1,838 research outputs found
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
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
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
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
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
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
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
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
- …