175 research outputs found
A study of the new X-ray transient RXTE J2123-058 during its post-outburst state
We carried out I, R, V and B photometric observations of the neutron star X-ray binary RXTE J2123â058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low-mass binary model to interpret our observations. After folding our data on the 0.24 821-d orbital period, and correcting for the steady brightness decline following the outburst, we observed sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations
The 2015 outburst of the accretion-powered pulsar IGR J00291+5934: INTEGRAL and Swift observations
The pulsar IGR J00291 + 5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on INTEGRAL and Swift observations during the 2015 outburst, which lasts for similar to 25 d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1-250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of NH4 approximate to 10(21) cm(-2), a plasma temperature of kT(e) approximate to 50 keV, and a Thomson optical depth of tau(T) approximate to 1. Pulsations at the known spin period of the source are detected in the INTEGRAL data up to the similar to 150 keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291 + 5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly 1 : 6% of the Eddington accretion rate. The properties of the burst suggest it is powered primarily by helium ignited at a depth of y(ign) approximate to 1.5 x 10(9) g cm(-2) following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first similar to 20 s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of d = 4 : 2 +/- 0.5 kpc
Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered
that disk-accreting neutron stars with weak magnetic fields produce three
distinct types of high-frequency X-ray oscillations. These oscillations are
powered by release of the binding energy of matter falling into the strong
gravitational field of the star or by the sudden nuclear burning of matter that
has accumulated in the outermost layers of the star. The frequencies of the
oscillations reflect the orbital frequencies of gas deep in the gravitational
field of the star and/or the spin frequency of the star. These oscillations can
therefore be used to explore fundamental physics, such as strong-field gravity
and the properties of matter under extreme conditions, and important
astrophysical questions, such as the formation and evolution of millisecond
pulsars. Observations using RossiXTE have shown that some two dozen neutron
stars in low-mass X-ray binary systems have the spin rates and magnetic fields
required to become millisecond radio-emitting pulsars when accretion ceases,
but that few have spin rates above about 600 Hz. The properties of these stars
show that the paucity of spin rates greater than 600 Hz is due in part to the
magnetic braking component of the accretion torque and to the limited amount of
angular momentum that can be accreted in such systems. Further study will show
whether braking by gravitational radiation is also a factor. Analysis of the
kilohertz oscillations has provided the first evidence for the existence of the
innermost stable circular orbit around dense relativistic stars that is
predicted by strong-field general relativity. It has also greatly narrowed the
possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to
appear in "Short-period Binary Stars: Observation, Analyses, and Results",
eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer,
http://www.springerlink.com
EXO 0748-676 Rules out Soft Equations of State for Neutron Star Matter
The interiors of neutron stars contain matter at very high densities, in a
state that differs greatly from those found in the early universe or achieved
at terrestrial experiments. Matter in these conditions can only be probed
through astrophysical observations that measure the mass and radius of neutron
stars with sufficient precision. Here I report for the first time a unique
determination of the mass and radius of the neutron star EXO 0748-676, which
appears to rule out all the soft equations of state of neutron star matter. If
this object is typical, then condensates and unconfined quarks do not exist in
the centers of neutron stars.Comment: To appear in Nature, press embargo until publicatio
Broadband X-ray spectra and timing of the accreting millisecond pulsar Swift J1756.9-2508 during its 2018 and 2019 outbursts
The accreting millisecond X-ray pulsar Swift J1756.9-2508 launched into an outburst in April 2018 and June 2019 - 8.7 years after the previous period of activity. We investigated the temporal, timing, and spectral properties of these two outbursts using data from NICER, XMM-Newton, NuSTAR, INTEGRAL, Swift, and Insight-HXMT. The two outbursts exhibited similar broadband spectra and X-ray pulse profiles. For the first time, we report the detection of the pulsed emission up to âŒ100 keV that was observed by Insight-HXMT during the 2018 outburst. We also found the pulsation up to âŒ60 keV that was observed by NICER and NuSTAR during the 2019 outburst. We performed a coherent timing analysis combining the data from the two outbursts. The binary system is well described by a constant orbital period over a time span of âŒ12 years. The time-averaged broadband spectra are well fitted by the absorbed thermal Comptonization model COMPPS in a slab geometry with an electron temperature, kTe=40-50 keV, Thomson optical depth Ï âŒ 1.3, blackbody seed photon temperature kTbb, seed  âŒ0.7-0.8 keV, and hydrogen column density of NH ⌠4.2x1022 cm-2. We searched the available data for type-I (thermonuclear) X-ray bursts, but found none, which is unsurprising given the estimated low peak accretion rate (â0.05 of the Eddington rate) and generally low expected burst rates for hydrogen-poor fuel. Based on the history of four outbursts to date, we estimate the long-term average accretion rate at roughly 5x10-12 Mâ yr-1 for an assumed distance of 8 kpc. The expected mass transfer rate driven by gravitational radiation in the binary implies the source may be no closer than 4 kpc. Swift J1756.9-2508 is the third low mass X-ray binary exhibiting "double" outbursts, which are separated by much shorter intervals than what we typically see and are likely to result from interruption of the accretion flow from the disk onto the neutron star. Such behavior may have important implications for the disk instability model.</p
Liverpool Telescope follow-up of candidate electromagnetic counterparts during the first run of Advanced LIGO
The first direct detection of gravitational waves was made in late 2015 with the Advanced LIGO detectors. By prior arrangement, a worldwide collaboration of electromagnetic follow-up observers were notified of candidate gravitational wave events during the first science run, and many facilities were engaged in the search for counterparts. No counterparts were identified, which is in line with expectations given that the events were classified as black hole - black hole mergers. However these searches laid the foundation for similar follow-up campaigns in future gravitational wave detector science runs, in which the detection of neutron star merger events with observable electromagnetic counterparts is much more likely. Three alerts were issued to the electromagnetic collaboration over the course of the first science run, which lasted from September 2015 to January 2016. Two of these alerts were associated with the gravitational wave events since named GW150914 and GW151226. In this paper we provide an overview of the Liverpool Telescope contribution to the follow-up campaign over this period. Given the hundreds of square degree uncertainty in the sky position of any gravitational wave event, efficient searching for candidate counterparts required survey telescopes with large (~degrees) fields-of-view. The role of the Liverpool Telescope was to provide follow-up classification spectroscopy of any candidates. We followed candidates associated with all three alerts, observing 1, 9 and 17 candidates respectively. We classify the majority of the transients we observed as supernovae
Accreting Millisecond X-Ray Pulsars
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations
and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer;
[revision with literature updated, several typos removed, 1 new AMXP added
De Novo Designed Peptide and Protein Hairpins SelfâAssemble into Sheets and Nanoparticles
The design and assembly of peptideâbased materials has advanced considerably, leading to a variety of fibrous, sheet, and nanoparticle structures. A remaining challenge is to account for and control different possible supramolecular outcomes accessible to the same or similar peptide building blocks. Here a de novo peptide system is presented that forms nanoparticles or sheets depending on the strategic placement of a âdisulfide pinâ between two elements of secondary structure that drive selfâassembly. Specifically, homodimerizing and homotrimerizing de novo coiledâcoil αâhelices are joined with a flexible linker to generate a series of linear peptides. The helices are pinned backâtoâback, constraining them as hairpins by a disulfide bond placed either proximal or distal to the linker. Computational modeling indicates, and advanced microscopy shows, that the proximally pinned hairpins selfâassemble into nanoparticles, whereas the distally pinned constructs form sheets. These peptides can be made synthetically or recombinantly to allow both chemical modifications and the introduction of whole protein cargoes as required
Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star
The fundamental properties of neutron stars provide a direct test of the
equation of state of cold nuclear matter, a relationship between pressure and
density that is determined by the physics of the strong interactions between
the particles that constitute the star. The most straightforward method of
determining these properties is by measuring the gravitational redshift of
spectral lines produced in the neutron star photosphere. The equation of state
implies a mass-radius relation, while a measurement of the gravitational
redshift at the surface of a neutron star provides a direct constraint on the
mass-to-radius ratio. Here we report the discovery of signficant absorption
lines in the spectra of 28 bursts of the low-mass X-ray binary EXO 0748-676. We
identify the most signficant features with the Fe XXVI and XXV n=2-3 and O VIII
n=1-2 transitions, all with a redshift of z=0.35, identical within small
uncertainties for the respective transitions. For an astrophysically plausible
range of masses (M ~ 1.3-2.0 M_solar), this value is completely consistent with
models of neutron stars composed of normal nuclear matter, while it excludes
some models in which the neutron stars are made of more exotic matter.Comment: Published in Nature (Nov 7, 2002
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