43 research outputs found
A search for the superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536
Burst oscillations are brightness asymmetries that develop in the burning
ocean during thermonuclear bursts on accreting neutron stars. They have been
observed during H/He-triggered (Type I) bursts and Carbon-triggered
superbursts. The mechanism responsible is not unknown, but the dominant burst
oscillation frequency is typically within a few Hz of the spin frequency, where
this is independently known. One of the best-studied burst oscillation sources,
4U 1636-536, has oscillations at in both its regular Type I
bursts and in one superburst. Recently however, Strohmayer \& Mahmoodifar
reported the discovery of an additional signal at a higher frequency,
, during the superburst. This higher frequency is consistent
with the predictions for several types of global ocean mode, one of the
possible burst oscillation mechanisms. If this is the case then the same
physical mechanism may operate in the normal Type I bursts of this source. In
this paper we report a stacked search for periodic signals in the regular Type
I bursts: we found no significant signal at the higher frequency, with upper
limits for the single trial root mean square (rms) fractional amplitude of
0.57(6)\%. Our analysis did however reveal that the dominant
burst oscillation signal is present at a weak level even in the sample of
bursts where it cannot be detected in individual bursts. This indicates that
any cutoff in the burst oscillation mechanism occurs below the detection
threshold of existing X-ray telescopes.Comment: 6 pages, 2 figures. Accepted for publication by Ap
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
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
Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27
The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in
2015. After the source transited to quiescence, we triggered our multi-year
Chandra monitoring programme to study its quiescent behaviour. The programme
was designed to follow the cooling of a potentially heated neutron-star crust
due to accretion of matter during the preceding outburst, similar to what we
potentially have observed before in two other Be/X-ray transients, namely 4U
0115+63 and V 0332+53. However, unlike for these other two systems, we do not
find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed
heated during the accretion phase. We detected the source at a rather low X-ray
luminosity (~10^33 erg/s) during only three of our five observations. When the
source was not detected it had very low-luminosity upper limits (<10^32 erg/s;
depending on assumed spectral model). We interpret these detections and the
variability observed as emission likely due to very low-level accretion onto
the neutron star. We also discuss why the neutron-star crust in GRO J1750-27
might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly
were.Comment: 13 pages, 6 figures, 5 tables. Accepted for A&
Unexpected late-time temperature increase observed in two neutron star crust cooling sources -- XTE~J1701-462 and EXO~0748-676
Transient LMXBs that host neutron stars (NSs) provide excellent laboratories
for probing the dense matter physics present in NS crusts. During accretion
outbursts in LMXBs, exothermic reactions may heat the NS crust, disrupting the
crust-core equilibrium. When the outburst ceases, the crust cools to restore
thermal equilibrium with the core. Monitoring this evolution allows us to probe
the dense matter physics in the crust. Properties of the deeper crustal layers
can be probed at later times after the end of the outburst. We report on the
unexpected late-time temperature evolution (>2000 days after the end of their
outbursts) of two NSs in LMXBs, XTE J1701-462 and EXO 0748-676. Although both
these sources exhibited very different outbursts (in terms of duration and the
average accretion rate), they exhibit an unusually steep decay of ~7 eV in the
observed effective temperature (occurring in a time span of ~700 days) around
~2000 days after the end of their outbursts. Furthermore, they both showed an
even more unexpected rise of ~3 eV in temperature (over a time period of
~500-2000 days) after this steep decay. This rise was significant at the
2.4{\sigma} and 8.5{\sigma} level for XTE J1701-462 and EXO 0748-676,
respectively. The physical explanation for such behaviour is unknown and cannot
be straightforwardly be explained within the cooling hypothesis. In addition,
this observed evolution cannot be well explained by low-level accretion either
without invoking many assumptions. We investigate the potential pathways in the
theoretical heating and cooling models that could reproduce this unusual
behaviour, which so far has been observed in two crust-cooling sources. Such a
temperature increase has not been observed in the other NS crust-cooling
sources at similarly late times, although it cannot be excluded that this might
be a result of the inadequate sampling obtained at such late times.Comment: accepted for publication by A&A letter
Consistent accretion-induced heating of the neutron-star crust in MXB 1659-29 during two different outbursts
Monitoring the cooling of neutron-star crusts heated during accretion
outbursts allows us to infer the physics of the dense matter present in the
crust. We examine the crust cooling evolution of the low-mass X-ray binary MXB
1659-29 up to ~505 days after the end of its 2015 outburst (hereafter outburst
II) and compare it with what we observed after its previous 1999 outburst
(hereafter outburst I) using data obtained from the Swift, XMM-Newton, and
Chandra observatories. The observed effective surface temperature of the
neutron star in MXB 1659-29 dropped from ~92 eV to ~56 eV from ~12 days to ~505
days after the end of outburst II. The most recently performed observation
after outburst II suggests that the crust is close to returning to thermal
equilibrium with the core. We model the crust heating and cooling for both its
outbursts collectively to understand the effect of parameters that may change
for every outburst (e.g., the average accretion rate, the length of outburst,
the envelope composition of the neutron star at the end of the outburst) and
those which can be assumed to remain the same during these two outbursts (e.g.,
the neutron star mass, its radius). Our modelling indicates that all parameters
were consistent between the two outbursts with no need for any significant
changes. In particular, the strength and the depth of the shallow heating
mechanism at work (in the crust) were inferred to be the same during both
outbursts, contrary to what has been found when modelling the cooling curves
after multiple outburst of another source, MAXI J0556-332. This difference in
source behaviour is not understood. We discuss our results in the context of
our current understanding of cooling of accretion-heated neutron-star crusts,
and in particular with respect to the unexplained shallow heating mechanism.Comment: Submitted to A&A. The supplementary video can be found at
https://www.youtube.com/watch?v=OpJ053zq9-
Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63
In 2017, the Be/X-ray transient 4U 0115+63 exhibited a new type-II outburst
that was two times fainter than its 2015 giant outburst (in the Swift/BAT count
rates). Despite this difference between the two bright events, the source
displayed similar X-ray behaviour after these periods. Once the outbursts
ceased, the source did not transit towards quiescence directly, but was
detected about a factor of 10 above its known quiescent level. It eventually
decayed back to quiescence over time scales of months. In this paper we present
the results of our Swift monitoring campaign, and an XMM-Newton observation of
4U 0115+63 during the decay of the 2017 type-II outburst, and its subsequent
low-luminosity behaviour. We discuss the possible origin of the decaying source
emission at this low-level luminosity, which has now been shown as a recurrent
phenomenon, in the framework of the two proposed scenarios to explain this
faint state: cooling from an accretion-heated neutron-star crust or continuous
low-level accretion. In addition, we compare the outcome of our study with the
results we obtained from the 2015/2016 monitoring campaign on this source.Comment: 12 pages, 5 figures, 3 tables. Accepted, Astronomy & Astrophysic
Continued cooling of the accretion-heated neutron star crust in the X-ray transient IGR J17480-2446 located in the globular cluster Terzan 5
We present a new Chandra observation (performed in July 2016) of the neutron
star X-ray transient IGR J17480-2446, located in the globular cluster Terzan 5.
We study the continued cooling of the neutron star crust in this system that
was heated during the 2010 outburst of the source. This new observation was
performed two years after the last observation of IGR J17480-2446, hence,
significantly extending the cooling baseline. We reanalysed all available
Chandra observations of the source (but excluding observations during which one
of the known transients in Terzan 5 was in outburst) and fitted the obtained
cooling curve with our cooling code NSCool, which allows for much improved
modelling than what was previously performed for the source. The data and our
fit models indicate that the crust was still cooling ~5.5 years after the
outburst ended. The neutron star crust has likely not reached crust-core
thermal equilibrium yet, and further cooling is predicted (which can be
confirmed with additional Chandra observations in >5 years). Intriguingly, we
find indications that the thermal conductivity might be relatively low in part
of the crust compared to what has been inferred for other crust-cooling sources
and tentatively suggest that this layer might be located around the neutron
drip. The reason for this difference is unclear, but might be related to the
fact that IGR J17480-2446 harbours a relatively slowly rotating neutron star
(with a spin of 11 Hz) that has a relatively strong inferred surface magnetic
field ( Gauss) compared to what is known or typically assumed for
other cooling sources.Comment: 17 pages, 10 figures, 4 tables, accepted for publication in MNRA