383 research outputs found
A study of the long term variability of RX J1856.5-3754 with XMM-Newton
We report on a detailed spectral analysis of all the available XMM-Newton
data of RX J1856.5-3754, the brightest and most extensively observed nearby,
thermally emitting neutron star. Very small variations (~1-2%) in the
single-blackbody temperature are detected, but are probably due to an
instrumental effect, since they correlate with the position of the source on
the detector. Restricting the analysis to a homogeneous subset of observations,
with the source at the same detector position, we place strong limits on
possible spectral or flux variations from March 2005 to present-day. A slightly
higher temperature (kT~61.5 eV, compared to the average value kT~61 eV) was
instead measured in April 2002. If this difference is not of instrumental
origin, it implies a rate of variation of about 0.15 eV/yr between April 2002
and March 2005. The high-statistics spectrum from the selected observations is
well fit by the sum of two blackbody models, which extrapolate to an optical
flux level in agreement with the observed value.Comment: 4 pages, to appear in the proceedings of the ERPM conference, Zielona
Gora, April 201
The Relative Importance of Type of Education and Subject Area: Empirical Evidence for Educational Decisions
The birthplace and age of the isolated neutron star RX J1856.5-3754
X-ray observations unveiled various types of radio-silent Isolated Neutron
Stars (INSs), phenomenologically very diverse, e.g. the Myr old X-ray Dim INS
(XDINSs) and the kyr old magnetars. Although their phenomenology is much
diverse, the similar periods (P=2--10 s) and magnetic fields (~10^{14} G)
suggest that XDINSs are evolved magnetars, possibly born from similar
populations of supermassive stars. One way to test this hypothesis is to
identify their parental star clusters by extrapolating backward the neutron
star velocity vector in the Galactic potential. By using the information on the
age and space velocity of the XDINS RX J1856.5-3754, we computed backwards its
orbit in the Galactic potential and searched for its parental stellar cluster
by means of a closest approach criterion. We found a very likely association
with the Upper Scorpius OB association, for a neutron star age of 0.42+/-0.08
Myr, a radial velocity V_r^NS =67+/- 13$ km s^{-1}, and a present-time
parallactic distance d_\pi^NS = 123^{+11}_{-15} pc. Our result confirms that
the "true" neutron star age is much lower than the spin-down age (tau_{sd}=3.8
Myrs), and is in good agreement with the cooling age, as computed within
standard cooling scenarios. The mismatch between the spin-down and the
dynamical/cooling age would require either an anomalously large breaking index
(n~20) or a decaying magnetic field with initial value B_0 ~ 10^{14} G.
Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB
association and the masses of its members we cannot yet draw firm conclusions
on the estimated mass of the RX J1856.5-3754 progenitor.Comment: 6 pages, accepted for publication on Monthly Notices of the Royal
Astronomical Societ
Observations of Isolated Neutron Stars with the ESO Multi-Conjugate Adaptive Optics Demonstrator
High-energy observations have unveiled peculiar classes of isolated neutron
stars which, at variance with radio pulsars, are mostly radio silent and not
powered by the star rotation. Among these objects are the magnetars,
hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray
emission, and neutron stars with purely thermal, and in most cases stationary,
X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While
apparently dissimilar in their high-energy behavior and age, both magnetars and
XDINSs have similar periods and unusually high magnetic fields. This suggests a
tantalizing scenario where the former evolve into the latter.Discovering so far
uninvestigated similarities between the multi-wavelength properties of these
two classes would be a further step forward to establish an evolutionary
scenario. A most promising channels is the near infrared (NIR) one, where
magnetars are characterized by a distinctive spectral flattening with respect
to the extrapolation of the soft X-ray spectrum.We observed the two XDINSs RX
J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics
Demonstrator (MAD) at the Very Large Telescope (VLT) as part of the instrument
guaranteed time observations program, to search for their NIR counterparts.
Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K_s ~20 and
Ks ~21.5, respectively. In order to constrain the relation between XDINSs and
magnetars it would be of importance to perform deeper NIR observations. A good
candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred
magnetic field.Comment: Accepted for publication in Astronomy and Astrophysic
X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion
The X-ray source RX J0648.0-4418 is the only confirmed binary system in which
a compact object, most likely a massive white dwarf, accretes from a hot
subdwarf companion, the bright sdO star HD 49798. The X-ray emission from this
system is characterized by two periodic modulations caused by an eclipse, at
the orbital period of 1.55 d, and by the rotation of the compact object with a
spin period of 13.2 s. In 2011 we obtained six short XMM-Newton observations
centered at orbital phase 0.75, in order to study the system during the
eclipse, and spaced at increasingly long time intervals in order to obtain an
accurate measure of the spin-period evolution through phase-connected timing.
The duration of the eclipse ingress and egress, 500 s, indicates the presence
of an X-ray emitting region with dimensions of the order of a few 10^4 km,
surrounding the pulsar and probably due to scattering in the companion's wind.
We derived an upper limit on the spin-period derivative |Pdot|<6x10^-15 s/s,
more than two orders of magnitude smaller than the previously available value.
Significant X-ray emission is detected also during the 1.2 hours-long eclipse,
with a luminosity of about 3x10^30 erg/s. The eclipse spectrum shows prominent
emission lines of H- and He-like nitrogen, an overabundant element in HD 49798.
These findings support the suggestion that the X-ray emission observed during
the eclipse originates in HD 49798 and that the processes responsible for X-ray
emission in the stellar winds of massive O stars are also at work in the much
weaker winds of hot subdwarfs.Comment: Accepted for publication in Astronomy and Astrophysic
Status of the EPIC thin and medium filters on-board XMM-Newton after more than 10 years of operation II: analysis of in-flight data
After more than ten years of operation of the EPIC camera on board the X-ray observatory XMM-Newton we have reviewed the status of its thin and medium filters by performing both analysis of data collected in-flight and laboratory measurements on on-ground back-up filters. We have investigated the status of the EPIC thin and medium filters by performing an analysis of the optical loading in the PN offset maps to gauge variations in the optical and UV transmission of the filters. We both investigated repeated observations of single optically bright targets and performed a statistical analysis of the extent of loading versus visual magnitude at different epochs. We report the results of these measurements
Probing isolated compact remnants with microlensing
We consider isolated compact remnants (ICoRs), i.e. neutrons stars and black
holes that do not reside in binary systems and therefore cannot be detected as
X-ray binaries. ICoRs may represent percent of the stellar mass
budget of the Galaxy, but they are very hard to detect. Here we explore the
possibility of using microlensing to identify ICoRs. In a previous paper we
described a simulation of neutron star evolution in phase space in the Galaxy,
taking into account the distribution of the progenitors and the kick at
formation. Here we first reconsider the evolution and distribution of neutron
stars and black holes adding a bulge component. From the new distributions we
calculate the microlensing optical depth, event rate and distribution of event
time scales, comparing and contrasting the case of ICoRs and "normal stars". We
find that the contribution of remnants to optical depth is slightly lower than
without kinematics, owing to the evaporation from the Galaxy. On the other
hand, the relative contribution to the rate of events is a factor
higher. In all, percent of the events are likely related to ICoRs.
In particular, percent of the events with duration days
are possibly related to black holes. It seems therefore that microlensing
observations are a suitable tool to probe the population of Galactic ICoRs.Comment: 7 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
Mechanisms of immune escape and resistance to checkpoint inhibitor therapies in mismatch repair deficient metastatic colorectal cancers
SIMPLE SUMMARY: A subset of colorectal cancers (CRCs) is characterized by a mismatch repair deficiency that is frequently associated with microsatellite instability (MSI). The compromised DNA repair machinery leads to the accumulation of tumor neoantigens affecting the sensitivity of MSI metastatic CRC to immune checkpoint inhibitors (CPIs), both upfront and in later lines of treatment. However, up to 30% of MSI CRCs exhibit primary resistance to frontline immune based therapy, and an additional subset develops acquired resistance. Here, we first discuss the clinical and molecular features of MSI CRCs and then we review how the loss of antigenicity, immunogenicity, and a hostile tumor microenvironment could influence primary and acquired resistance to CPIs. Finally, we describe strategies to improve the outcome of MSI CRC patients upon CPI treatment. ABSTRACT: Immune checkpoint inhibitors (CPIs) represent an effective therapeutic strategy for several different types of solid tumors and are remarkably effective in mismatch repair deficient (MMRd) tumors, including colorectal cancer (CRC). The prevalent view is that the elevated and dynamic neoantigen burden associated with the mutator phenotype of MMRd fosters enhanced immune surveillance of these cancers. In addition, recent findings suggest that MMRd tumors have increased cytosolic DNA, which triggers the cGAS STING pathway, leading to interferon-mediated immune response. Unfortunately, approximately 30% of MMRd CRC exhibit primary resistance to CPIs, while a substantial fraction of tumors acquires resistance after an initial benefit. Profiling of clinical samples and preclinical studies suggests that alterations in the Wnt and the JAK-STAT signaling pathways are associated with refractoriness to CPIs. Intriguingly, mutations in the antigen presentation machinery, such as loss of MHC or Beta-2 microglobulin (B2M), are implicated in initial immune evasion but do not impair response to CPIs. In this review, we outline how understanding the mechanistic basis of immune evasion and CPI resistance in MMRd CRC provides the rationale for innovative strategies to increase the subset of patients benefiting from CPIs
Halo millisecond pulsars ejected by intermediate mass black holes in globular clusters
Intermediate mass black holes (IMBHs) are among the most elusive objects in
contemporary astrophysics. Both theoretical and observational evidence of their
existence is subject of debate. Conversely, both theory and observations
confirm the presence of a large population of millisecond pulsars (MSPs) with
low mass companions residing in globular cluster (GC) centers. If IMBHs are
common in GC centers as well, then dynamical interactions will inevitably break
up many of these binaries, causing the ejection of several fast MSPs in the
Galactic halo. Such population of fast halo MSPs, hard to produce with
'standard' MSP generation mechanisms, would provide a strong, albeit indirect,
evidence of the presence of a substantial population of IMBHs in GCs. In this
paper we study in detail the dynamical formation and evolution of such fast
MSPs population, highlighting the relevant observational properties and
assessing detection prospects with forthcoming radio surveys.Comment: 13 pages, 7 figures, 2 tables, accepted for publication in MNRA
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