1,256 research outputs found
Two fast X-ray transients in archival Chandra data
We present the discovery of two new X-ray transients in archival Chandra
data. The first transient, XRT 110103, occurred in January 2011 and shows a
sharp rise of at least three orders of magnitude in count rate in less than 10
s, a flat peak for about 20 s and decays by two orders of magnitude in the next
60 s. We find no optical or infrared counterpart to this event in preexisting
survey data or in an observation taken by the SIRIUS instrument at the Infrared
Survey Facility 2.1 yr after the transient, providing limiting magnitudes of
J>18.1, H>17.6 and Ks>16.3. This event shows similarities to the transient
previously reported in Jonker et al. which was interpreted as the possible
tidal disruption of a white dwarf by an intermediate mass black hole. We
discuss the possibility that these transients originate from the same type of
event. If we assume these events are related a rough estimate of the rates
gives 1.4*10^5 per year over the whole sky with a peak 0.3-7 keV X-ray flux
greater than 2*10^-10 erg cm^-2 s^-1 . The second transient, XRT 120830,
occurred in August 2012 and shows a rise of at least three orders of magnitude
in count rate and a subsequent decay of around one order of magnitude all
within 10 s, followed by a slower quasi-exponential decay over the remaining 30
ks of the observation. We detect a likely infrared counterpart with magnitudes
J=16.70+/-0.06, H=15.92+/-0.04 and Ks=15.37+/-0.06 which shows an average
proper motion of 74+/-19 milliarcsec per year compared to archival 2MASS
observations. The JHKs magnitudes, proper motion and X-ray flux of XRT 120830
are consistent with a bright flare from a nearby late M or early L dwarf.Comment: Accepted for publication in MNRAS, 6 pages, 5 figure
Laboratory Measurements Of White Dwarf Photospheric Spectral Lines: H Beta
We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. Here we present time-resolved measurements of H beta and fit this line using different theoretical line profiles to diagnose electron density, n(e), and n = 2 level population, n2. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n(e) similar to 4 to similar to 30 x 10(16) cm(-3) throughout a 120-ns evolution of our plasma. Also, we observe n(2) to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within similar to 55 ns to become consistent with LTE. This supports our electrontemperature determination of T-e similar to 1.3 eV (similar to 15,000 K) after this time. At n(e) greater than or similar to 10(17) cm(-3), we find that computer-simulation-based line-profile calculations provide better fits (lower reduced chi(2)) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.Laboratory Directed Research and Development programUnited States Department of Energy DE-AC04-94AL85000, DE-SC0010623National Science Foundation DGE-1110007Astronom
Chemo-Sensitive Running Droplet
Chemical control of the spontaneous motion of a reactive oil droplet moving
on a glass substrate under an aqueous phase is reported. Experimental results
show that the self-motion of an oil droplet is confined on an acid-treated
glass surface. The transient behavior of oil-droplet motion is also observed
with a high-speed video camera. A mathematical model that incorporates the
effect of the glass surface charge is built based on the experimental
observation of oil-droplet motion. A numerical simulation of this mathematical
model reproduced the essential features concerning confinement within a certain
chemical territory of oil-droplet motion, and also its transient behavior. Our
results may shed light on physical aspects of reactive spreading and a
chemotaxis in living things.Comment: 17 pages, 10 figure
The expanding bipolar shell of the helium nova V445 Puppis
From multi-epoch adaptive optics imaging and integral field unit spectroscopy, we report the discovery of an expanding and narrowly confined bipolar shell surrounding the helium nova V445 Puppis (Nova Puppis 2000). An equatorial dust disc obscures the nova remnant, and the outflow is characterized by a large polar outflow velocity of 6720 +/- 650 km s(-1) and knots moving at even larger velocities of 8450 +/- 570 km s(-1). We derive an expansion parallax distance of 8.2 +/- 0.5 kpc and deduce a pre-outburst luminosity of the underlying binary of log L/L-circle dot = 4.34 +/- 0.36. The derived luminosity suggests that V445 Puppis probably contains a massive white dwarf accreting at high rate from a helium star companion making it part of a population of binary stars that potentially lead to supernova Ia explosions due to accumulation of helium-rich material on the surface of a massive white dwarf
Tomography of full sawtooth crashes on the Tokamak Fusion Test Reactor
Full sawtooth crashes in high temperature plasmas have been investigated on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)]. A strong asymmetry in the direction of major radius, a feature of the ballooning mode, and a remaining m=1 region after the crash have been observed with electron cyclotron emission image reconstructions. The TFTR data is not consistent with two-dimensional (2-D) models; it rather suggests a three-dimensional (3-D) localized reconnection arising on the bad curvature side. This process explains the phenomenon of fast heat transfer which keeps the condition q0<1
Tomography of (2, 1) and (3, 2) magnetic island structures on Tokamak Fusion Test Reactor
High-resolution electron cyclotron emission (ECE) image reconstruction has been used to observe (m,n)=(2,1) and (3, 2) island structures on Tokamak Fusion Test Reactor [Plasma Phys. Controlled. Fusion 33, 1509 (1991)], where m and n are the poloidal and the toroidal mode number, respectively. The observed island structure is compared with other diagnostics, such as soft x-ray tomography and magnetic measurements. A cold elliptic island is observed after lithium pellet injection. Evidence for the enhancement of the heat transfer due to the island is observed. A relaxation phenomenon due to the m=2 mode is newly observed in Ohmic plasmas
The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C
The detection of GeV photons from gamma-ray bursts (GRBs) has important
consequences for the interpretation and modelling of these most-energetic
cosmological explosions. The full exploitation of the high-energy measurements
relies, however, on the accurate knowledge of the distance to the events. Here
we report on the discovery of the afterglow and subsequent redshift
determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray
Space Telescope with high significance detection of photons at >0.1 GeV.
Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO
telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South
Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift
of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray
Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5%
most distant GRBs, and makes it the most energetic GRB known to date. The
detection of GeV photons from such a distant event is rather surprising.
The observed gamma-ray variability in the prompt emission together with the
redshift suggests a lower limit for the Lorentz factor of the
ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous
measurements of Gamma in GRBs and strengthens the extreme nature of GRB
080916C.Comment: 6 pages, 5 figures; subm. to A&
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