648 research outputs found
The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
We used a combination of deep optical and Halpha images of the Galactic
globular cluster NGC 6440, acquired with the Hubble Space Telescope, to
identify the optical counterpart to the accreting millisecond X-ray pulsar SAX
J1748.9-2021during quiescence. A strong Halpha emission has been detected from
a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"
from the nominal position of the X-ray source. The position of the star also
agrees with the optical counterpart found by Verbunt et al. (2000) during an
outburst. We propose this star as the most likely optical counterpart to the
binary system. By direct comparison with isochrones, we estimated that COM-SAX
J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun
and a superficial temperature of 5250pm80 K. These parameters combined with the
orbital characteristics of the binary suggest that the system is observed at a
very low inclination angle (~8 deg -14 deg) and that the star is filling or
even overflowing its Roche Lobe. This, together with the equivalent width of
the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The
possibile presence of such a on-going mass transfer during a quiescence state
also suggests that the radio pulsar is not active yet and thus this system,
despite its similarity with the class of redback millisecond pulsars, is not a
transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
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
Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821
We present a detailed study of the X-ray energy and power spectral properties
of the neutron star transient IGR J17191-2821. We discovered four instances of
pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The
frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We
also report on the detection of five thermonuclear type-I X-ray bursts and the
discovery of burst oscillations at ~294 Hz during three of them. Finally, we
report on a faint and short outburst precursor, which occurred about two months
before the main outburst. Our results on the broadband spectral and variability
properties allow us to firmly establish the atoll source nature of IGR
J17191-2821.Comment: 9 pages, 7 figures - accepted for publication in MNRA
The Radiative Efficiency of a Radiatively Inefficient Accretion Flow
A recent joint XMM-Newton/Nuclear Spectroscopic Telescope Array (NuSTAR)
observation of the accreting neutron star Cen X-4 () revealed a hard power-law component
(-) with a relatively low cut-off energy (~10 keV),
suggesting bremsstrahlung emission. The physical requirements for
bremsstrahlung combined with other observed properties of Cen X-4 suggest the
emission comes from a boundary layer rather than the accretion flow. The
accretion flow itself is thus undetected (with an upper limit of ). A deep search for coherent pulsations (which
would indicate a strong magnetic field) places a 6 per cent upper limit on the
fractional amplitude of pulsations, suggesting the flow is not magnetically
regulated. Considering the expected energy balance between the accretion flow
and the boundary layer for different values of the neutron star parameters
(size, magnetic field, and spin) we use the upper limit on to
set an upper limit of for the intrinsic radiative
efficiency of the accretion flow for the most likely model of a fast-spinning,
non-magnetic neutron star. The non-detection of the accretion flow provides the
first direct evidence that this flow is indeed 'radiatively inefficient', i.e.
most of the gravitational potential energy lost by the flow before it hits the
star is not emitted as radiation.Comment: 15 pages, 3 figures - minor modifications to match published versio
Secular spin-down of the AMP XTE J1751-305
Context. Of the 13 known accreting millisecond pulsars, only a few showed
more than one outburst during the RXTE era. XTE J1751-305 showed, after the
main outburst in 2002, other three dim outbursts. We report on the timing
analysis of the latest one, occurred on October 8, 2009 and serendipitously
observed from its very beginning by RXTE. Aims. The detection of the pulsation
during more than one outburst permits to obtain a better constraint of the
orbital parameters and their evolution as well as to track the secular spin
frequency evolution of the source. Methods. Using the RXTE data of the last
outburst of the AMP XTE J1751-305, we performed a timing analysis to improve
the orbital parameters. Because of the low statistics, we used an epoch folding
search technique on the whole data set to improve the local estimate of the
time of ascending node passage. Results. Using this new orbital solution we
epoch folded data obtaining three pulse phase delays on a time span of 1.2
days, that we fitted using a constant spin frequency model. Comparing this
barycentric spin frequency with that of the 2002 outburst, we obtained a
secular spin frequency derivative of -0.55(12) x 10^{-14} Hz s^{-1}. In the
hypothesis that the secular spin-down is due to a rotating magneto-dipole
emission, consistently with what is assumed for radio pulsars, we estimate the
pulsar's magnetic dipole value. We derive an estimate of the magnetic field
strength at the polar cap of B_{PC} = 4.0(4) x 10^8 Gauss, for a neutron star
mass of 1.4M\odot, assuming the Friedman Pandharipande Skyrme equation of
state.Comment: 7 pages, 4 figures, accepted for publication on A&
Spectral evidence for jets from Accreting Millisecond X-ray Pulsars
Transient radio emission from X-ray binaries is associated with synchrotron
emission from collimated jets that escape the system, and accreting millisecond
X-ray pulsars (AMXPs) are no exception. Although jets from black hole X-ray
binaries are well-studied, those from neutron star systems appear much fainter,
for reasons yet uncertain. Jets are usually undetectable at higher frequencies
because of the relative brightness of other components such as the accretion
disc. AMXPs generally have small orbital separations compared with other X-ray
binaries and as such their discs are relatively faint. Here, I present data
that imply jets in fact dominate the radio-to-optical spectrum of outbursting
AMXPs. They therefore may provide the best opportunity to study the behaviour
of jets produced by accreting neutron stars, and compare them to those produced
by black hole systems.Comment: 4 pages, 2 figures, to appear in the proceedings of "A Decade of
Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands
et al. (AIP Conf. Proc.
The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480-2446, during its 2010 outburst
(abridged) We analyse the spectral and pulse properties of the 11 Hz
transient accreting pulsar, IGR J17480-2446, in the globular cluster Terzan 5,
considering all the available RXTE, Swift and INTEGRAL observations performed
between October and November, 2010.
By measuring the pulse phase evolution we conclude that the NS spun up at an
average rate of =1.48(2)E-12 Hz/s, compatible with the accretion of the
Keplerian angular momentum of matter at the inner disc boundary. Similar to
other accreting pulsars, the stability of the pulse phases determined by using
the second harmonic component is higher than that of the phases based on the
fundamental frequency. Under the assumption that the second harmonic is a good
tracer of the neutron star spin frequency, we successfully model its evolution
in terms of a luminosity dependent accretion torque. If the NS accretes the
specific Keplerian angular momentum of the in-flowing matter, we estimate the
inner disc radius to lie between 47 and 93 km when the luminosity attains its
peak value. Smaller values are obtained if the interaction between the magnetic
field lines and the plasma in the disc is considered.
The phase-averaged spectrum is described by thermal Comptonization of photons
with energy of ~1 keV. A hard to soft state transition is observed during the
outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an
electron temperature of ~20 keV towards an optically denser cloud at kT_e~3
keV. At the same time, the pulse amplitude decreases from 27% to few per cent
and becomes strongly energy dependent. We discuss various possibilities to
explain such a behaviour, proposing that at large accretion luminosities a
significant fraction of the in-falling matter is not channelled towards the
magnetic poles, but rather accretes more evenly onto the NS surface.Comment: To appear in MNRA
Binary evolution with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
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