2,917 research outputs found
Optical spectroscopy of (candidate) ultra-compact X-ray binaries: constraints on the composition of the donor stars
We present optical spectroscopy of several (candidate) ultra-compact X-ray
binaries (UCXBs) obtained with the ESO VLT and Gemini-North telescopes. In only
one of five observed UCXB candidates did we find evidence for H in its spectrum
(4U 1556-60). For XB 1905+00 the optical counterpart is not detected. For the
known UCXBs 4U 1626-67 and XB 1916-05 we find spectra consistent with a C/O and
a He/N accretion disc respectively, the latter is the first optical spectrum of
a He-rich donor in an UCXB. Interestingly, the C/O spectrum of 4U 1626-67 shows
both similarities as well as marked differences from the optical C/O spectrum
of 4U 0614+09. We obtained phase resolved spectroscopy of 4U 0614+09 and the 44
min transient XTE J0929-314. In neither object were we able to detect clear
orbital periodicities, highlighting the difficulties of period determinations
in UCXBs. We reanalysed the spectra of XTE J0929-314 that were taken close to
the peak of its 2003 X-ray outburst and do not confirm the detection of Halpha
emission as was claimed in the literature. The peak spectra do show strong C or
N emission around 4640A, as has also been detected in other UCXBs. We discuss
the implications of our findings for our understanding of the formation of
UCXBs and the Galactic population of UCXBs. At the moment all studied systems
are consistent with having white dwarf donors, the majority being C/O rich.Comment: Accepted for publication in MNRA
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
The mass function of GX 339-4 from spectroscopic observations of its donor star
We obtained 16 VLT/X-shooter observations of GX 339-4 in quiescence in the
period May - September 2016 and detected absorption lines from the donor star
in its NIR spectrum. This allows us to measure the radial velocity curve and
projected rotational velocity of the donor for the first time. We confirm the
1.76 day orbital period and we find that = km s,
km s and km s. From
these values we compute a mass function , a
factor lower than previously reported, and a mass ratio . We confirm the donor is a K-type star and estimate that it contributes
of the light in the - and H-band. We constrain the binary
inclination to and the black hole mass to
. GX 339-4 may therefore be the
first black hole to fall in the 'mass-gap' of .Comment: 11 pages, 7 figures, accepted for publication in Ap
Characterisation of a candidate dual AGN
We present Chandra and optical observations of a candidate dual AGN
discovered serendipitously while searching for recoiling black holes via a
cross-correlation between the serendipitous XMM source catalog (2XMMi) and
SDSS-DR7 galaxies with a separation no larger than ten times the sum of their
Petrosian radii. The system has a stellar mass ratio M/M. One of the galaxies (Source 1) shows clear evidence for AGN activity in
the form of hard X-ray emission and optical emission-line diagnostics typical
of AGN ionisation. The nucleus of the other galaxy (Source 2) has a soft X-ray
spectrum, bluer colours, and optical emission line ratios dominated by stellar
photoionisation with a "composite" signature, which might indicate the presence
of a weak AGN. When plotted on a diagram with X-ray luminosity vs [OIII]
luminosity both nuclei fall within the locus defined by local Seyfert galaxies.
From the optical spectrum we estimate the electron densities finding n e cm and n e cm. From a 2D
decomposition of the surface brightness distribution we infer that both
galaxies host rotationally supported bulges (Sersic index ). While the
active nature of Source 1 can be established with confidence, whether the
nucleus of Source 2 is active remains a matter of debate. Evidence that a faint
AGN might reside in its nucleus is, however, tantalising.Comment: 16 pages, 9 figures. Accepted for publication on MNRAS. Comments
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Two fast X-ray transients in archival Chandra data
We present the discovery of two new X-ray transients in archival Chandra
data. The first transient, XRT 110103, occurred in January 2011 and shows a
sharp rise of at least three orders of magnitude in count rate in less than 10
s, a flat peak for about 20 s and decays by two orders of magnitude in the next
60 s. We find no optical or infrared counterpart to this event in preexisting
survey data or in an observation taken by the SIRIUS instrument at the Infrared
Survey Facility 2.1 yr after the transient, providing limiting magnitudes of
J>18.1, H>17.6 and Ks>16.3. This event shows similarities to the transient
previously reported in Jonker et al. which was interpreted as the possible
tidal disruption of a white dwarf by an intermediate mass black hole. We
discuss the possibility that these transients originate from the same type of
event. If we assume these events are related a rough estimate of the rates
gives 1.4*10^5 per year over the whole sky with a peak 0.3-7 keV X-ray flux
greater than 2*10^-10 erg cm^-2 s^-1 . The second transient, XRT 120830,
occurred in August 2012 and shows a rise of at least three orders of magnitude
in count rate and a subsequent decay of around one order of magnitude all
within 10 s, followed by a slower quasi-exponential decay over the remaining 30
ks of the observation. We detect a likely infrared counterpart with magnitudes
J=16.70+/-0.06, H=15.92+/-0.04 and Ks=15.37+/-0.06 which shows an average
proper motion of 74+/-19 milliarcsec per year compared to archival 2MASS
observations. The JHKs magnitudes, proper motion and X-ray flux of XRT 120830
are consistent with a bright flare from a nearby late M or early L dwarf.Comment: Accepted for publication in MNRAS, 6 pages, 5 figure
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
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
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