973 research outputs found
Swift observations of the dwarf nova ASASSN-18fs
The All Sky Automated Survey for SuperNovae (ASAS-SN) reported a possible
Galactic dwarf nova ASASSN-18fs on 2018 March 19 at 13.2 mag in the V
band, with a quiescent magnitude of V17.6. Here we report on the follow-up
photometry using the {\it Neil Gehrels Swift Observatory}.Comment: Published by AAS Research Note
Variable quiescent state for the neutron-star X-ray transient SAX J1750.8-2900: not such a hot neutron star after all?
We monitored the neutron star low-mass X-ray binary SAX J1750.8-2900 after
the end of its 2015/2016 outburst using the X-ray Telescope (XRT) aboard Swift
to detect possible post-outburst 'rebrightenings', similar to those seen after
its 2008 outburst. We did not detect any rebrightening behaviour, suggesting
that the physical mechanism behind the rebrightening events is not always
active after each outburst of the source. Any model attempting to explain these
rebrightenings should thus be able to reproduce the different outburst profiles
of the source at different times. Surprisingly, our Swift/XRT observations were
unable to detect the source, contrary to previous Swift/XRT observations in
quiescence. We determined a temperature upper limit of 106 eV, much
colder than the post 2008 outburst value of 145 eV. We also report on an
archival Chandra observation of the source after its 2011 outburst and found a
temperature of 126 eV. These different temperatures, including the
non-detection very close after the end of the 2015/2016 outburst, are difficult
to explain in any model assuming we observe the cooling emission from a neutron
star core or an accretion-heated crust. We discuss our observations in the
context of a change in envelope (the outer 100 m of the crust)
composition and (possibly in combination with) a cooling crust. Both hypotheses
cannot explain our results unless potentially unrealistic assumptions are made.
Irrespective of what causes the temperature variability, it is clear that the
neutron star in SAX J1750.8-2900 may not be as hot as previously assumed.Comment: Accepted for publication in MNRA
XMM-Newton observations of two transient millisecond X-ray pulsars in quiescence
We report on XMM-Newton observations of two X-ray transient millisecond
pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of
\~7x10^{31} erg/s. (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent
spectrum is consistent with a simple power law spectrum. The upper limit on the
flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE
J1807-294 instead was not detected. We can put an upper limit on the source
quiescent 0.5-10 keV unabsorbed luminosity <4x10^{31} erg/s at 8 kpc. These
observations strenghten the idea that XRTMSPs have quiescent luminosities
significantly lower than classical neutron star transients.Comment: 4 pages including 1 figures. Accepted for publication in A&A Letter
The aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658
We studied the aperiodic X-ray timing behaviour of the accreting millisecond
pulsar SAX J1808.4-3658. The source was recently found to be the first
accreting millisecond pulsar that shows the kilohertz quasi-periodic
oscillations (kilohertz QPOs) that are found in many other X-ray binaries with
accreting neutron stars. The high frequency of these signals reflects the short
dynamical time scales in the region near the compact object where they
originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658
shows several low frequency timing features, based on which the source can be
classified as a so-called atoll source. The frequencies of the variability
components of the atoll sources follow a universal scheme of correlations. The
correlations in SAX J1808.4-3658 are similar but show a shift in upper
kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or
differently configured magnetic field.Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless
High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den
Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijer
A cooling neutron star crust after recurrent outbursts: Modelling the accretion outburst history of Aql X-1
With our neutron star crust cooling code {\tt NSCool} we track the thermal
evolution of the neutron star in Aql X-1 over the full accretion outburst
history from 1996 until 2015. For the first time, we model many outbursts (23
outbursts were detected) collectively and in great detail. This allows us to
investigate the influence of previous outbursts on the internal temperature
evolution and to test different neutron star crust cooling scenarios. Aql X-1
is an ideal test source for this purpose, because it shows frequent, short
outbursts and thermally dominated quiescence spectra. The source goes into
outburst roughly once a year for a few months.
Assuming that the quiescent {\it Swift}/XRT observations of Aql X-1 can be
explained within the crust cooling scenario (Waterhouse et al. 2016), we find
three main conclusions. Firstly, the data are well reproduced by our model if
the envelope composition and shallow heating parameters are allowed to change
between outbursts. This is not the case if both shallow heating parameters
(strength and depth) are tied throughout all accretion episodes, supporting
earlier results that the properties of the shallow heating mechanism are not
constant between outbursts. Second, from our models shallow heating could not
be connected to one specific spectral state during outburst. Third, and most
importantly, we find that the neutron star in Aql X-1 does not have enough time
between outbursts to cool down to crust-core equilibrium and that heating
during one outburst influences the cooling curves of the next.Comment: 20 pages, 8 figures, 4 tables, accepted for publication in MNRA
Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52
We present new results based on observations carried out with the Rossi X-ray
Timing Explorer during the decay of an outburst of the low-mass X-ray binary
(LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in
4U 1608-52, one of the long-standing issues about the phenomenology of the
kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique
relation between the frequency of the kHz QPOs and the X-ray flux. We show that
despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs
is monotonically related to the position of the source in the color-color
diagram. Our findings strengthen the idea that, as in the case of Z sources, in
the atoll sources the X-ray flux is not a good indicator of , and that
the observed changes in the frequency of the kHz QPOs in LMXBs are driven by
changes in . These results raise some concern about the recently
reported detection of the orbital frequency at the innermost stable orbit in 4U
1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses
AAS LaTex v4.0 (5 pages plus 4 postscript figures
Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9342058
We report on two new quiescent {\it XMM-Newton} observations (in addition to
the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling
neutron star crust in the low-mass X-ray binary 1RXS J180408.9342058. Its
crust was heated during the 4.5 month accretion outburst of the source.
From our quiescent observations, fitting the spectra with a neutron star
atmosphere model, we found that the crust had cooled from 100 eV to
73 eV from 8 days to 479 days after the end of its outburst.
However, during the most recent observation, taken 860 days after the end
of the outburst, we found that the crust appeared not to have cooled further.
This suggested that the crust had returned to thermal equilibrium with the
neutron star core. We model the quiescent thermal evolution with the
theoretical crustal cooling code NSCool and find that the source requires a
shallow heat source, in addition to the standard deep crustal heating
processes, contributing 0.9 MeV per accreted nucleon during outburst to
explain its observed temperature decay. Our high quality {\it XMM-Newton} data
required an additional hard component to adequately fit the spectra. This
slightly complicates our interpretation of the quiescent data of 1RXS
J180408.9342058. The origin of this component is not fully understood.Comment: Accepted for publication by MNRA
Linking burst-only X-ray binary sources to faint X-ray transients
Burst-only sources are X-ray sources showing up only during short bursts but
with no persistent emission (at least with the monitoring instrument which led
to their discovery). These bursts have spectral characteristics consistent with
thermonuclear (type I) burst from the neutron star surface, linking burst-only
sources to neutron star X-ray binary transients. We have carried out a series
of snapshot observations of the entire sample of burst-only sources with the
Swift satellite. We found a few sources in outburst and detect faint candidates
likely representing their quiescent counterparts. In addition, we observed
three quasi-persistent faint X-ray binary transients. Finally we discuss
burst-only sources and quasi-persistent sources in the framework of neutron
star transients.Comment: 9 pages, 8 figures. Accepted for publication on Ap
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
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