2,043 research outputs found
Remote lightning monitor system
An apparatus for monitoring, analyzing and accurately determining the value of peak current, the peak rate of change in current with respect to time and the rise time of the electrical currents generated in an electrical conductive mast that is located in the vicinity where lightning is to be monitored is described. The apparatus includes an electrical coil for sensing the change in current flowing through the mast and generating a voltage responsive. An on-site recorder and a recorder control system records the voltages produced responsive to lightning strikes and converts the voltage to digital signals for being transmitted back to the remote command station responsive to command signals. The recorder and the recorder control system are carried within an RFI proof environmental housing into which the command signals are fed by means of a fiber optic cable so as to minimize electrical interference
Lymphotoxins and cytomegalovirus cooperatively induce interferon-beta, establishing host-virus détente
Tumor necrosis factor (TNF)-related cytokines regulate cell death and survival and provide strong selective pressures for viruses, such as cytomegalovirus (CMV), to evolve counterstrategies in order to persist in immune-competent hosts. Signaling by the lymphotoxin (LT)-β receptor or TNF receptor-1, but not Fas or TRAIL receptors, inhibits the cytopathicity and replication of human CMV by a nonapoptotic, reversible process that requires nuclear factor κB (NF-κB)-dependent induction of interferon-β (IFN-β). Efficient induction of IFN-β requires virus infection and LT signaling, demonstrating the need for both host and viral factors in the curtailment of viral replication without cellular elimination. LTα-deficient mice and LTβR-Fc transgenic mice were profoundly susceptible to murine CMV infection. Together, these results reveal an essential and conserved role for LTs in establishing host defense to CMV
The Chandra Galactic Bulge Survey: optical catalogue and point-source counterparts to X-ray sources
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of
optical sources in the GBS footprint. This consists of two regions centered at
Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning
(l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of
observations for each line of sight in r', i' and H{\alpha} filters. It is
complete down to r' = 20.2 and i' = 19.2 mag; the mean 5{\sigma} depth is r' =
22.5 and i' = 21.1 mag. The mean root-mean-square residuals of the astrometric
solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the
1640 unique X-ray sources detected in Chandra observations of the GBS area, and
find candidate optical counterparts to 1480 X-ray sources. We use a false alarm
probability analysis to estimate the contamination by interlopers, and expect ~
10 per cent of optical counterparts to be chance alignments. To determine the
most likely counterpart for each X-ray source, we compute the likelihood ratio
for all optical sources within the 4{\sigma} X-ray error circle. This analysis
yields 1480 potential counterparts (~ 90 per cent of the sample). 584
counterparts have saturated photometry (r'<17, i'<16), indicating these objects
are likely foreground sources and the real counterparts. 171 candidate
counterparts are detected only in the i'-band. These sources are good qLMXB and
CV candidates as they are X-ray bright and likely located in the Bulge.Comment: 18 pages, 18 figures. Published in MNRAS. 2016MNRAS.458.4530
The Relationship Between X-ray Luminosity and Duty Cycle for Dwarf Novae and their Specific Frequency in the Inner Galaxy
We measure the duty cycles for an existing sample of well observed, nearby
dwarf novae using data from AAVSO, and present a quantitative empirical
relation between the duty cycle of dwarf novae outbursts and the X-ray
luminosity of the system in quiescence. We have found that , where
DC stands for duty cycle. We note that there is intrinsic scatter in this
relation greater than what is expected from purely statistical errors. Using
the dwarf nova X-ray luminosity functions from \citet{Pretorius12} and
\citet{Byckling10}, we compare this relation to the number of dwarf novae in
the Galactic Bulge Survey which were identified through optical outbursts
during an 8-day long monitoring campaign. We find a specific frequency of X-ray
bright () Cataclysmic Variables undergoing
Dwarf Novae outbursts in the direction of the Galactic Bulge of
. Such a specific frequency would give
a Solar neighborhood space density of long period CVs of
pc. We advocate the use of specific
frequency in future work, given that projects like LSST will detect DNe well
outside the distance range over which .Comment: 9 pagers, 4 figures Accepted for publication in MNRA
Constraining the nature of the accreting binary in CXOGBS J174623.5-310550
We report optical and infrared observations of the X-ray source CXOGBS
J174623.5-310550. This Galactic object was identified as a potential quiescent
low-mass X-ray binary accreting from an M-type donor on the basis of optical
spectroscopy and the broad Halpha emission line. The analysis of X-shooter
spectroscopy covering 3 consecutive nights supports an M2/3-type spectral
classification. Neither radial velocity variations nor rotational broadening is
detected in the photospheric lines. No periodic variability is found in I- and
r'-band light curves. We derive r' = 20.8, I = 19.2 and Ks = 16.6 for the
optical and infrared counterparts with the M-type star contributing 90% to the
I-band light. We estimate its distance to be 1.3-1.8 kpc. The lack of radial
velocity variations implies that the M-type star is not the donor star in the
X-ray binary. This could be an interloper or the outer body in a hierarchical
triple. We constrain the accreting binary to be a < 2.2 hr orbital period
eclipsing cataclysmic variable or a low-mass X-ray binary lying in the
foreground of the Galactic Bulge.Comment: (9 pages, 5 figures, accepted for publication in MNRAS
Spectroscopic classification of X-ray sources in the Galactic Bulge Survey
We present the classification of 26 optical counterparts to X-ray sources
discovered in the Galactic Bulge Survey. We use (time-resolved) photometric and
spectroscopic observations to classify the X-ray sources based on their
multi-wavelength properties. We find a variety of source classes, spanning
different phases of stellar/binary evolution. We classify CX21 as a quiescent
cataclysmic variable (CV) below the period gap, and CX118 as a high accretion
rate (nova-like) CV. CXB12 displays excess UV emission, and could contain a
compact object with a giant star companion, making it a candidate symbiotic
binary or quiescent low mass X-ray binary (although other scenarios cannot be
ruled out). CXB34 is a magnetic CV (polar) that shows photometric evidence for
a change in accretion state. The magnetic classification is based on the
detection of X-ray pulsations with a period of 81 2 min. CXB42 is
identified as a young stellar object, namely a weak-lined T Tauri star
exhibiting (to date unexplained) UX Ori-like photometric variability. The
optical spectrum of CXB43 contains two (resolved) unidentified double-peaked
emission lines. No known scenario, such as an AGN or symbiotic binary, can
easily explain its characteristics. We additionally classify 20 objects as
likely active stars based on optical spectroscopy, their X-ray to optical flux
ratios and photometric variability. In 4 cases we identify the sources as
binary stars.Comment: Accepted for publication in MNRA
Variability of Optical Counterparts in the Chandra Galactic Bulge Survey
We present optical lightcurves of variable stars consistent with the
positions of X-ray sources identified with the Chandra X-ray Observatory for
the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on
the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on
timescales from hr to 8 days over the of the X-ray survey
containing sources from the initial GBS catalog. Among the lightcurve
morphologies we identify are flickering in interacting binaries, eclipsing
sources, dwarf nova outbursts, ellipsoidal variations, long period variables,
spotted stars, and flare stars. of X-ray sources have at least one
potential optical counterpart. of these candidate counterparts are
detectably variable; a much greater fraction than expected for randomly
selected field stars, which suggests that most of these variables are real
counterparts. We discuss individual sources of interest, provide variability
information on candidate counterparts, and discuss the characteristics of the
variable population.Comment: Accepted for publication in the Astrophysical Journal Supplement
The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
The All-Sky Automated Survey for Supernovae (ASAS-SN) is working towards
imaging the entire visible sky every night to a depth of V~17 mag. The present
data covers the sky and spans ~2-5~years with ~100-400 epochs of observation.
The data should contain some ~1 million variable sources, and the ultimate goal
is to have a database of these observations publicly accessible. We describe
here a first step, a simple but unprecedented web interface
https://asas-sn.osu.edu/ that provides an up to date aperture photometry light
curve for any user-selected sky coordinate. Because the light curves are
produced in real time, this web tool is relatively slow and can only be used
for small samples of objects. However, it also imposes no selection bias on the
part of the ASAS-SN team, allowing the user to obtain a light curve for any
point on the celestial sphere. We present the tool, describe its capabilities,
limitations, and known issues, and provide a few illustrative examples.Comment: 8 pages, 9 figures, submitted to PAS
HD 145263: Spectral Observations of Silica Debris Disk Formation via Extreme Space Weathering?
We report here time domain infrared spectroscopy and optical photometry of
the HD145263 silica-rich circumstellar disk system taken from 2003 through
2014. We find an F4V host star surrounded by a stable, massive 1e22 - 1e23 kg
(M_Moon to M_Mars) dust disk. No disk gas was detected, and the primary star
was seen rotating with a rapid ~1.75 day period. After resolving a problem with
previously reported observations, we find the silica, Mg-olivine, and
Fe-pyroxene mineralogy of the dust disk to be stable throughout, and very
unusual compared to the ferromagnesian silicates typically found in primordial
and debris disks. By comparison with mid-infrared spectral features of
primitive solar system dust, we explore the possibility that HD 145263's
circumstellar dust mineralogy occurred with preferential destruction of
Fe-bearing olivines, metal sulfides, and water ice in an initially comet-like
mineral mix and their replacement by Fe-bearing pyroxenes, amorphous pyroxene,
and silica. We reject models based on vaporizing optical stellar megaflares,
aqueous alteration, or giant hypervelocity impacts as unable to produce the
observed mineralogy. Scenarios involving unusually high Si abundances are at
odds with the normal stellar absorption near-infrared feature strengths for Mg,
Fe, and Si. Models involving intense space weathering of a thin surface patina
via moderate (T < 1300 K) heating and energetic ion sputtering due to a stellar
superflare from the F4V primary are consistent with the observations. The space
weathered patina should be reddened, contain copious amounts of nanophase Fe,
and should be transient on timescales of decades unless replenished.Comment: 41 Pages, 5 Figures, 5 Tables, Accepted for publication in the
Astrophysical Journa
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