127,991 research outputs found
News on the X-ray emission from hot subdwarf stars
In latest years, the high sensitivity of the instruments on-board the
XMM-Newton and Chandra satellites allowed us to explore the properties of the
X-ray emission from hot subdwarf stars. The small but growing sample of X-ray
detected hot subdwarfs includes binary systems, in which the X-ray emission is
due to wind accretion onto a compact companion (white dwarf or neutron star),
as well as isolated sdO stars, in which X-rays are probably due to shock
instabilities in the wind. X-ray observations of these low-mass stars provide
information which can be useful for our understanding of the weak winds of this
type of stars and can lead to the discovery of particularly interesting binary
systems. Here we report the most recent results we have recently obtained in
this research area.Comment: 8 pages, 3 figures. To appear in the Proceedings of the 8th Meeting
on Hot Subdwarf Stars and Related Objects, 9-15 July 2017, Cracow, Poland.
Eds. A. Baran, A. E. Lynas-Gray, Open Astronomy, in pres
Swift monitoring of the massive X-ray binary SAX J0635.2+0533
SAX J0635.2+0533 is a binary pulsar with a very short pulsation period ( =
33.8 ms) and a high long-term spin down ( 3.8 s
s), which suggests a rotation-powered (instead of an accretion-powered)
nature for this source. While it was discovered at a flux level around
10 erg cm s, between 2003 and 2004 this source was
detected with XMM-Newton at an average flux of about 10 erg cm
s; moreover, the flux varied of over one order of magnitude on time
scales of a few days, sometimes decreasing below erg
cm s. Since both the rotation-powered and the accretion-powered
scenarios have difficulties to explain these properties, the nature of SAX
J0635.2+0533 is still unclear. Here we report on our recent long-term
monitoring campaign on SAX J0635.2+0533 carried out with Swift and on a
systematic reanalysis of all the RXTE observations performed between 1999 and
2001. We found that during this time interval the source remained almost always
active at a flux level above 10 erg cm s.Comment: 8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy &
Astrophysic
Top Quark as a Dark Portal and Neutrino Mass Generation
We present a new model for radiatively generating Majorana active neutrino
masses while incorporating a viable dark matter candidate. This is possible by
extending the Standard Model with a single Majorana neutrino endowed with a
dark parity, a colour electroweak singlet scalar, as well as a colour
electroweak triplet scalar. Within this framework, the -type quarks play a
special role, serving as a portal for dark matter, and a messenger for neutrino
mass generation. We consider three benchmark scenarios where the abundance of
dark matter can match the latest experimental results, while generating
neutrino masses in the milli-electronvolt range. We show how constraints from
lepton flavour violation, in particular the branching fraction of , can place lower bounds on the coupling between our dark matter
candidate and top quarks. Furthermore, we show that this coupling can also be
constrained using collider data from the Tevatron and the LHC.Comment: 8 captions, 10 figure
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Coronavirus mRNA synthesis: identification of novel transcription initiation signals which are differentially regulated by different leader sequences.
The mRNA synthesis of mouse hepatitis virus (MHV) has been proposed to be the result of interaction between the leader RNA and the intergenic sites. Previously, we have identified a transcription initiation site (for mRNA 2-1), which is more efficiently transcribed by viruses containing two copies of UCUAA sequence in the leader RNA than by those with three copies. In this study, we have identified several sites which are regulated in the opposite way, namely, they are efficiently transcribed by the leader RNA with three UCUAA copies but not by those with two copies. These sites were characterized by primer extension and amplification by polymerase chain reaction. One of these sites is in the gene 3 region of a recombinant virus between A59 and JHM strains of MHV. Another is in the gene 2 region of MHV-1 strain. Both of these sites have a sequence similar to but different from the consensus transcription initiation signal (UCUAAUCUAUC and UUUAAUCUU, as opposed to UCUAAAC). These two novel intergenic sequences are not present in the genome of the JHM strain, consistent with the absence of these mRNAs in the JHM-infected cells. The discovery of this type of transcription initiation site provides additional evidence for the importance of the leader RNA in the transcription initiation of MHV mRNAs
A closer look at string resonances in dijet events at the LHC
The first string excited state can be observed as a resonance in dijet
invariant mass distributions at the LHC, if the scenario of low-scale string
with large extra dimensions is realized. A distinguished property of the dijet
resonance by string excited states from that the other "new physics" is that
many almost degenerate states with various spin compose a single resonance
structure. It is examined that how we can obtain evidences of low-scale string
models through the analysis of angular distributions of dijet events at the
LHC. Some string resonance states of color singlet can obtain large mass shifts
through the open string one-loop effect, or through the mixing with closed
string states, and the shape of resonance structure can be distorted. Although
the distortion is not very large (10% for the mass squared), it might be able
to observe the effect at the LHC, if gluon jets and quark jets could be
distinguished in a certain level of efficiency.Comment: 12 pages, 8 figure
Dark Matter and IMF normalization in Virgo dwarf early-type galaxies
In this work we analyze the dark matter (DM) fraction, , and
mass-to-light ratio mismatch parameter, (computed with respect
to a Milky-Way-like IMF), for a sample of 39 dwarf early-type galaxies (dEs) in
the Virgo cluster. Both and are estimated within the
central (one effective radius) galaxy regions, with a Jeans dynamical analysis
that relies on galaxy velocity dispersions, structural parameters, and stellar
M/L ratios from the SMAKCED survey. In this first attempt to constrain,
simultaneously, the IMF normalization and the DM content, we explore the impact
of different assumptions on the DM model profile. On average, for a NFW
profile, the is consistent with a Chabrier-like normalization
(), with . One of the main results of
the present work is that for at least a few systems the is
heavier than the MW-like value (i.e. either top- or bottom-heavy). When
introducing tangential anisotropy, larger and smaller
are derived. Adopting a steeper concentration-mass relation than that from
simulations, we find lower () and larger . A
constant M/L profile with null gives the heaviest
(). In the MONDian framework, we find consistent results to those for
our reference NFW model. If confirmed, the large scatter of for
dEs would provide (further) evidence for a non-universal IMF in early-type
systems. On average, our reference estimates are consistent with those
found for low- () early-type
galaxies (ETGs). Furthermore, we find consistent with values from the
SMAKCED survey, and find a double-value behavior of with stellar mass,
which mirrors the trend of dynamical M/L and global star formation efficiency
with mass.Comment: 11 pages, 3 figures, 1 table, published on MNRAS. Figure 1 has been
updated with respect to version 1, including the range of values found if the
S\'ersic index, n, is varied from 0.5 to 2 (dark-green curves
Improving Transient Performance of Adaptive Control Architectures using Frequency-Limited System Error Dynamics
We develop an adaptive control architecture to achieve stabilization and
command following of uncertain dynamical systems with improved transient
performance. Our framework consists of a new reference system and an adaptive
controller. The proposed reference system captures a desired closed-loop
dynamical system behavior modified by a mismatch term representing the
high-frequency content between the uncertain dynamical system and this
reference system, i.e., the system error. In particular, this mismatch term
allows to limit the frequency content of the system error dynamics, which is
used to drive the adaptive controller. It is shown that this key feature of our
framework yields fast adaptation with- out incurring high-frequency
oscillations in the transient performance. We further show the effects of
design parameters on the system performance, analyze closeness of the uncertain
dynamical system to the unmodified (ideal) reference system, discuss robustness
of the proposed approach with respect to time-varying uncertainties and
disturbances, and make connections to gradient minimization and classical
control theory.Comment: 27 pages, 7 figure
Three new X-ray emitting sdO stars discovered with Chandra
X-ray observations of sdO stars are a useful tool to investigate their
properties, but so far only two sdO stars were detected at X-rays. We observed
a complete flux-limited sample of 19 sdO stars with the Chandra HRC-I camera to
measure the count rate of the detected sources or to set a tight upper limit on
it for the undetected sources. We obtained a robust detection of BD+37 1977 and
Feige 34 and a marginal detection of BD+28 4211. The estimated luminosity of
BD+37 1977 is above 10^31 erg/s, which is high enough to suggest the possible
presence of an accreting compact companion. This possibility is unlikely for
all the other targets (both detected and undetected), since in their case L_X <
10^30 erg/s. On the other hand, for all 19 targets the estimated value of L_X
(or its upper limit) implies an X-ray/bolometric flux ratio that agrees with
log(L_X/L_bol) = -6.7 +/- 0.5, which is the range of values typical of
main-sequence and giant O stars. Therefore, for Feige 34 and BD+28 4211 the
observed X-ray flux is most probably due to intrinsic emission. The same is
possibile for the 16 undetected stars.Comment: 6 pages. Accepted for publication by Astronomy and Astrophysic
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