Physics
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...182.0702a&link_type=abstract
American Astronomical Society, 182nd AAS Meeting, #07.02; Bulletin of the American Astronomical Society, Vol. 25, p.801
Physics
Scientific paper
Recent HST and IUE observations show ultraviolet (1400 Angstroms) and visible (4000 Angstroms) light from the X-ray burster 4U1820--30 in the core of the globular cluster NGC 6624. The spectrum in the optical and the UV is Rayleigh--Jeans, with f_nu (1400 Angstroms)\> ~ 2 mJy. We show that this flux is due to the reprocessing of X-rays incident on an accretion disk around the neutron star, as well as on the low mass, degenerate dwarf companion. Approximately 90% of the light at 1400 Angstroms comes from reprocessing of X-rays in the disk. The disk is small in this ultracompact binary (P_orbit ~ 11 minutes), with disk radius varpi_D = (0.5--1) times 10(10) cm. The small disk size causes the spectrum to be almost Rayleigh--Jeans at wavelengths longer than 1200 Angstroms. The ``standard'' thin disk spectrum f_nu ~ nu (1/3) would not appear until wavelengths shorter than 130 Angstroms. The lack of X-ray eclipses implies i < 80(deg) . The reprocessed light from the star varies at the orbital period, with the orbitally modulated fraction ~ 7sin i % of the flux at 1400 Angstroms. Measurement of this modulation would constrain the orbital geometry. The X-ray flux also shows small amplitude quasi-periodic oscillations (QPO) peaking at f ~ 50 Hz as well as red noise extending down to a few Hz. If the disk is axisymmetric and reprocesses instantaneously, the Rayleigh--Jeans part of the disk spectrum has a fluctuation power spectrum P_r (f) = T(ft_D) P_X(f), where P_X (f) is the X-ray power spectrum and t_D = varpi_D /c. T -> 1/16 for f << 1/2pi t_D ~ 2 Hz, while for f >> 1/2pi t_D, T ~ (ft_D)(-3) . The red noise in the X-rays might appear in the reprocessed light at the ~ 0.1 % level, while QPO in the reprocessed light should be much weaker. Such reprocessing fluctuations can yield valuable insight into the physics of the disk, especially if the finite reprocessing time in the disk and departures from axisymmetry introduce further features into the transfer function T(f).
Arons Jonathan
King Ivan R.
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