177 research outputs found
Mass transfer from the donor of GRS 1915+105
A scenario for a periodic filling and emptying of the accretion disc of the microquasar GRS 1915+105 is proposed, by computing the mass transfer rate from the evolving low mass red giant donor (Greiner at al. 2001) and comparing it with the observed accretion rate onto the primary black hole. We propose a duty-cycle with (5-10)(eta/0.1) per cent active ON-state where eta is the efficiency of converting accretion into radiation. The duration of the quiescent recurrent OFF-state is identified as the viscosity time scale at the circularization radius and equals 370(alpha/0.001)^(-4/5) years, where alpha is the viscosity parameter in the alpha-prescription of a classical disc. If the viscosity at the outer edge of the disc is small and eta is close to the maximum available potential energy (per rest mass energy) at the innermost stable orbit, the present active phase may last another 10 - 20 years
Microflaring of a solar Bright point
A solar X-ray Bright point (BP) was observed with the SUMER-spectrograph of
the SOHO-observatory. The data consist of four far-UV spectral lines formed
between 2 10^4 - 6 10^5 K, with 2 arcsec spatial, 2.8 min temporal and 4 km/s
spectral resolution. A striking feature is the strong microflaring and
appearance of several short lived transients. Using simultaneous magnetic field
measurements the region observed seemed to lie above a cancelling flux region.
With respect to the filling factor and emission measure this particular BP was
similar to the average surface of a moderately active solar type star.Comment: 7 pages, 6 figures, in press Astronomy and Astrophysics; for Fig.3 it
is recommended to download separately the colour version h3653f3.pd
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
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