication in MNRAS
(Accepted 2016 January 26. Received 2016 January 25 ; in original form 2015
December 15
INTEGRAL discovery of unusually long broad-band X-ray activity from the Supergiant Fast X-ray Transient IGR J18483-0311
We report on a broad-band X-ray study (0.5-250 keV) of the Supergiant Fast
X-ray Transient IGR J18483-0311 using archival INTEGRAL data and a new targeted
XMM-Newton observation. Our INTEGRAL investigation discovered for the first
time an unusually long X-ray activity (3-60 keV) which continuously lasted for
at least 11 days, i.e. a significant fraction (about 60%) of the entire orbital
period, and spanned orbital phases corresponding to both periastron and
apastron passages. This prolongated X-ray activity is at odds with the much
shorter durations marking outbursts from classical SFXTs especially above 20
keV, as such it represents a departure from their nominal behavior and it adds
a further extreme characteristic to the already extreme SFXT IGR J18483-0311.
Our IBIS/ISGRI high energy investigation (100-250 keV) of archival outbursts
activity from the source showed that the recently reported hint of a possible
hard X-ray tail is not real and it is likely due to noisy background. The new
XMM-Newton targeted observation did not detect any sign of strong X-ray
outburst activity from the source despite being performed close to its
periastron passage, on the contrary IGR J18483-0311 was caught during the
common intermediate X-ray state with a low luminosity value of 3x10^33 erg s^-1
(0.5-10 keV). We discuss all the reported results in the framework of both
spherically symmetric clumpy wind scenario and quasi-spherical settling
accretion model.Comment: Accepted for publication on MNRAS. 10 pages, 7 figures, 1 tabl
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
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
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
XMM-Newton and NuSTAR simultaneous X-ray observations of IGR J11215-5952
We report the results of an XMM-Newton and NuSTAR coordinated observation of
the Supergiant Fast X-ray Transient (SFXT) IGRJ11215-5952, performed on
February 14, 2016, during the expected peak of its brief outburst, which
repeats every about 165 days. Timing and spectral analysis were performed
simultaneously in the energy band 0.4-78 keV. A spin period of 187.0 +/- 0.4 s
was measured, consistent with previous observations performed in 2007. The
X-ray intensity shows a large variability (more than one order of magnitude) on
timescales longer than the spin period, with several luminous X-ray flares
which repeat every 2-2.5 ks, some of which simultaneously observed by both
satellites. The broad-band (0.4-78 keV) time-averaged spectrum was well
deconvolved with a double-component model (a blackbody plus a power-law with a
high energy cutoff) together with a weak iron line in emission at 6.4 keV
(equivalent width, EW, of 40+/-10 eV). Alternatively, a partial covering model
also resulted in an adequate description of the data. The source time-averaged
X-ray luminosity was 1E36 erg/s (0.1-100 keV; assuming 7 kpc). We discuss the
results of these observations in the framework of the different models proposed
to explain SFXTs, supporting a quasi-spherical settling accretion regime,
although alternative possibilities (e.g. centrifugal barrier) cannot be ruled
out.Comment: 13 pages, 11 figures, accepted for publication on The Astrophysical
Journa
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