920 research outputs found
Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory
There are currently over 160 known gamma-ray pulsars. While most of them are
detected only from space, at least two are now seen also from the ground. MAGIC
and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up
to hundreds of GeV and more recently MAGIC has reported emission at
TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela
pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with
pulsars has been detected by many groups, including the Milagro Collaboration.
These GeV-TeV observations open the possibility of searching for
very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC
field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015),
The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution
Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars
The low luminosity, X-ray flaring activity, of the sub-class of high mass
X-ray binaries called Supergiant Fast X-ray Transients, has been investigated
using XMM-Newton public observations, taking advantage of the products made
publicly available by the EXTraS project. One of the goals of EXTraS was to
extract from the XMM-Newton public archive information on the aperiodic
variability of all sources observed in the soft X-ray range with EPIC (0.2-12
keV). Adopting a Bayesian block decomposition of the X-ray light curves of a
sample of SFXTs, we picked out 144 X-ray flares, covering a large range of soft
X-ray luminosities (1e32-1e36 erg/s). We measured temporal quantities, like the
rise time to and the decay time from the peak of the flares, their duration and
the time interval between adjacent flares. We also estimated the peak
luminosity, average accretion rate and energy release in the flares. The
observed soft X-ray properties of low-luminosity flaring activity from SFXTs is
in qualitative agreement with what is expected by the application of the
Rayleigh-Taylor instability model in accreting plasma near the neutron star
magnetosphere. In the case of rapidly rotating neutron stars, sporadic
accretion from temporary discs cannot be excluded.Comment: Accepted for publication in MNRAS (accepted 2019 May 1; received 2019
April 30; in original form 2019 February 25). 22 pages, 16 figures, 3 tables
A candidate optical counterpart to the middle-aged gamma-ray pulsar PSR J1741-2054
We carried out deep optical observations of the middle-aged -ray
pulsar PSR J1741-2054 with the Very Large Telescope (VLT). We identified two
objects, of magnitudes and , at positions
consistent with the very accurate Chandra coordinates of the pulsar, the
faintest of which is more likely to be its counterpart. From the VLT images we
also detected the known bow-shock nebula around PSR J1741-2054. The nebula is
displaced by \sim 0\farcs9 (at the confidence level) with respect
to its position measured in archival data, showing that the shock propagates in
the interstellar medium consistently with the pulsar proper motion. Finally, we
could not find evidence of large-scale extended optical emission associated
with the pulsar wind nebula detected by Chandra, down to a surface brightness
limit of magnitudes arcsec. Future observations are needed
to confirm the optical identification of PSR J1741-2054 and characterise the
spectrum of its counterpart.Comment: 8 pages, 3 figures, Astrophysical Journal, in pres
Analytical Modeling of a New Compliant Microsystem for Atherectomy Operations
This work offers a new alternative tool for atherectomy operations, with the purpose of minimizing the risks for the patients and maximizing the number of clinical cases for which the system can be used, thanks to the possibility of scaling its size down to lumen reduced to a few tenths
of mm. The development of this microsystem has presented a certain theoretical work during the kinematic synthesis and the design stages. In the first stage a new multi-loop mechanism with a
Stephenson’s kinematic chain (KC) was found and then adopted as the so-called pseudo-rigid body mechanism (PRBM). Analytical modeling was necessary to verify the synthesis requirements. In
the second stage, the joint replacement method was applied to the PRBM to obtain a corresponding and equivalent compliant mechanism with lumped compliance. The latter presents two loops and
six elastic joints and so the evaluation of the microsystem mechanical advantage (MA) had to be calculated by taking into account the accumulation of elastic energy in the elastic joints. Hence, a new
closed form expression of the microsystem MA was found with a method that presents some new aspects in the approach. The results obtained with Finite Element Analysis (FEA) were compared to
those obtained with the analytical model. Finally, it is worth noting that a microsystem prototype can
be fabricated by using MEMS Technology classical methods, while the microsystem packaging could be a further development for the present investigation
X-ray observations and the search for Fermi-LAT gamma-ray pulsars
The Large Area Telescope (LAT) on Fermi has detected ~150 gamma-ray pulsars,
about a third of which were discovered in blind searches of the -ray
data. Because the angular resolution of the LAT is relatively poor and blind
searches for pulsars (especially millisecond pulsars, MSPs) are very sensitive
to an error in the position, one must typically scan large numbers of
locations. Identifying plausible X-ray counterparts of a putative pulsar
drastically reduces the number of trials, thus improving the sensitivity of
pulsar blind searches with the LAT. I discuss our ongoing program of Swift,
XMM-Newton, and Chandra observations of LAT unassociated sources in the context
of our blind searches for gamma-ray pulsars.Comment: 6 pages, 5 Figures, submitted to AN, proceedings of the workshop "The
Fast and the Furious: Energetic Phenomena in Isolated Neutron Stars, Pulsar
Wind Nebulae and Supernova Remnants" ESAC, Madrid, Spain 22nd - 24th May 201
Multi-wavelength observations of 3FGL J2039.6-5618: a candidate redback millisecond pulsar
We present multi-wavelength observations of the unassociated gamma-ray source
3FGL J2039.6-5618 detected by the Fermi Large Area Telescope. The source
gamma-ray properties suggest that it is a pulsar, most likely a millisecond
pulsar, for which neither radio nor -ray pulsations have been detected
yet. We observed 3FGL J2039.6-5618 with XMM-Newton and discovered several
candidate X-ray counterparts within/close to the gamma-ray error box. The
brightest of these X-ray sources is variable with a period of 0.22450.0081
d. Its X-ray spectrum can be described by a power law with photon index
, and hydrogen column density cm, which gives an unabsorbed 0.3--10 keV X-ray flux of erg cm s. Observations with the Gamma-Ray Burst
Optical/Near-Infrared Detector (GROND) discovered an optical counterpart to
this X-ray source, with a time-average magnitude . The counterpart
features a flux modulation with a period of 0.227480.00043 d that
coincides, within the errors, with that of the X-ray source, confirming the
association based on the positional coincidence. We interpret the observed
X-ray/optical periodicity as the orbital period of a close binary system where
one of the two members is a neutron star. The light curve profile of the
companion star, with two asymmetric peaks, suggests that the optical emission
comes from two regions at different temperatures on its tidally-distorted
surface. Based upon its X-ray and optical properties, we consider this source
as the most likely X-ray counterpart to 3FGL J2039.6-5618, which we propose to
be a new redback system.Comment: 35 pages, 8 figures, accepted for publication on Astrophysical
Journa
Malignant hypertension and hyperreninemia: Primary or secondary hypertension? A case report
Malignant hypertension is a rare condition characterized by severe hypertension and multi-organ ischemic damage. Marked activation of the renin-angiotensin system is observed in many patients, but its persistence over time is not known. We report a case of a 42-year-old woman who presented with severe hypertension and multi-organ damage. Initial evaluation showed an elevated value of direct renin concentration with normal plasma aldosterone concentration and a nodular lesion in the left adrenal gland. The differential diagnosis between the primary and secondary form of hypertension had to be questioned. Consequently, the patient was followed up for 20 months. Repeated checks showed a significant increase in renin levels with a normal aldosterone concentration and regression of organ damage. After 20 months, renin values returned within normal range. Hyperreninemia persisting over a long period of time has not been fully explained. Long-term follow-up allowed us to attribute malignant hypertension to de novo essential hypertension
Grasping and releasing agarose micro beads in water drops
The micromanipulation of micro objects is nowadays the focus of several investigations, specially in biomedical applications. Therefore, some manipulation tasks are required to be in aqueous environment and become more challenging because they depend upon observation and actuation methods that are compatible with MEMS Technology based micromanipulators. This paper describes how three grasping-releasing based tasks have been successfully applied to agarose micro beads whose average size is about 60 \u3bcm: (i) the extraction of a single micro bead from a water drop; (ii) the insertion of a single micro bead into the drop; (iii) the grasping of a single micro bead inside the drop. The success of the performed tasks rely on the use of a microgripper previously designed, fabricated, and tested
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