Other
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003basbr..23...93b&link_type=abstract
Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 93-94
Other
Scientific paper
Tidally induced spiral shocks are expected to appear in dwarf novae discs during outburst as the disc expands and its outer parts feel more effectively the gravitational attraction of the secondary star. Indeed, Doppler tomography of emission lines revealed the presence of conspicuous spiral structures during outbursts of the dwarf novae IP Pegasi and U Geminorum. Eclipse mapping of IP Pegasi during outburst helped to constrain the location and orientation of the spiral structures and to show that the gas in the spiral shocks has sub-Keplerian velocities. However, recent eclipse mapping of IP Pegasi in a different outburst seems to indicate that only one arm was visible at that epoch, raising the question whether this is a limitation of the method or a real effect. In this work I report the results of a comprehensive study aiming to investigate the ability of the eclipse mapping method to recover spiral structures in accretion discs as a function of (i) the relative brightness of the spiral arms to that of the underlying disc, (ii) the orientation of the spiral arms, and (iii) the signal-to-noise ratio and the phase resolution of the eclipse light curves. It is shown that the orientation and radial position of the spiral arms are well recovered in eclipse maps from light curves of signal-to-noise ratio as low as s/n = 25 and even in the case where the emission from the spirals is diluted by a symmetric disc component of up to 60 per cent of the total brightness, regardless of the orientation of the spirals. On the other hand, simulations with models in which one of the arms is made fainter than the other by a factor of 2 reveal that the brightness contrast between the two arms in the eclipse map is much more pronounced than the real flux ratio, and that the fainter arm starts to disappear into the underlying symmetric disc component even for light curves of s/n = 50. Therefore, the absence of one spiral arm in an eclipse map is not an artefact of the method, but probably an indication of a real difference in apparent brightness between the two arms. The "missing" arm could be caused by obscuration of its emission by intervening optically thick disc material along the line of sight to the observer at orbital phases around the eclipse.
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