11,341 research outputs found
XMM-Newton observations of two transient millisecond X-ray pulsars in quiescence
We report on XMM-Newton observations of two X-ray transient millisecond
pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of
\~7x10^{31} erg/s. (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent
spectrum is consistent with a simple power law spectrum. The upper limit on the
flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE
J1807-294 instead was not detected. We can put an upper limit on the source
quiescent 0.5-10 keV unabsorbed luminosity <4x10^{31} erg/s at 8 kpc. These
observations strenghten the idea that XRTMSPs have quiescent luminosities
significantly lower than classical neutron star transients.Comment: 4 pages including 1 figures. Accepted for publication in A&A Letter
Double solid twistor spaces: the case of arbitrary signature
In a recent paper (math.DG/0701278) we constructed a series of new Moishezon
twistor spaces which is a kind of variant of the famous LeBrun twistor spaces.
In this paper we explicitly give projective models of another series of
Moishezon twistor spaces on nCP^2 for arbitrary n>2, which can be regarded as a
generalization of the twistor spaces of a 'double solid type' on 3CP^2 studied
by Kreussler, Kurke, Poon and the author. Similarly to the twistor spaces of
'double solid type' on 3CP^2, projective models of present twistor spaces have
a natural structure of double covering of a CP^2-bundle over CP^1. We
explicitly give a defining polynomial of the branch divisor of the double
covering whose restriction to fibers are degree four. If n>3 these are new
twistor spaces, to the best of the author's knowledge. We also compute the
dimension of the moduli space of these twistor spaces. Differently from
math.DG/0701278, the present investigation is based on analysis of
pluri-(half-)anticanonical systems of the twistor spaces.Comment: 30 pages, 3 figures; v2: title changed (the original title was
"Explicit construction of new Moishezon twistor spaces, II".
The Structure of 1-\u3cem\u3etert\u3c/em\u3e-butyl-2,2,3,4,4-pentamethylphosphetane 1-oxide, C\u3csub\u3e12\u3c/sub\u3eH\u3csub\u3e25\u3c/sub\u3eOP
The quiescent X-ray emission of three transient X-ray pulsars
We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients
in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U
0115+63 and V 0332+53. These observations allowed us to study these transients
at the faintest flux levels thus far. Spectra are remarkably different from the
ones obtained at luminosities a factor >10 higher, testifying that the
quiescent emission mechanism is different. Pulsations were not detected in any
of the sources, indicating that accretion of matter down to the neutron star
surface has ceased. We conclude that the quiescent emission of the three X-ray
transients likely originates from accretion onto the magnetospheric boundary in
the propeller regime and/or from deep crustal heating resulting from
pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures
The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232
During a systematic search for periodic signals in a sample of ROSAT PSPC
(0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray
pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the
galactic plane. The energy spectrum is well fit by a power law with a photon
index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1.
The source is probably a long period, low luminosity X-ray pulsar, similar to X
Per, or an intermediate polar.Comment: 5 pages (figures included). Accepted for publication on MNRA
Deciphering the large-scale environment of radio galaxies in the local Universe: where do they born, grow and die?
The role played by the large-scale environment on the nuclear activity of
radio galaxies (RGs), is still not completely understood. Accretion mode, jet
power and galaxy evolution are connected with their large-scale environment
from tens to hundreds of kpc. Here we present a detailed, statistical, analysis
of the large-scale environment for two samples of RGs up to redshifts
=0.15. The main advantages of our study, with respect to those
already present in the literature, are due to the extremely homogeneous
selection criteria of catalogs adopted to perform our investigation. This is
also coupled with the use of several clustering algorithms. We performed a
direct search of galaxy-rich environments around RGs using them as beacon. To
perform this study we also developed a new method that does not appear to
suffer by a strong dependence as other algorithms. We conclude
that, despite their radio morphological (FR\,I FR\,II) and/or their
optical (HERG LERG) classification, RGs in the local Universe tend to live
in galaxy-rich large-scale environments having similar characteristics and
richness. We highlight that the fraction of FR\,Is-LERG, inhabiting galaxy rich
environments, appears larger than that of FR\,IIs-LERG. We also found that 5
out of 7 FR\,II-HERGs, with 0.11, lie in groups/clusters of
galaxies. However, we recognize that, despite the high level of completeness of
our catalogs, when restricting to the local Universe, the low number of HERGs
(10\% of the total FR\,IIs investigated) prevent us to make a strong
statistical conclusion about this source class.Comment: 21 pages, 25 figures, accepted for publication on the Astrophysical
Journal Supplement Series - pre-proof versio
A Search for Pulsars in Quiescent Soft X-Ray Transients. I
We have carried out a deep search at 1.4 GHz for radio pulsed emission from
six soft X-ray transient sources observed during their X-ray quiescent phase.
The commonly accepted model for the formation of the millisecond radio pulsars
predicts the presence of a rapidly rotating, weakly magnetized neutron star in
the core of these systems. The sudden drop in accretion rate associated with
the end of an X-ray outburst causes the Alfv\`en surface to move outside the
light cylinder, allowing the pulsar emission process to operate. No pulsed
signal was detected from the sources in our sample. We discuss several
mechanisms that could hamper the detection and suggest that free-free
absorption from material ejected from the system by the pulsar radiation
pressure could explain our null result.Comment: accepted by Ap
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
Optimisation of the Swift X-ray follow-up of Advanced LIGO and Virgo gravitational wave triggers in 2015--16
One of the most exciting near-term prospects in physics is the potential
discovery of gravitational waves by the advanced LIGO and Virgo detectors. To
maximise both the confidence of the detection and the science return, it is
essential to identify an electromagnetic counterpart. This is not trivial, as
the events are expected to be poorly localised, particularly in the near-term,
with error regions covering hundreds or even thousands of square degrees. In
this paper we discuss the prospects for finding an X-ray counterpart to a
gravitational wave trigger with the Swift X-ray Telescope, using the assumption
that the trigger is caused by a binary neutron star merger which also produces
a short gamma-ray burst. We show that it is beneficial to target galaxies
within the GW error region, highlighting the need for substantially complete
galaxy catalogues out to distances of 300 Mpc. We also show that nearby,
on-axis short GRBs are either extremely rare, or are systematically less
luminous than those detected to date. We consider the prospects for detecting
afterglow emission from an an off-axis GRB which triggered the GW facilities,
finding that the detectability, and the best time to look, are strongly
dependent on the characteristics of the burst such as circumburst density and
our viewing angle.Comment: 17 pages, 14 figures. Accepted for publication in MNRA
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