616 research outputs found
Cooling of the crust in the neutron star low-mass X-ray binary MXB 1659-29
In quasi-persistent neutron star transients, long outbursts cause the neutron
star crust to be heated out of thermal equilibrium with the rest of the star.
During quiescence, the crust then cools back down. Such crustal cooling has
been observed in two quasi-persistent sources: KS 1731-260 and MXB 1659-29.
Here we present an additional Chandra observation of MXB 1659-29 in quiescence,
which extends the baseline of monitoring to 6.6 yr after the end of the
outburst. This new observation strongly suggests that the crust has thermally
relaxed, with the temperature remaining consistent over 1000 days. Fitting the
temperature cooling curve with an exponential plus constant model we determine
an e-folding timescale of 465 +/- 25 days, with the crust cooling to a constant
surface temperature of kT = 54 +/- 2 eV (assuming D=10 kpc). From this, we
infer a core temperature in the range 3.5E7-8.3E7 K (assuming D=10 kpc), with
the uncertainty due to the surface composition. Importantly, we tested two
neutron star atmosphere models as well as a blackbody model, and found that the
thermal relaxation time of the crust is independent of the chosen model and the
assumed distance.Comment: accepted for publication in ApJL, 4 pages, 1 figure
An X-ray-UV correlation in Cen X-4 during quiescence
Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is
seen to be variable on timescales from hundreds of seconds to years, suggesting
that at least in this object, low-level accretion is important during
quiescence. Here we present results from recent XMM-Newton and Swift
observations of Cen X-4, where the X-ray flux (0.5 - 10 keV) varies by a factor
of 6.5 between the brightest and faintest states. We find a positive
correlation between the X-ray flux and the simultaneous near-UV flux, where as
there is no significant correlation between the X-ray and simultaneous optical
(V, B) fluxes. This suggests that while the X-ray and UV emitting regions are
somehow linked, the optical region originates elsewhere. Comparing the
luminosities, it is plausible that the UV emission originates due to
reprocessing of the X-ray flux by the accretion disk, with the hot inner region
of the disk being a possible location for the UV emitting region. The optical
emission, however, could be dominated by the donor star. The X-ray/UV
correlation does not favour the accretion stream-impact point as the source of
the UV emission.Comment: 8 pages, 3 figures, accepted for publication in MNRA
X-ray and UV correlation in the quiescent emission of Cen X-4, evidence of accretion and reprocessing
We conducted the first long-term (60 days), multiwavelength (optical,
ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift
observations, with the goal of understanding variability in the low mass X-ray
binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all
energy bands on timescales from days to months, with the strongest quiescent
variability a factor of 22 drop in the X-ray count rate in only 4 days. The
X-ray, UV and optical (V band) emission are correlated on timescales down to
less than 110 s. The shape of the correlation is a power law with index gamma
about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere
(kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the
spectral shape remaining constant as the flux varies. Both components vary in
tandem, with each responsible for about 50% of the total X-ray flux, implying
that they are physically linked. We conclude that the X-rays are likely
generated by matter accreting down to the NS surface. Moreover, based on the
short timescale of the correlation, we also unambiguously demonstrate that the
UV emission can not be due to either thermal emission from the stream impact
point, or a standard optically thick, geometrically thin disc. The spectral
energy distribution shows a small UV emitting region, too hot to arise from the
accretion disk, that we identified as a hot spot on the companion star.
Therefore, the UV emission is most likely produced by reprocessing from the
companion star, indeed the vertical size of the disc is small and can only
reprocess a marginal fraction of the X-ray emission. We also found the
accretion disc in quiescence to likely be UV faint, with a minimal contribution
to the whole UV flux.Comment: 5 pages, 4 figures, submitted to Proc. Int. Conf. Physics at the
Magnetospheric Boundary, Geneva, Switzerland (25-28 June, 2013
Efficient interface conditions for the semi-vectorial finite-difference beam propagation method
Efficient interface conditions (EICs) are derived for the propagation equation using the slowly varying envelope approximation for the dominant electric field component. At the interface between two different media, the two lateral second derivatives in the discretized propagation equation are adapted such that the discretized modal field equation is correct up to second order in the lateral grid spacing. Since the error term is then of the order of the lateral grid spacing, our EICs are first-order EICs. These interface conditions are compared with well-known zero-order EICs derived by Stern and Kim and Ramaswamy. It is shown that the first-order EICs yield faster convergence to the exact effective index value as the lateral grid spacing is decreased than do the zero-order EICs. It turns out that our EICs are very much like those derived by Vassallo. Using essentially the same method, he derived EICs of second and first order for the field component respectively parallel and perpendicular, to the interface. Hence the accuracy of his EICs is one order higher for the field component parallel to the interface, although it introduces an extra asymmetry in the propagation matrix
Daily, multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence
We conducted the first long-term (60 days), multiwavelength (optical,
ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift
observations from June to August 2012, with the goal of understanding
variability in the low mass X-ray binary Cen X-4 during quiescence. We found
Cen X-4 to be highly variable in all energy bands on timescales from days to
months, with the strongest quiescent variability a factor of 22 drop in the
X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission
are correlated on timescales down to less than 110 s. The shape of the
correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum
is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with
spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as
the flux varies. Both components vary in tandem, with each responsible for
about 50% of the total X-ray flux, implying that they are physically linked. We
conclude that the X-rays are likely generated by matter accreting down to the
NS surface. Moreover, based on the short timescale of the correlation, we also
unambiguously demonstrate that the UV emission can not be due to either thermal
emission from the stream impact point, or a standard optically thick,
geometrically thin disc. The spectral energy distribution shows a small UV
emitting region, too hot to arise from the accretion disk, that we identified
as a hot spot on the companion star. Therefore, the UV emission is most likely
produced by reprocessing from the companion star, indeed the vertical size of
the disc is small and can only reprocess a marginal fraction of the X-ray
emission. We also found the accretion disc in quiescence to likely be UV faint,
with a minimal contribution to the whole UV flux.Comment: 19 pages, 6 figures, 4 table
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
Kilohertz QPOs in Neutron Star Binaries modeled as Keplerian Oscillations in a Rotating Frame of Reference
Since the discovery of kHz quasi-periodic oscillations (QPO) in neutron star
binaries, the difference between peak frequencies of two modes in the upper
part of the spectrum, i.e. Delta (omega)=omega_h-omega_K has been studied
extensively. The idea that the difference Delta(omega) is constant and (as a
beat frequency) is related to the rotational frequency of the neutron star has
been tested previously. The observed decrease of Delta(omega) when omega_h and
omega_k increase has weakened the beat frequency interpretation. We put forward
a different paradigm: a Keplerian oscillator under the influence of the
Coriolis force. For such an oscillator, omega_h and the assumed Keplerian
frequency omega_k hold an upper hybrid frequency relation:
omega^2_h-omega^2_K=4*Omega^2, where Omega is the rotational frequency of the
star's magnetosphere near the equatorial plane. For three sources (Sco X-1, 4U
1608-52 and 4U 1702-429), we demonstrate that the solid body rotation
Omega=Omega_0=const. is a good first order approximation. Within the second
order approximation, the slow variation of Omega as a function of omega_K
reveals the structure of the magnetospheric differential rotation. For Sco X-1,
the QPO have frequencies approximately 45 and 90 Hz which we interpret as the
1st and 2nd harmonics of the lower branch of the Keplerian oscillations for the
rotator with vector Omega not aligned with the normal of the disk: omega_L/2
pi=(Omega/pi)(omega_K/omega_h)sin(delta) where delta is the angle between
vector Omega and the vector normal to the disk.Comment: 13 pages, 3 figures, accepted for publications in ApJ Letter
Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that
exhibits an accretion outburst approximately once every year. Whether the
thermal X-rays detected in intervening quiescent episodes are the result of
cooling of the neutron star or due to continued low-level accretion remains
unclear. In this work we use Swift data obtained after the long and bright 2011
and 2013 outbursts, as well as the short and faint 2015 outburst, to
investigate the hypothesis that cooling of the accretion-heated neutron star
crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that
the X-ray light curves and measured neutron star surface temperatures are
consistent with the expectations of the crust cooling paradigm. By using a
thermal evolution code, we find that ~1.2-3.2 MeV/nucleon of shallow heat
release describes the observational data well, depending on the assumed
mass-accretion rate and temperature of the stellar core. We find no evidence
for varying strengths of this shallow heating after different outbursts, but
this could be due to limitations of the data. We argue that monitoring Aql X-1
for up to ~1 year after future outbursts can be a powerful tool to break model
degeneracies and solve open questions about the magnitude, depth and origin of
shallow heating in neutron star crusts.Comment: 14 pages, 5 figures, 3 tables, accepted to MNRA
A XMM-Newton observation during the 2000 outburst of SAX J1808.4-3658
I present a XMM-Newton observation of the accretion driven millisecond X-ray
pulsar SAX J1808.4-3658 during its 2000 outburst. The source was conclusively
detected, albeit at a level of only ~2 x 10^{32} erg/s. The source spectrum
could be fitted with a power-law model (with a photon index of ~2.2), a neutron
star atmosphere model (with a temperature of ~0.2 keV), or with a combination
of a thermal (either a black-body or an atmosphere model) and a power-law
component. During a XMM-Newton observation taken approximately one year later,
the source was in quiescence and its luminosity was a factor of ~4 lower. It is
possible that the source spectrum during the 2000 outburst was softer than its
quiescent 2001 spectrum, however, the statistics of the data do not allow to
make a firm conclusion. The results obtained are discussed in the context of
the 2000 outburst of SAX J1808.4-3658 and the quiescent properties of the
source.Comment: Accepted for publication in ApJ, 15 January 200
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
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