1,372 research outputs found
A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900
We report on a series of Swift/XRT observations, performed between February
and 22 March 2012, during the quiescent state of the neutron-star X-ray binary
SAX J1750.8-2900. In these observations, the source was either just detected or
undetected, depending on the exposure length (which ranged from ~0.3 to ~3.8
ks). The upper limits for the non-detections were consistent with the detected
luminosities (when fitting a thermal model to the spectrum) of ~1E34 erg/s
(0.5-10 keV). This level is consistent with what has been measured previously
for this source in quiescence. However, on March 17 the source was found to
have an order of magnitude larger count rate. When fitting the flare spectrum
with an absorbed power-law model, we obtained a flare luminosity of (3-4) 1E34
erg/s (0.5-10 keV). Follow-up Swift observations showed that this flare lasted
<16 days. This event was very likely due to a brief episode of low-level
accretion onto the neutron star and provides further evidence that the
quiescent state of neutron-star X-ray transients might not be as quiet as is
generally assumed. The detection of this low-level accretion flare raises the
question whether the quiescent emission of the source (outside the flare) could
also be due to residual accretion, albeit continuous instead of episodic.
However, we provide arguments which would suggest that the lowest intensity
level might instead represent the cooling of the accretion-heated neutron star.Comment: Accepted for publication in MNRAS Main Journal on June 18th, 2013.
Minor changes to the original submission to incorporate the comments of the
refere
A peculiar thermonuclear X-ray burst from the transiently accreting neutron star SAX J1810.8-2609
We report on a thermonuclear (type-I) X-ray burst that was detected from the
neutron star low-mass X-ray binary SAX J1810.8-2609 in 2007 with Swift. This
event was longer (~20 min) and more energetic (a radiated energy of Eb~6.5E39
erg) than other X-ray bursts observed from this source. A possible explanation
for the peculiar properties is that the X-ray burst occurred during the early
stage of the outburst when the neutron star was relatively cold, which allows
for the accumulation of a thicker layer of fuel. We also report on a new
accretion outburst of SAX J1810.8-2609 that was observed with MAXI and Swift in
2012. The outburst had a duration of ~17 days and reached a 2-10 keV peak
luminosity of Lx~3E37(D/5.7kpc)^2 erg/s. This is a factor >10 more luminous
than the two previous outbursts observed from the source, and classifies it as
a bright rather than a faint X-ray transient.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and
Opportunities after 80 years", J. van Leeuwen (ed.); 4 pages, 1 figure, 2
tables, contributed tal
Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63
The Be/X-ray transients V0332+53 and 4U 0115+63 exhibited giant, type-II
outbursts in 2015. Here we present Swift/XRT follow-up observations at the end
of those outbursts. Surprisingly, the sources did not decay back to their known
quiescent levels but stalled at a (slowly decaying) meta-stable state with
luminosities ~10 times that observed in quiescence. The spectra in these states
are considerably softer than the outburst spectra and appear to soften in time
when the luminosity decreases. The physical mechanism behind these meta-stable
states is unclear and they could be due to low-level accretion (either directly
onto the neutron stars or onto their magnetospheres) or due to cooling of the
accretion-heated neutron star crusts. Based on the spectra, the slowly
decreasing luminosities, and the spectral softening, we favour the crust
cooling hypothesis but we cannot exclude the accretion scenarios. On top of
this meta-stable state, weak accretion events were observed that occurred at
periastron passage and may thus be related to regular type-I outbursts.Comment: Accepted for publication in MNRAS Letter
Cooling of Accretion-Heated Neutron Stars
We present a brief, observational review about the study of the cooling
behaviour of accretion-heated neutron stars and the inferences about the
neutron-star crust and core that have been obtained from these studies.
Accretion of matter during outbursts can heat the crust out of thermal
equilibrium with the core and after the accretion episodes are over, the crust
will cool down until crust-core equilibrium is restored. We discuss the
observed properties of the crust cooling sources and what has been learned
about the physics of neutron-star crusts. We also briefly discuss those systems
that have been observed long after their outbursts were over, i.e, during times
when the crust and core are expected to be in thermal equilibrium. The surface
temperature is then a direct probe for the core temperature. By comparing the
expected temperatures based on estimates of the accretion history of the
targets with the observed ones, the physics of neutron-star cores can be
investigated. Finally, we discuss similar studies performed for strongly
magnetized neutron stars in which the magnetic field might play an important
role in the heating and cooling of the neutron stars.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue
on 'Physics of Neutron Stars and Related Objects', celebrating the 75th
birth-year of G. Srinivasan. In case of missing sources and/or references in
the tables, please contact the first author and they will be included in
updated versions of this revie
The X-ray spectral properties of very-faint persistent neutron star X-ray binaries
AX J1754.2-2754, 1RXS J171824.2-402934 and 1RXH J173523.7-354013 are three
persistent neutron star low-mass X-ray binaries that display a 2--10 keV
accretion luminosity Lx of only (1-10)x1E34 erg s-1 (i.e., only ~0.005-0.05 %
of the Eddington limit). The phenomenology of accreting neutron stars which
accrete at such low accretion rates is not yet well known and the reason why
they have such low accretion rates is also not clear. Therefore, we have
obtained XMM-Newton data of these three sources and here we report our analysis
of the high-quality X-ray spectra we have obtained for them. We find that AX
J1754.2-2754 has Lx~1E35 erg s-1, while the other two have X-ray luminosities
about an order of magnitude lower. However, all sources have a similar,
relatively soft, spectrum with a photon index of 2.3-2.5, when the spectrum is
fitted with an absorbed power-law model. This model fits the data of AX
J1754.2-2754 adequately, but it cannot fit the data obtained for 1RXS
J171824.2-402934 and 1RXH J173523.7-354013. For those sources a clear soft
thermal component is needed to fit their spectra. This soft component
contributes 40% - 50% to the 0.5-10 keV flux of the sources. When including
this additional spectral component, the power-law photon indices are
significantly lower. It can be excluded that a similar component with similar
contributions to the 2-10 keV X-ray flux is present for AX J1754.2-2754,
indicating that the soft spectrum of this source is mostly due to the fact that
the power-law component itself is not hard. We note that we cannot excluded
that weaker soft component is present in the spectrum of this source which only
contributes up to ~25% to the 0.5-10 keV X-ray flux. We discuss our results in
the context of what is known of accreting neutron stars at very low accretion
rate.Comment: 9 pages, 2 tables, 1 figure. Aceppted for publication in MNRA
XMM-Newton and Swift spectroscopy of the newly discovered very-faint X-ray transient IGR J17494-3030
A growing group of low-mass X-ray binaries are found to be accreting at
very-faint X-ray luminosities of <1E36 erg/s (2-10 keV). Once such system is
the new X-ray transient IGR J17494-3030. We present Swift and XMM-Newton
observations obtained during its 2012 discovery outburst. The Swift
observations trace the peak of the outburst, which reached a luminosity of ~7
E35 (D/8 kpc)^2 erg/s (2-10 keV). The XMM-Newton data were obtained when the
outburst had decayed to an intensity of ~ 8 E34 (D/8 kpc)^2 erg/s. The spectrum
can be described by a power-law with an index of ~1.7 and requires an
additional soft component with a black-body temperature of ~0.37 keV
(contributing ~20% to the total unabsorbed flux in the 0.5-10 keV band). Given
the similarities with high-quality spectra of very-faint neutron star low-mass
X-ray binaries, we suggest that the compact primary in IGR J17494-3030 is a
neutron star. Interestingly, the source intensity decreased rapidly during the
~12 hr XMM-Newton observation, which was accompanied by a decrease in inferred
temperature. We interpret the soft spectral component as arising from the
neutron star surface due to low-level accretion, and propose that the observed
decline in intensity was the result of a decrease in the mass-accretion rate
onto the neutron star.Comment: 3 figures, 2 tables, accepted in MNRAS letter, in pres
The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421
SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary.
Albeit discovered when it exhibited a ~10-s long thermonuclear X-ray burst, it
had faded to a 0.5-10 keV luminosity of Lx<8E32 (D/7.1 kpc)^2 erg/s only ~8 hr
later. It is generally assumed that neutron stars are quiescent (i.e., not
accreting) at such an intensity, raising questions about the trigger conditions
of the X-ray burst and the origin of the faint persistent emission. We report
on a ~51 ks XMM-Newton observation aimed to find clues explaining the unusual
behavior of SAX J2224.9+5421. We identify a likely counterpart that is detected
at Lx~5E31 (D/7.1 kpc)^2 erg/s (0.5-10 keV) and has a soft X-ray spectrum that
can be described by a neutron star atmosphere model with a temperature of ~50
eV. This would suggest that SAX J2224.9+5421 is a transient source that was in
quiescence during our XMM-Newton observation and experienced a very faint
(ceasing) accretion outburst at the time of the X-ray burst detection. We
consider one other potential counterpart that is detected at Lx~5E32 (D/7.1
kpc)^2 erg/s and displays an X-ray spectrum that is best described by power law
with a photon index of ~1.7. Similarly hard X-ray spectra are seen for a few
quiescent neutron stars and may be indicative of a relatively strong magnetic
field or the occurrence of low-level accretion.Comment: 7 pages, 2 figures, 2 tables. Accepted to Ap
A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267
The heating and cooling of transiently accreting neutron stars provides a
powerful probe of the structure and composition of their crust. Observations of
superbursts and crust cooling of accretion-heated neutron stars require more
heat release than is accounted for in current models. Obtaining firm
constraints on the depth and magnitude of this extra heat is challenging and
therefore its origin remains uncertain. We report on Swift and XMM-Newton
observations of the transient neutron star low-mass X-ray binary XTE J1709-267,
which were made in 2012 September-October when it transitioned to quiescence
after a ~10-week long accretion outburst. The source is detected with
XMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-ray
spectrum consists of a thermal component that fits to a neutron star atmosphere
model and a non-thermal emission tail, which each contribute ~50% to the total
emission. The neutron star temperature decreases from ~158 to ~152 eV during
the ~8-hour long observation. This can be interpreted as cooling of a crustal
layer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutron
star), which is just below the ignition depth of superbursts. The required heat
generation in the layers on top would be ~0.06-0.13 MeV per accreted nucleon.
The magnitude and depth rule out electron captures and nuclear fusion reactions
as the heat source, but it may be accounted for by chemical separation of light
and heavy nuclei. Low-level accretion offers an alternative explanation for the
observed variability.Comment: 6 pages, 4 figures, 1 table, accepted to ApJ Letters. Minor changes
according to referee report, revised version includes a discussion on the
alternative interpretation of residual accretio
Hard state neutron star and black hole X-ray binaries in the radio:X-ray luminosity plane
Motivated by the large body of literature around the phenomenological
properties of accreting black hole (BH) and neutron star (NS) X-ray binaries in
the radio:X-ray luminosity plane, we carry out a comparative regression
analysis on 36 BHs and 41 NSs in hard X-ray states, with data over 7 dex in
X-ray luminosity for both. The BHs follow a radio to X-ray (logarithmic)
luminosity relation with slope , consistent with the NSs'
slope () within 2.5. The best-fitting
intercept for the BHs significantly exceeds that for the NSs, cementing BHs as
more radio loud, by a factor 22. \This discrepancy can not be fully
accounted for by the mass or bolometric correction gap, nor by the NS boundary
layer contribution to the X-rays, and is likely to reflect physical differences
in the accretion flow efficiency, or the jet powering mechanism. Once
importance sampling is implemented to account for the different luminosity
distributions, the slopes of the non-pulsating and pulsating NS subsamples are
formally inconsistent (), unless the transitional millisecond pulsars
(whose incoherent radio emission mechanism is not firmly established) are
excluded from the analysis. We confirm the lack of a robust partitioning of the
BH data set into separate luminosity tracks.Comment: Accepted by MNRAS as a Lette
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