1,039 research outputs found
A transient high-coherence oscillation in 4U 1820-30
We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the
atoll source 4U 1820-30 in order to investigate the detailed time-frequency
properties of the source during the intervals when a ~7 Hz QPO was detected by
Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in
addition to a QPO signal lasting a couple of minutes as previously reported,
there is a much narrower transient oscillation with a life time of only a few
seconds. Within this time, the oscillation is consistent with being coherent.
Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz
in the two observations. We discuss the possible association of this QPO with
other oscillations known both in Neutron-Star and Black-Hole systems,
concentrating on the similarities with the narrow 5-7 Hz oscillations observed
at high flux in Black-Hole Candidates.Comment: 5 pages, 3 figures, accepted for publication in A&A. FIgure 1 is
reduced in resolution, full-resolution version of this text available at
http://www.merate.mi.astro.it/~belloni/ms0335.ps.g
A simple model for the complex lag structure of microquasars
The phase lag structure between the hard and soft X-ray photons observed in
GRS 1915+105 and XTE J1550+564 has been said to be ``complex'' because the
phase of the Quasi-Periodic Oscillation fundamental Fourier mode changes with
time and because the even and odd harmonics signs behave differentely.
From simultaneous X-ray and radio observations this seems to be related to
the presence of a jet (level of radio emission). We propose a simple idea where
a partial absorption of the signal can shift the phases of the Fourier modes
and account for the phase lag reversal. We also briefly discuss a possible
physical mechanism that could lead to such an absorption of the quasi-periodic
oscillation modulation.Comment: accepted by A&A Letter
A continuous Flaring- to Normal-branch transition in Sco X-1
We report the first resolved rapid transition from a Flaring Branch
Oscillation to a Normal Branch Oscillation in the RXTE data of the Z source Sco
X-1. The transition took place on a time scale of ~100 seconds and was clearly
associated to the Normal Branch-Flaring Branch vertex in the color-color
diagram. We discuss the results in the context of the possible association of
the Normal Branch Oscillation with other oscillations known both in
Neutron-Star and Black-Hole systems, concentrating on the similarities with the
narrow 4-6 Hz oscillations observed at high flux in Black-Hole Candidates.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Optical/infrared observations of the X-ray burster KS1731-260 in quiescence
We performed an optical/infrared study of the counterpart of the low-mass
X-ray binary KS1731-260 to test its identification and obtain information about
the donor. Optical and infrared images of the counterpart of KS1731-260 were
taken in two different epochs (2001 and 2007) after the source returned to
quiescence in X-rays. We compared those observations with obtained when KS
1731-260 was still active. We confirm the identification of KS1731-260 with the
previously proposed counterpart and improve its position to RA=17:34:13.46 and
DEC=-26:05:18.60. The H-band magnitude of this candidate showed a decline of
~1.7 mags from outburst to quiescence. In 2007 April we obtained R=22.8+-0.1
and I=20.9+-0.1 for KS1731-260. Similar optical brightness was measured in June
2001 and July 2007. The intrinsic optical color R-I is consistent with spectral
types from F to G for the secondary although there is a large excess over that
from the secondary at the infrared wavelengths. This may be due to emission
from the cooler outer regions of the accretion disk. We cannot rule out a brown
dwarf as a donor star, although it would require that the distance to the
source is significantly lower than the 7 kpc reported by Muno et al. 2000.Comment: Accepted for publication in A&
The faint neutron star soft X-ray transient SAX J1810.8-2609 in quiescence
We present the analysis of a 35 ksec long Chandra observation of the neutron
star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in
the field of view. The position of one of them is consistent with the location
of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of
that source coincides with the position of the proposed optical counterpart,
strengthening the identification as the counterpart. We detected the neutron
star SXT system in quiescence at an unabsorbed luminosity of ~1x10^32 erg s^-1
(assuming a distance of 4.9 kpc). This luminosity is at the low-end of
quiescent luminosities found in other neutron star SXTs. This renders support
to the existence of a group of faint soft X-ray transients of which the
accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent
member. The quiescent spectrum of SAX J1810.8-2609 is well-fit with an absorbed
power law with photon index of 3.3+-0.5. With a value of 3.3x10^21 cm^-2 the
Galactic absorption is consistent with the value derived in outburst. Since the
spectra of quiescent neutron star SXTs are often fit with an absorbed blackbody
or neutron star atmosphere plus power-law model we also fitted the spectrum
using those fit functions. Both models provide a good fit to the data. If
cooling of the neutron star core and/or crust is responsible for the soft part
of the spectrum the time averaged mass accretion rate must have been very low
(~5.7x10^-13 Msun yr^-1; assuming standard core cooling only) or the neutron
star must be massive. We also discuss the possibility that the thermal spectral
component in neutron stars in quiescence is produced by residual accretion.Comment: 5 pages, 1 figure, accepted for publication by MNRA
Good practices in de 'spotlight'
Vanuit de projecten ‘Duurzaam telen, begint bij jou’ en ‘Telen met Toekomst’ is gezamenlijk een keuze gemaakt om per sector vijf Good Practices extra onder de aandacht te brengen. Per sector aandacht voor gewasbescherming en teeltsystemen
Further Constraints on Thermal Quiescent X-ray Emission from SAX J1808.4-3658
We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar,
in deep quiescence with XMM-Newton and (near-simultaneously) Gemini-South. The
X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and
2006, describable by an absorbed power-law with photon index 1.74+-0.11 and
unabsorbed X-ray luminosity L_X=7.9+-0.7*10^{31} ergs/s, for N_H=1.3*10^{21}
cm^{-2}. Fitting all the quiescent XMM-Newton X-ray spectra with a power-law,
we constrain any thermally emitting neutron star with a hydrogen atmosphere to
have a temperature less than 30 eV and L_{NS}(0.01-10 keV)<6.2*10^{30} ergs/s.
A thermal plasma model also gives an acceptable fit to the continuum. Adding a
neutron star component to the plasma model produces less stringent constraints
on the neutron star; a temperature of 36^{+4}_{-8} eV and L_{NS}(0.01-10
keV)=1.3^{+0.6}_{-0.8}*10^{31} ergs/s. In the framework of the current theory
of neutron star heating and cooling, the constraints on the thermal luminosity
of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these
neutron stars.
We compile data from the literature on the mass transfer rates and quiescent
thermal flux of the largest possible sample of transient neutron star LMXBs. We
identify a thermal component in the quiescent spectrum of the accreting
millisecond pulsar IGR J00291+5934, which is consistent with the standard
cooling model. The contrast between the cooling rates of IGR J00291+5934 and
1808 suggests that 1808 may have a significantly larger mass. This can be
interpreted as arising from differences in the binary evolution history or
initial neutron star mass in these otherwise similar systems.Comment: ApJ in press, 7 pages, 2 color figure
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
A Prototype ATM Network for Real Time Control of the LHC
The LHC accelerator aims at injecting, accelerating and colliding beams with very well controlled beam parameters (e.g. momentum, orbit, tune and chromaticity). This is a non-trivial task since the super conducting main bending magnets will generate field errors with dynamic effects that may result in beam loss. To overcome this problem, real time control of beam parameters via the Power Converters has been proposed. This requires site wide deterministic communication of control data. In this paper we will outline some aspects of a prototype deterministic network for the LHC with a core based on ATM (Asynchronous Transfer Mode) communication technology
Swift J1357.2-0933: the faintest black hole?
Swift J1357.2-0933 is the first confirmed very faint black hole X-ray
transient and has a short estimated orbital period of 2.8 hr. We observed Swift
J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent
state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of
~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as
suggested in the literature), this would imply a luminosity of ~8x10^29 erg
s-1, making it the faintest detected quiescent black hole LMXB. This would also
imply that there is no indication of a reversal in the quiescence X-ray
luminosity versus orbital period diagram down to 2.8 hr, as has been predicted
theoretically and recently supported by the detection of the 2.4 hr orbital
period black hole MAXI J1659-152 at a 0.5-10 keV X-ray luminosity of ~ 1.2 x
10^31 erg s-1. However, there is considerable uncertainty in the distance of
Swift J1357.2-0933 and it may be as distant as 6 kpc. In this case, its
quiescent luminosity would be Lx ~ 1.3 x 10^31 erg s-1, i.e., similar to MAXI
J1659-152 and hence it would support the existence of such a bifurcation
period. We also detected the source in optical at r' ~22.3 mag with the
Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical
luminosity ratio of Swift J1357.2-0933 agrees with the expected value for a
black hole at this range of quiescent X-ray luminosities.Comment: 5 pages, 3 figures, Accepted for publication in MNRA
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