5,032 research outputs found
An ``outside-in'' outburst of Aql X--1
We present optical spectroscopy and optical and infrared photometry of the
neutron star soft X-ray transient Aql X--1 during its X-ray outburst of August
1997. By modelling the X-ray, optical, and IR light curves, we find a 3 day
delay between the IR and X-ray rise times, analogous to the UV-optical delay
seen in dwarf novae outbursts and black hole X-ray transients. We interpret
this delay as the signature of an ``outside-in'' outburst, in which a thermal
instability in the outer disc propagates inward. This outburst is the first of
this type definitively identified in a neutron star X-ray transient.Comment: 6 pages latex, 2 figures, Accepted by MNRA
Momentum distributions of electron-positron pairs annihilating at vacancy clusters in Si
We report calculations of momentum densities of electron-positron pairs annihilating at various vacancy clusters in Si. The densities integrated along one direction, i.e., those corresponding to the spectra measured by the two-dimensional angular correlation of the annihilation radiation method, are shown to be isotropic if the positron is captured by a small vacancy cluster. The densities integrated along two directions correspond to the one-dimensional angular-correlation distribution or the Doppler-broadened annihilation radiation line shape. The characteristic narrowing of these one-dimensional distributions at vacancy clusters is demonstrated. The line-shape parameters S and W describing mainly the annihilations with valence and core electrons, respectively, are calculated and compared quantitatively with experimental data. We find a systematic increase in the S and a decrease in the W parameter as the size of the vacancy increases.Peer reviewe
UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star
We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P=
0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR
Hubble Space Telescope with the time and spectral resolutions of 27 s and 15
km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV
doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K).
The mean quiescent CIV flux state was close to the saturated value and 100
times the solar one. The line profile (after removing the rotational and
instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM =
70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two
separate broadening mechanisms and velocity fields at the coronal base. It is
possible that TR transient events (random multiple velocities), with a large
surface coverage, give rise to the broadening of the narrow component,while
true microflaring is responsible for the broad one.
The transition region was observed to flare frequently on different time
scales and magnitudes. The largest impulsive flare seen in the CIV 1549
emission reached in less than one minute the peak differential emission measure
(10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower
quiescent level.The 3 min average line profile of the flare was blue-shifted
(-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have
been due to a chromospheric heating and subsequent evaporation by an electron
beam, accelerated (by reconnection) at the apex of a coronal loop.Comment: to be published in AJ (April 98), 3 tables and 7 figures as separate
PS-files, print Table 2 as a landscap
UBVRI photopolarimetry of the long period eclipsing AM Herculis binary V1309
We report simultaneous UBVRI photo-polarimetric observations of the long
period (7.98 h) AM Her binary V1309 Ori. The length and shape of the eclipse
ingress and egress varies from night to night. We suggest this is due to the
variation in the brightness of the accretion stream. By comparing the phases of
circular polarization zero-crossovers with previous observations, we confirm
that V1309 Ori is well synchronized, and find an upper limit of 0.002 percent
for the difference between the spin and orbital periods. We model the
polarimetry data using a model consisting of two cyclotron emission regions at
almost diametrically opposite locations, and centered at colatitude 35 (deg)
and 145 (deg) on the surface of the white dwarf. We also present archive X-ray
observations which show that the negatively polarised accretion region is X-ray
bright.Comment: 11 pages, 12 figures (2 colour), Fig1 and Fig 4 are in lower
resolution than in original paper, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
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