878 research outputs found
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We
analysed a sample of 20 Swift observations, taken between 2013 April and
August, and two Chandra observations taken in 2001 September and 2004 June. We
found only marginal hints of a limited number of dips in the light curve,
previously reported to occur in this source, although the uncertainties due to
the low counting statistics of the data are large. The Chandra and Swift
spectra showed clearly spectral variability which resembles those observed in
other ULXs. We can account for this spectral variability in terms of changes in
both the normalization and intrinsic column density of a two-components model
consisting of a blackbody (for the soft component) and a multicolour accretion
disc (for the hard component). We discuss the possibility that strong outflows
ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA
VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774
X-ray observations performed with ROSAT led to the discovery of a group
(seven to date) of X-ray dim and radio-silent middle-aged isolated neutron
stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra
(kT~40-100 eV), long X-ray pulsations (P=3-12 s), and appear to be endowed with
relatively high magnetic fields, (B~10d13-14 G). RBS 1774 is one of the few
XDINSs with a candidate optical counterpart, which we discovered with the VLT.
We performed deep observations of RBS 1774 in the R band with the VLT to
disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose
contributions are expected to be very much different in the red part of the
spectrum. We did not detect the RBS 1774 candidate counterpart down to a 3
sigma limiting magnitude of R~27. The constraint on its colour, (B-R)<0.6,
rules out that it is a background object, positionally coincident with the
X-ray source. Our R-band upper limit is consistent with the extrapolation of
the B-band flux (assuming a 3 sigma uncertainty) for a set of power-laws F_nu
~nu^alpha with spectral indeces alpha<0.07. If the optical spectrum of RBS 1774
were non-thermal, its power-law slope would be very much unlike those of all
isolated neutron stars with non-thermal optical emission, suggesting that it is
most likely thermal. For instance, a Rayleigh-Jeans with temperature T_O = 11
eV, for an optically emitting radius r_O=15 km and a source distance d=150 pc,
would be consistent with the optical measurements. The implied low distance is
compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is
nearly aligned with the magnetic axis or with the line of sight, or it is
slightly misaligned with respect to both the magnetic axis and the line of
sight by 5-10 degreesComment: 8 pages, 8 postscript figures, accepted for publication in Astronomy
& Astrophysic
The two Ultraluminous X-ray sources in the galaxy NGC 925
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC
925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality,
simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first
glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because
of its high X-ray luminosity ( erg s) and its
spectral/temporal features, a closer inspection shows that its properties are
more similar to those of a typical super-Eddington accreting stellar black hole
and we classify it as a `broadened disc' ultraluminous X-ray source. Based on
the physical interpretation of this spectral state, we suggest that ULX-1 is
seen at small inclination angles, possibly through the evacuated cone of a
powerful wind originating in the accretion disc. The spectral classification of
ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington
rates as none of its spectral/temporal properties can be associated to either
the soft or hard state of Galactic accreting black hole binaries.Comment: Accepted on MNRAS with very minor comments, 7 pages, 5 figures, 1
tabl
On the stability of self-gravitating accreting flows
Analytic methods show stability of the stationary accretion of test fluids
but they are inconclusive in the case of self-gravitating stationary flows. We
investigate numerically stability of those stationary flows onto compact
objects that are transonic and rich in gas. In all studied examples solutions
appear stable. Numerical investigation suggests also that the analogy between
sonic and event horizons holds for small perturbations of compact support but
fails in the case of finite perturbations.Comment: 10 pages, accepted for publication in PR
Poly(ADP-ribosyl)ation Acts in the DNA Demethylation of Mouse Primordial Germ Cells Also with DNA Damage-Independent Roles
Poly(ADP-ribosyl)ation regulates chromatin structure and transcription driving epigenetic events. In particular, Parp1 is able to directly influence DNA methylation patterns controlling transcription and activity of Dnmt1. Here, we show that ADP-ribose polymer levels and Parp1 expression are noticeably high in mouse primordial germ cells (PGCs) when the bulk of DNA demethylation occurs during germline epigenetic reprogramming in the embryo. Notably, Parp1 activity is stimulated in PGCs even before its participation in the DNA damage response associated with active DNA demethylation. We demonstrate that PARP inhibition impairs both genome-wide and locus-specific DNA methylation erasure in PGCs. Moreover, we evidence that impairment of PARP activity causes a significant reduction of expression of the gene coding for Tet1 hydroxylases involved in active DNA demethylation. Taken together these results demonstrate new and adjuvant roles of poly(ADP-ribosyl)ation during germline DNA demethylation and suggest its possible more general involvement in genome reprogramming
A very faint core-collapse supernova in M85
An anomalous transient in the early Hubble-type (S0) galaxy Messier 85 (M85)
in the Virgo cluster was discovered by Kulkarni et al. (2007) on 7 January 2006
that had very low luminosity (peak absolute R-band magnitude MR of about -12)
that was constant over more than 80 days, red colour and narrow spectral lines,
which seem inconsistent with those observed in any known class of transient
events. Kulkarni et al. (2007) suggest an exotic stellar merger as the possible
origin. An alternative explanation is that the transient in M85 was a type
II-plateau supernova of extremely low luminosity, exploding in a lenticular
galaxy with residual star-forming activity. This intriguing transient might be
the faintest supernova that has ever been discovered.Comment: 7 pages, 2 figures. Submitted to Nature "Brief Communication Arising"
on 18 July 2007, Accepted on 17 August 2007. Arising from: Kulkarni et al.
2007, Nature, 447, 458-46
Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
NGC 7793 P13 is a variable (luminosity range ~100) ultraluminous X-ray source
(ULX) proposed to host a stellar-mass black hole of less than 15 M in
a binary system with orbital period of 64 d and a 18-23 M B9Ia
companion. Within the EXTraS project we discovered pulsations at a period of
~0.42 s in two XMM-Newton observations of NGC 7793 P13, during which the source
was detected at and erg
s (0.3-10 keV band). These findings unambiguously demonstrate that the
compact object in NGC 7793 P13 is a neutron star accreting at super-Eddington
rates. While standard accretion models face difficulties accounting for the
pulsar X-ray luminosity, the presence of a multipolar magnetic field with ~
few 10 G close to the base of the accretion column appears to
be in agreement with the properties of the system.Comment: 5 pages, 5 figures, 2 tables; Version accepted for publication in
MNRAS Letter
Pulsating in unison at optical and X-ray energies: simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038
PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the
visible band; such a detection took place when the pulsar was surrounded by an
accretion disk and also showed X-ray pulsations. We report on the first high
time resolution observational campaign of this transitional pulsar in the disk
state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray
(XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and
X-ray pulsations were detected simultaneously in the X-ray high intensity mode
in which the source spends 70% of the time, and both disappeared in the
low mode, indicating a common underlying physical mechanism. In addition,
optical and X-ray pulses were emitted within a few km, had similar pulse shape
and distribution of the pulsed flux density compatible with a power-law
relation connecting the optical and the 0.3-45 keV
X-ray band. Optical pulses were detected also during flares with a pulsed flux
reduced by one third with respect to the high mode; the lack of a simultaneous
detection of X-ray pulses is compatible with the lower photon statistics. We
show that magnetically channeled accretion of plasma onto the surface of the
neutron star cannot account for the optical pulsed luminosity (
erg/s). On the other hand, magnetospheric rotation-powered pulsar emission
would require an extremely efficient conversion of spin-down power into pulsed
optical and X-ray emission. We then propose that optical and X-ray pulses are
instead produced by synchrotron emission from the intrabinary shock that forms
where a striped pulsar wind meets the accretion disk, within a few light
cylinder radii away, 100 km, from the pulsar.Comment: 26 pages, 14 figures, first submitted to ApJ on 2019, January 1
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