75 research outputs found
The Ultraluminous X-ray Source in Holmberg IX and its Environment
We present optical observations of an ultraluminous X-ray source (ULX) in
Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity
of some 10^{40} erg/s. It is located in a huge (400pc x 300pc) ionized nebula
being much larger than normal supernova remnants. From the observed emission
lines (widths and ratios) we find that the structure is due to collisional
excitation by shocks, rather than by photoionization.
We identify the optical counterpart to be a 22.8 mag blue star (M_V=-5.0)
belonging to a small stellar cluster. From isochrone fitting of our
multi-colour photometry we determine a cluster age of 20 to 50 Myr. We also
discovered strong stellar HeII4686 emission (equivalent width of 10 A) which
proves the identification with the X-ray source, and which suggests the
presence of an X-ray heated accretion disc around the putative black hole.Comment: 2 pages, 2 figures, to appear in the proceedings of the IAU Symposium
230, "Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
Ultraluminous X-ray Sources: Bubbles and Optical Counterparts
Optical studies of ultraluminous X-ray sources (ULX) in nearby galaxies have
turned out to be instrumental in discriminating between various models
including the much advertised intermediate mass black hole hypothesis and
various beaming scenarios. Here we report on ESO VLT and SUBARU observations of
ULX that have revealed the parent stellar clusters with ages of some 60 million
years in two cases. Thus we are able to derive upper limits of about 8 M_sun
for the mass donors in these systems. The optical counterparts are dominated by
X-ray heated accretion disks, and the discovery of the HeII4686 emission line
now allows to derive dynamical masses in these systems. Apparent radial
velocity variations of 300 km/s have been detected in NGC 1313 X-2 which, if
confirmed by further observations, would exclude the presence of IMBH in these
systems.Comment: 4 pages, to appear in the proceedings of IAU Symposium 230,
"Populations of High Energy Sources in Galaxies", Dublin, 15-19 Aug 200
Building an Archive with Saada
Saada transforms a set of heterogeneous FITS files or VOTables of various
categories (images, tables, spectra ...) in a database without writing code.
Databases created with Saada come with a rich Web interface and an Application
Programming Interface (API). They support the four most common VO services.
Such databases can mix various categories of data in multiple collections. They
allow a direct access to the original data while providing a homogenous view
thanks to an internal data model compatible with the characterization axis
defined by the VO. The data collections can be bound to each other with
persistent links making relevant browsing paths and allowing data-mining
oriented queries.Comment: 18 pages, 5 figures Special VO issu
Classifying the zoo of ultraluminous X-ray sources
Ultraluminous X-ray sources (ULXs) are likely to include different physical
types of objects. We discuss some possible subclasses, reviewing the properties
of a sample of ULXs recently observed by Chandra and XMM-Newton. Sources with
an isotropic X-ray luminosity up to a few times 10^{39} erg/s are consistent
with ``normal'' stellar-mass X-ray binaries (mostly high-mass X-ray binaries in
star-forming regions). Higher black hole (BH) masses (~ 50-100 M_sun) may be
the end product of massive stellar evolution in peculiar environments: they may
explain ULXs with luminosities ~ 1-2 x 10^{40} erg/s. Only a handful of ULXs
require a true intermediate-mass BH (M >~ 500 M_sun). Finally, a small subclass
of ULXs shows flaring or rapid variability in its power-law spectral component.Comment: Chinese Journal of Astronomy & Astrophysics, accepte
Searching for new thermally emitting isolated neutron stars in the 2XMMp catalogue - Discovery of a promising candidate
The group of 7 thermally emitting and radio-quiet isolated neutron stars
(INSs) discovered by ROSAT constitutes a nearby population which locally
appears to be as numerous as that of the classical radio pulsars. So far,
attempts to enlarge this particular group of INSs finding more remote objects
failed to confirm any candidate. We found in the 2XMMp catalogue a handful of
sources with no catalogued counterparts and with X-ray spectra similar to those
of the ROSAT discovered INSs, but seen at larger distances and thus undergoing
higher interstellar absorptions. In order to rule out alternative
identifications such as an AGN or a CV, we obtained deep ESO-VLT and SOAR
optical imaging for the X-ray brightest candidates. We report here on the
current status of our search and discuss the possible nature of our candidates.
We focus particularly on the X-ray brightest source of our sample, 2XMM
J104608.7-594306, observed serendipitously over more than four years by the
XMM-Newton Observatory. A lower limit on the X-ray to optical flux ratio of ~
300 together with a stable flux and soft X-ray spectrum make it the most
promising thermally emitting INS candidate. Beyond the finding of new members,
our study aims at constraining the space density of this population at large
distances and at determining whether their apparently high local density is an
anomaly or not.Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of
Pulsars", 12-17 August 2007, Montreal, Canad
XMM-Newton reveals a candidate period for the spin of the "Magnificent Seven" neutron star RX J1605.3+3249
The group of thermally emitting isolated neutron stars (INSs) known as the
"Magnificent Seven" (M7) is unique among the various neutron star populations.
Crustal heating by means of magnetic field decay and an evolutionary link with
magnetars may explain why these objects rotate more slowly and have higher
thermal luminosities and magnetic field intensities than standard pulsars of
similar age. The third brightest INS, RX J1605.3+3249, is the only object
amidst the seven still lacking a detected periodicity. We observed the source
with the XMM-Newton Observatory for 60 ks aiming at unveiling the neutron star
rotation rate and investigating its spectrum in detail. A periodic signal at
P=3.387864(16) s, most likely the neutron star spin period, is detected at the
4-sigma confidence level. The coherent combination of the new data with a past
XMM-Newton EPIC-pn observation of the source constrains the pulsar spin-down
rate at the 2-sigma confidence level, implying a dipolar magnetic field of
B~7.4e13 G. If confirmed, RX J1605.3+3249 would be the neutron star with the
highest dipolar field amongst the M7. The spectrum of the source shows evidence
of a cool blackbody component, as well as for the presence of two broad
absorption features. Furthermore, high-resolution spectroscopy with the RGS
cameras confirms the presence of a narrow absorption feature at energy 0.57 keV
in the co-added spectrum of the source, also seen in other thermally emitting
isolated neutron stars. Phase-resolved spectroscopy, as well as a dedicated
observing campaign aimed at determining a timing solution, will give invaluable
constraints on the neutron star geometry and will allow one to confirm the high
value of spin down, which would place the source closer to a magnetar than any
other M7 INS.Comment: 12 pages, 6 figures; accepted for publication in A&A (revised version
after language editing; results unchanged
Timing analysis of the isolated neutron star RX J0720.4-3125
We present a combined analysis of XMM-Newton, Chandra and Rosat observations
of the isolated neutron star RXJ0720.4-3125, spanning a total period of \sim 7
years. We develop a maximum likelihood periodogramme for our analysis based on
the \Delta C-statistic and the maximum likelihood method, which are appropriate
for the treatment of sparse event lists. Our results have been checked "a
posteriori" by folding a further BeppoSAX dataset with the period predicted at
the time of that observation: the phase is found to be consistent. The study of
the spin history and the measure of the spin-down rate is of extreme importance
in discriminating between the possible mechanisms suggested for the nature of
the X-ray emission. The value of \dot P, here measured for the first time, is
\approx 10^{-14} s/s. This value can not be explained in terms of torque from a
fossil disk. When interpreted in terms of dipolar losses, it gives a magnetic
field of B \approx 10^{13} G, making also implausible that the source is
accreting from the underdense surroundings. On the other hand, we also find
unlikely that the field decayed from a much larger value (B\approx 10^{15} G,
as expected for a magnetar powered by dissipation of a superstrong field) since
this scenario predicts a source age of \approx 10^4 yrs, too young to match the
observed X-ray luminosity. The observed properties are more compatible with a
scenario in which the source is \approx 10^6 yrs old, and its magnetic field
has not changed substantially over the lifetime.Comment: 11 Pages, 6 Figures. Accepted for publication in MNRA
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