359 research outputs found
The return to quiescence of Aql X-1 following the 2010 outburst
Aql X-1 is the most prolific low mass X-ray binary transient hosting a
neutron star. In this paper we focus on the return to quiescence following the
2010 outburst of the source. This decay was monitored thanks to 11 pointed
observations taken with XMM-Newton, Chandra and Swift. The decay from outburst
to quiescence is very fast, with an exponential decay characteristic time scale
of ~2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with
no further signs of variability or decay. The comparison with the only other
well-monitored outburst from Aql X-1 (1997) is tail-telling. The luminosities
at which the fast decay starts are fully compatible for the two outbursts,
hinting at a mechanism intrinsic to the system and possibly related to the
neutron star rotation and magnetic field (i.e., the propeller effect). In
addition, for both outbursts, the decay profiles are also very similar, likely
resulting from the shut-off of the accretion process onto the neutron star
surface. Finally, the quiescent neutron star temperatures at the end of the
outbursts are well consistent with one another, suggesting a hot neutron star
core dominating the thermal balance. Small differences in the quiescent X-ray
luminosity among the two outbursts can be attributed to a different level of
the power law component.Comment: MNRAS accepted (4 figures and 6 tables
Probing the Crust of the Neutron Star in EXO 0748-676
X-ray observations of quiescent X-ray binaries have the potential to provide
insight into the structure and the composition of neutron stars. EXO 0748-676
had been actively accreting for over 24 yr before its outburst ceased in late
2008. Subsequent X-ray monitoring revealed a gradual decay of the quiescent
thermal emission that can be attributed to cooling of the accretion-heated
neutron star crust. In this work, we report on new Chandra and Swift
observations that extend the quiescent monitoring to ~5 yr post-outburst. We
find that the neutron star temperature remained at ~117 eV between 2009 and
2011, but had decreased to ~110 eV in 2013. This suggests that the crust has
not fully cooled yet, which is supported by the lower temperature of ~95 eV
that was measured ~4 yr prior to the accretion phase in 1980. Comparing the
data to thermal evolution simulations reveals that the apparent lack of cooling
between 2009 and 2011 could possibly be a signature of convection driven by
phase separation of light and heavy nuclei in the outer layers of the neutron
star.Comment: 9 pages, 4 tables, 3 figures. Minor revisions according to referee
report. Accepted to Ap
Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing
binary EXO 0748-676 started a transition from outburst to quiescence, after it
had been actively accreting for more than 24 years. In a previous work, we
discussed Chandra and Swift observations obtained during the first five months
after this transition. Here, we report on further X-ray observations of EXO
0748-676, extending the quiescent monitoring to 1.6 years. Chandra and
XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal
component that is well-fitted by a neutron star atmosphere model. An additional
hard powerlaw tail is detected that changes non-monotonically over time,
contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux.
The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal
bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the
inferred neutron star effective temperature decreases from ~124 to 109 eV. We
interpret the observed decay as cooling of the neutron star crust and show that
the fractional quiescent temperature change of EXO 0748-676 is markedly smaller
than observed for three other neutron star X-ray binaries that underwent
prolonged accretion outbursts.Comment: Moderate textual revisions according to referee report, accepted for
publication in MNRA
Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
The quasi-persistent neutron star X-ray transient and eclipsing binary EXO
0748-676 recently started the transition to quiescence following an accretion
outburst that lasted more than 24 years. We report on two Chandra and twelve
Swift observations performed within five months after the end of the outburst.
The Chandra spectrum is composed of a soft, thermal component that fits to a
neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw
tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The
combined Chandra/Swift data set reveals a relatively hot and luminous quiescent
system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal
luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the
context of cooling neutron star models.Comment: Accepted for publication in MNRAS Letters, moderate revision
according to referee report, added one plot to figure 2 and included new
Swift observations, 5 pages, 2 figure
Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?
The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR
J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion
outburst in 2010. Chandra observations performed within five months after the
end of the outburst revealed evidence that the crust of the neutron star became
substantially heated during the accretion episode and was subsequently cooling
in quiescence. This provides the rare opportunity to probe the structure and
composition of the crust. Here, we report on new Chandra observations of Terzan
5 that extend the monitoring to ~2.2 yr into quiescence. We find that the
thermal flux and neutron star temperature have continued to decrease, but
remain significantly above the values that were measured before the 2010
accretion phase. This suggests that the crust has not thermally relaxed yet,
and may continue to cool. Such behavior is difficult to explain within our
current understanding of heating and cooling of transiently accreting neutron
stars. Alternatively, the quiescent emission may have settled at a higher
observed equilibrium level (for the same interior temperature), in which case
the neutron star crust may have fully cooled.Comment: Accepted to ApJ without revision. Updated references and fixed few
typos to match published version. 7 pages, 3 figures, 3 table
Studies of orbital parameters and pulse profile of the accreting millisecond pulsar XTE J1807-294
The accreting millisecond pulsar XTE J1807-294 was observed by XMM-Newton on
March 22, 2003 after its discovery on February 21, 2003 by RXTE. The source was
detected in its bright phase with an observed average count rate of 33.3 cts/s
in the EPIC-pn camera in the 0.5-10 keV energy band (3.7 mCrab). Using the
earlier established best-fit orbital period of 40.0741+/-0.0005 minutes from
RXTE observations and considering a circular binary orbit as first
approximation, we derived a value of 4.8+/-0.1 lt-ms for the projected orbital
radius of the binary system and an epoch of the orbital phase of MJD
52720.67415(16). The barycentric mean spin period of the pulsar was derived as
5.2459427+/-0.0000004 ms. The pulsar's spin-pulse profile showed a prominent
(1.5 ms FWHM) pulse, with energy and orbital phase dependence in the amplitude
and shape. The measured pulsed fraction in four energy bands was found to be
3.1+/-0.2 % (0.5-3.0 keV), 5.4+/-0.4 % (3.0-6.0 keV), 5.1+/-0.7 % (6.0-10.0
keV) and 3.7+/-0.2 % (0.5-10.0 keV), respectively. Studies of spin-profiles
with orbital phase and energy showed significant increase in its pulsed
fraction during the second observed orbit of the neutron star, gradually
declining in the subsequent two orbits, which was associated with sudden but
marginal increase in mass accretion. From our investigations of orbital
parameters and estimation of other properties of this compact binary system, we
conclude that XTE J1807-294 is very likely a candidate for a millisecond radio
pulsar.Comment: 4 pages, 4 figures, Accepted for publication in Astronomy and
Astrophysics letter
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
X-ray variability during the quiescent state of the neutron-star X-ray transient in the globular cluster NGC 6440
The globular cluster NGC 6440 is known to harbor a bright neutron-star X-ray
transient. We observed the globular cluster with Chandra on two occasions when
the bright transient was in its quiescent state in July 2000 and June 2003
(both observations were made nearly 2 years after the end of their preceding
outbursts). The quiescent spectrum during the first observation is well
represented by a two component model (a neutron-star atmosphere model plus a
power-law component which dominates at energies above 2 keV). During the second
observation (which was roughly of equal duration to the first observation) we
found that the power-law component could no longer be detected. Our spectral
fits indicate that the effective temperature of the neutron-star surface was
consistent between the two observations. We conclude that the effect of the
change in power-law component caused the 0.5-10 keV flux to be a factor of ~2
lower during the second observation compared to the first observation. We
discuss plausible explanations for the variations, including variable residual
accretion onto the neutron star magnetosphere or some variation in the
interaction of the pulsar wind with the matter still outflowing from the
companion star.Comment: 18 pages, 3 color figs, 1 b&w figures, 3 tables; discussion expanded;
accepted for publication in Ap
High frequency quasi-periodic oscillations in the black hole X-ray transient XTE J1650-500
We report the detection of high frequency variability in the black hole X-ray
transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz
during a transition from the hard to the soft state. We also detected less
coherent variability around 50 Hz, that disappeared when the 250 Hz QPO showed
up. There are indications that when the energy spectrum hardened the QPO
frequency increased from ~110 Hz to ~270 Hz, although the observed frequencies
are also consistent with being 1:2:3 harmonics of each other. Interpreting the
250 Hz as the orbital frequency at the innermost stable orbit around a
Schwarzschild black hole leads to a mass estimate of 8.2 Msun. The spectral
results by Miller et al.(2002, ApJ, 570, L69), which suggest considerable black
hole spin, would imply a higher mass.Comment: Submitted to ApJ, 12 pages including 2 figure
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