294 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
1RXS J180408.9-342058: an ultra compact X-ray binary candidate with a transient jet
We present a detailed NIR/optical/UV study of the transient low mass X-ray
binary 1RXS J180408.9-342058 performed during its 2015 outburst, aimed at
determining the nature of its companion star. We obtained three optical spectra
at the 2.1 m San Pedro Martir Observatory telescope (Mexico). We performed
optical and NIR photometric observations with both the REM telescope and the
New Technology Telescope (NTT) in La Silla. We obtained optical and UV
observations from the Swift archive. Finally, we performed optical polarimetry
of the source by using the EFOSC2 instrument mounted on the NTT. The optical
spectrum of the source is almost featureless since the hydrogen and He I
emissions lines, typically observed in LMXBs, are not detected. Similarly,
carbon and oxygen lines are neither observed. We marginally detect the He II
4686 AA emission line, suggesting the presence of helium in the accretion disc.
No significant optical polarisation level was observed. The lack of hydrogen
and He I emission lines in the spectrum implies that the companion is likely
not a main sequence star. Driven by the tentative detection of the He II 4686
AA emission line, we suggest that the system could harbour a helium white
dwarf. If this is the case, 1RXS J180408.9-342058 would be an ultra-compact
X-ray binary. By combining an estimate of the mass accretion rate together with
evolutionary tracks for a He white dwarf, we obtain a tentative orbital period
of ~ 40 min. On the other hand, we also built the NIR-optical-UV spectral
energy distribution (SED) of the source at two different epochs. One SED was
gathered when the source was in the soft X-ray state, and it is consistent with
the presence of a single thermal component. The second SED, obtained when the
source was in the hard X-ray state, shows a thermal component together with a
tail in the NIR, likely indicating the presence of a (transient) jet.Comment: 8 pages, 5 figures, 4 tables. Accepted for publication in Astronomy &
Astrophysics (Section 7
The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
We present an optical (gri) study during quiescence of the accreting
millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran
Telescopio Canarias (GTC) in August 2014. Despite the source being in
quiescence at the time of our observations, it showed a strong optical flaring
activity, more pronounced at higher frequencies (i.e. the g band). Once the
flares were subtracted, we tentatively recovered a sinusoidal modulation at the
system orbital period in all bands, even if a significant phase shift with
respect to an irradiated star, typical of accreting millisecond X-ray pulsars
is detected. We conclude that the observed flaring could be a manifestation of
the presence of an accretion disc in the system. The observed light curve
variability could be explained by the presence of a superhump, which might be
another proof of the formation of an accretion disc. In particular, the disc at
the time of our observations was probably preparing to the new outburst of the
source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&
Searching for supergiant fast X-ray transients with Swift
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs)
hosting a neutron star and an OB supergiant companion. We examine the available
Swift data, as well as other new or archival/serendipitous data, on three
sources: IGR J17407-2808, 2XMM J185114.3-000004, and IGR J18175-2419, whose
X-ray characteristics qualify them as candidate SFXT, in order to explore their
properties and test whether they are consistent with an SFXT nature. As IGR
J17407-2808 and 2XMM J185114.3-000004 triggered the Burst Alert Telescope on
board Swift, the Swift data allow us to provide their first arcsecond
localisations, leading to an unequivocal identification of the source CXOU
J174042.0-280724 as the soft X-ray counterpart of IGR J17407-2808, as well as
their first broadband spectra, which can be fit with models generally
describing accreting neutron stars in HMXBs. While still lacking optical
spectroscopy to assess the spectral type of the companion, we propose 2XMM
J185114.3-000004 as a very strong SFXT candidate. The nature of IGR J17407-2808
remains, instead, more uncertain. Its broad band properties cannot exclude that
the emission originates from either a HMXB (and in that case, a SFXT) or, more
likely, a low mass X-ray binary. Finally, based on the deep non-detection in
our XRT monitoring campaign and a careful reanalysis of the original Integral
data in which the discovery of the source was first reported, we show that IGR
J18175-2419 is likely a spurious detection.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages, 11
figures, 6 table
Differential lipid dependence of function of bacterial sodium channel homologues
The lipid bilayer is important for maintaining the integrity of cellular compartments and plays a vital role in maintaining the hydrophobic/charged interactions necessary for structure, conformational flexibility and function. Despite the intimate relationship between ion channels and the membranes in which they are embedded, challenges resulting from the dynamic and complex nature of cellular membranes have limited our ability to address the functional role of these interactions. To directly assess lipid dependence of activity, we examined channel function ofthree purified bacterial sodium channel orthologues (NaChBac, NavMs, and NavSp) by cumulative 22Na+ uptake into proteoliposomes containing a 3:1 ratio of POPE and another glycerophospholipid (POPC, POPG, POPS, Cardiolipin (CL), POPA, or PI). We observed a unique lipid dependence for each homologue tested. Common to each was a low level of activity above background (uptake into protein free liposomes) when the second lipid was a zwitterionic lipid such as POPE and POPC. Maximal activity for full-length NaChBac and NavMs proteins was observed in POPE + POPG liposomes. On the other hand, full-length NavSp channels possessed a different lipid dependence, with maximal activity in liposomes containing POPE + PI. No strong lipid dependence was observed for pore-only constructs of NavMs or NavSp, that lacked the S1-S4 segments, suggesting that the lipid dependence of sodium channels may arise from their abilities to affect the voltage-sensing domains. The effect may be maximized by specific lipid-protein interactions that are uniquely favourable in each homologue, giving rise to differing lipid dependences
Engulfing a radio pulsar: the case of PSR J1023+0038
The binary millisecond radio pulsar PSR J1023+0038 has been recently the
subject of multiwavelength monitoring campaigns which revealed that an
accretion disc has formed around the neutron star (since 2013 June). We present
here the results of X-ray and UV observations carried out by the Swift
satellite between 2013 October and 2014 May, and of optical and NIR
observations performed with the REM telescope, the Liverpool Telescope, the
2.1-m telescope at the San Pedro M\'artir Observatory and the 1.52-m telescope
at the Loiano observing station. The X-ray spectrum is well described by an
absorbed power law, which is softer than the previous quiescent epoch (up to
2013 June). The strong correlation between the X-ray and the UV emissions
indicates that the same mechanism should be responsible for part of the
emission in these bands. Optical and infrared photometric observations show
that the companion star is strongly irradiated. Double-peaked emission lines in
the optical spectra provide compelling evidence for the presence of an outer
accretion disc too. The spectral energy distribution from IR to X-rays is well
modelled if the contributions from the companion, the disc and the intra-binary
shock emission are all considered. Our extensive data set can be interpreted in
terms of an engulfed radio pulsar: the radio pulsar is still active, but
undetectable in the radio band due to a large amount of ionized material
surrounding the compact object. X-rays and gamma-rays are produced in an
intra-binary shock front between the relativistic pulsar wind and matter from
the companion and an outer accretion disc. The intense spin-down power
irradiates the disc and the companion star, accounting for the UV and optical
emissions.Comment: 11 pages, 8 figures, 5 tables; accepted for publication on MNRA
Multi-wavelength observations of 1RXH J173523.7-354013: revealing an unusual bursting neutron star
On 2008 May 14, the Burst Alert Telescope aboard the Swift mission triggered
on a type-I X-ray burst from the previously unclassified ROSAT object 1RXH
J173523.7-354013, establishing the source as a neutron star X-ray binary. We
report on X-ray, optical and near-infrared observations of this system. The
X-ray burst had a duration of ~2 h and belongs to the class of rare,
intermediately long type-I X-ray bursts. From the bolometric peak flux of
~3.5E-8 erg/cm^2/s, we infer a source distance of D<9.5 kpc. Photometry of the
field reveals an optical counterpart that declined from R=15.9 during the X-ray
burst to R=18.9 thereafter. Analysis of post-burst Swift/XRT observations, as
well as archival XMM-Newton and ROSAT data suggests that the system is
persistent at a 0.5-10 keV luminosity of ~2E35 (D/9.5 kpc)^2 erg/s. Optical and
infrared photometry together with the detection of a narrow Halpha emission
line (FWHM=292+/-9 km/s, EW=-9.0+/-0.4 Angstrom) in the optical spectrum
confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary.
The Halpha emission demonstrates that the donor star is hydrogen-rich, which
effectively rules out that this system is an ultra-compact X-ray binary.Comment: Accepted for publication in MNRAS, 13 pages, 6 figures, 5 table
Swift J201424.9+152930: discovery of a new deeply eclipsing binary with 491 s and 3.4 h modulations
We report on the discovery of a new X-ray pulsator, Swift J201424.9+152930
(Sw J2014). Owing to its X-ray modulation at 491 s, it was discovered in a
systematic search for coherent signals in the archival data of the Swift X-ray
Telescope. To investigate the nature of Sw J2014, we performed multi-wavelength
follow-up observations with space-borne (Swift and XMM-Newton) and ground-based
(the 1.5-m Loiano Telescope and the 3.6-m Telescopio Nazionale Galileo)
instruments. The X-ray spectrum of Sw J2014 can be described by a hard and
highly absorbed power law. The optical observations made it possible to single
out the optical counterpart to this source, which displays several variable
emission lines and total eclipses lasting ~20 min. Total eclipses of similar
length were observed also in X-rays. The study of the eclipses, allowed us to
infer a second periodicity of 3.44 h, which we interpret as the orbital period
of a close binary system. We also found that the period has not significantly
changed over a ~7 yr timespan. Based on the timing signatures of Sw J2014, and
its optical and X-ray spectral properties, we suggest that it is a close binary
hosting an accreting magnetic white dwarf. The system is therefore a
cataclysmic variable of the intermediate polar type and one of the very few
showing deep eclipses.Comment: 12 pages, 8 figures, 5 tables; minor changes to match the final MNRAS
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