481 research outputs found
Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?
In this Paper we report on phase resolved I-band optical spectroscopic and
photometric observations of CirX-1 obtained with the Very Large Telescope. The
spectra are dominated by Paschen absorption lines at nearly all orbital phases
except near phase zero (coinciding with the X-ray dip) when the absorption
lines are filled-in by broad Paschen emission lines. The radial velocity curve
of the absorption lines corresponds to an eccentric orbit (e=0.45) whose period
and time of periastron passage are consistent with the period and phase
predicted by the most recent X-ray dip ephemeris. We found that the I-band
magnitude decreases from 17.6 to ~16.8 near phase 0.9-1.0, this brightening
coincides in phase with the X-ray dip. Even though it is likely that the
absorption line spectrum is associated with the companion star of CirX-1, we
cannot exclude the possibility that the spectrum originates in the accretion
disc. However, if the spectrum belongs to the companion star, it must be a
supergiant of spectral type B5-A0. If we assume that the compact object does
not move through the companion star at periastron, the companion star mass is
constrained to ~<10 Msun for a 1.4 Msun neutron star, whereas the inclination
has to be ~> 13.7 degrees. Alternatively, the measured absorption lines and
their radial velocity curve can be associated with the accretion disc
surrounding a 1.4 Msun neutron star and its motion around the centre of mass.
An absorption line spectrum from an accretion disc is typically found when our
line-of-sight passes through the accretion disc rim implying a high
inclination. However, from radio observations it was found that the angle
between the line-of-sight and the jet axis is smaller than 5 degrees implying
that the jet ploughs through the accretion disc in this scenario.Comment: 8 pages, 4 figures, 3 tables, accepted by MNRA
Constraining the neutron star equation of state using quiescent low-mass X-ray binaries
Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can
provide important constraints on the equation of state of neutron stars. The
mass and radius of the neutron star can potentially be determined from fitting
a neutron star atmosphere model to the observed X-ray spectrum. For a radius
measurement it is of critical importance that the distance to the source is
well constrained since the fractional uncertainty in the radius is at least as
large as the fractional uncertainty in the distance. Uncertainties in modelling
the neutron star atmosphere remain. At this stage it is not yet clear if the
soft thermal component in the spectra of many quiescent X-ray binaries is
variable on timescales too short to be accommodated by the cooling neutron star
scenario. This can be tested with a long XMM-Newton observation of the neutron
star X-ray transient CenX-4 in quiescence. With such an observation one can use
the Reflection Grating Spectrometer spectrum to constrain the interstellar
extinction to the source. This removes this parameter from the X-ray spectral
fitting of the EPIC pn and MOS spectra and allows one to investigate whether
the variability observed in the quiescent X-ray spectrum of this source is due
to variations in the soft thermal spectral component or variations in the power
law spectral component coupled with variations in N_H. This will test whether
the soft thermal component can indeed be due to the hot thermal glow of the
neutron star. Irrespective of the outcome of such a study, the observed cooling
in quiescence in sources for which the crust is significantly out of thermal
equilibrium with the core due to a prolonged outburst, such as KS 1731-260,
seem excellent candidates for mass and radius determinations through modelling
the observed X-rays with a neutron star atmosphere model.Comment: 7 pages, 3 figures, proceedings "40 years of pulsars" conferenc
X-ray and Near-IR Variability of the Anomalous X-ray Pulsar 1E 1048.1-5937: From Quiescence Back to Activity
(Abridged) We report on new and archival X-ray and near-infrared observations
of the anomalous X-ray pulsar 1E 1048.1-5937 performed between 2001-2007 with
RXTE, CXO, Swift, HST, and VLT. During its ~2001-2004 active period, 1E
1048.-5937 exhibited two large, long-term X-ray pulsed-flux flares as well as
short bursts, and large (>10x) torque changes. Monitoring with RXTE revealed
that the source entered a phase of timing stability in 2004; at the same time,
a series of four simultaneous observations with CXO and HST in 2006 showed that
its X-ray flux and spectrum and near-IR flux, all variable prior to 2005,
stabilized. The near-IR flux, when detected by HST (H~22.7 mag) and VLT
(K_S~21.0 mag), was considerably fainter than previously measured. Recently, in
2007 March, this newfound quiescence was interrupted by a sudden flux
enhancement, X-ray spectral changes and a pulse morphology change, simultaneous
with a large spin-up glitch and near-IR enhancement. Our RXTE observations
revealed a sudden pulsed flux increase by a factor of ~3 in the 2-10 keV band.
In observations with CXO and Swift, we found that the total X-ray flux
increased much more than the pulsed flux, reaching a peak value of >7 times the
quiescent value (2-10 keV). With these recent data, we find a strong
anti-correlation between X-ray flux and pulsed fraction, and a correlation
between X-ray spectral hardness and flux. Simultaneously with the radiative and
timing changes, we observed a significant X-ray pulse morphology change such
that the profile went from nearly sinusoidal to having multiple peaks. We
compare these remarkable events with other AXP outbursts and discuss
implications in the context of the magnetar model and other models of AXP
emission.Comment: 13 pages (6 figures) in emulateapj style. Accepted for publication in
ApJ. New version includes referee's corrections; split Figure 1 into 2
figures; modified Figs. 4b and 6b; rearranged and renumbered of some figures
and sections; added an X-ray dataset; improved analysis of pulse morphology
and pulsed fraction; added paragraph to sec. 3.2.
The neutron star soft X-ray transient 1H1905+000 in quiescence
In this Paper we report on our analysis of a ~25 ksec. Chandra X-ray
observation of the neutron star soft X-ray transient (SXT) 1H1905+000 in
quiescence. Furthermore, we discuss our findings of the analysis of optical
photometric observations which we obtained using the Magellan telescope and
photometric and spectroscopic observations which we obtained using the Very
Large Telescope at Paranal. The X-ray counterpart of 1H1905+000 was not
detected in our Chandra data, with a 95 per cent confidence limit to the source
count rate of 1.2x10^-4 counts s^-1. For different spectral models this yields
an upper limit on the luminosity of 1.8x10^31 erg s^-1 (for an upper limit on
the distance of 10 kpc.) This luminosity limit makes 1H1905+000 the faintest
neutron star SXT in quiescence observed to date. The neutron star luminosity is
so low that it is similar to the lowest luminosities derived for black hole
SXTs in quiescence. This low luminosity for a neutron star SXT challanges the
hypothesis presented in the literature that black hole SXTs in quiescence have
lower luminosities than neutron star SXTs as a result of the presence of a
black hole event horizon. Furthermore, the limit on the neutron star luminosity
obtained less than 20 years after the outburst has ceased, constrains the
thermal conductivity of the neutron star crust. Finally, the neutron star core
must be so cold that unless the time averaged mass accretion rate is lower than
2x10^-12 M_sun yr^-1, core cooling has to proceed via enhanced neutrino
emission processes. We derive a limit on the absolute I-band magnitude of the
quiescent counterpart of M_I>7.8 assuming the source is at 10 kpc. This is in
line with 1H1905+000 being an ultra-compact X-ray binary, as has been proposed
based on the low outburst V-band absolute magnitude.Comment: 10 pages, 5 figures, accepted for publication in MNRA
The quasi-persistent neutron star soft X-ray transient 1M 1716-315 in quiescence
We report on our analysis of a 20 ksec Chandra X-ray observation of the
quasi-persistent neutron star soft X-ray transient (SXT) 1M1716-315 in
quiescence. Only one source was detected in the HEAO-I error region. Its
luminosity is 1.6E32-1.3E33 erg s-1. In this the range is dominated by the
uncertainty in the source distance. The source spectrum is well described by an
absorbed soft spectrum, e.g. a neutron star atmosphere or black body model. No
optical or near-infrared counterpart is present at the location of the X-ray
source, down to a magnitude limit of I> 23.5 and K_s> 19.5. The positional
evidence, the soft X-ray spectrum together with the optical and near-infrared
non-detections provide strong evidence that this source is the quiescent
neutron star SXT. The source is 10-100 times too bright in X-rays in order to
be explained by stellar coronal X-ray emission. Together with the interstellar
extinction measured in outburst and estimates for the source distance, the
reported optical and near-infrared limit give an upper limit on the absolute
magnitude of the counterpart of I>8.6 and K_s>5.1. This implies that the system
is either an ultra-compact X-ray binary having P_orb<1 hr or the companion star
is an M-dwarf. We reconstructed the long term X-ray lightcurve of the source.
1M1716-315 has been active for more than 12 years before returning to
quiescence, the reported Chandra observation started 16.9+-4.1 years after the
outburst ended.Comment: 7 pages, 4 figures, accepted to MNRA
Oral medicine case book 69: Burkitt lymphoma of the oral cavity
A 25-year-old female was referred to the Haematology
Unit at Tygerberg Hospital for further management of a
rapidly expanding and large submandibular mass which
on fine needle aspiration was suggestive of lymphoma . Five months earlier she had been diagnosed
with pulmonary tuberculosis and was confirmed to be
HIV positive with a CD4 count of 17. She was placed on
anti-retroviral (ARV) and antituberculous therapy (the ARV
therapy included efavirenz, emtricitabine and tenofivir).
Her CD4 count, at the time of the current consultation,
was 204 and the viral load was suppressed. Lumbar
puncture was normal. Significant clinical findings were
a large right submandibular mass and right cervical and
axillary lymphadenopathy. The submandibular mass was
removed and submitted for histological examination.DHE
The Long-term Radiative Evolution of Anomalous X-ray Pulsar 1E 2259+586 after its 2002 Outburst
We present an analysis of five X-ray Multi-Mirror Mission (XMM) observations
of the anomalous X-ray pulsar (AXP) 1E 2259+586 taken in 2004 and 2005 during
its relaxation following its 2002 outburst. We compare these data with those of
five previous XMM observations taken in 2002 and 2003, and find the observed
flux decay is well described by a power-law of index -0.69+/-0.03. As of
mid-2005, the source may still have been brighter than preoutburst, and was
certainly hotter. We find a strong correlation between hardness and flux, as
seen in other AXP outbursts. We discuss the implications of these results for
the magnetar model.Comment: 23 Pages, 4 figures, 3 tables, published on Ap
SAX J1808.4-3657 in Quiescence: A Keystone for Neutron Star Science
The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object
between accreting X-ray binaries and millisecond radio pulsars. We have
constrained the thermal radiation from its surface through XMM-Newton X-ray
observations, providing strong evidence for neutrino cooling processes from the
neutron star core. We have also undertaken simultaneous X-ray and optical
(Gemini) observations, shedding light on whether the strong heating of the
companion star in quiescence may be due to X-ray irradiation, or to a radio
pulsar turning on when accretion stops.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond
Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 4
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