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Session 26 - Proto Stars to Supernovae.
Oral session, Monday, June 08

[26.05] Rings in the RXTE/2-colour diagram of GRS 1915+105

O. Vilhu (Univ. Helsinki), J. Nevalainen (CfA)

The Galactic superluminal source GRS 1915+105 was found to experience a very peculiar variability in a narrow PCA interval 9300 - 11300 cts/s. This can be seen as a ring-shaped pattern in the 2-colour diagram, where the hard hardness is plotted against the soft hardness. The system runs one cycle with quasi-periods ranging between 40 - 100 sec. We model this behaviour with the help of a self-consistent 2-phase model where soft seed photons from an optically thick classical disc are Comptonized in a hot spherical flow inside this disc (see Poutanen and Svensson, 1996, ApJ 470, 249; Vilhu et al. 1997, 4th Compton Symp., astro-ph 9707094).

In the model, changes of two parameters regulate the paths in the 2-colour diagram: the black body temperature (T_in) at the inner disc and the optical depth of the hot phase (\tau_e). Both of these oscillate with the same period but with a small phase-shift between each other, causing the ring-shaped pattern. Since there is a strong correlation between \tau_e and the hard tail photon index \alpha (large \tau_e - small \alpha), the model can be compared with other studies using other sets of observations and diskbb + power law modelling. We found that our model produced qualitatively the same behaviour of T_in and \alpha as found by Belloni et al. (1997, ApJ, 479, L145) and by Craig Markwardt (1997, private comm.) for their particular observations. A more detailed spectral fitting is in progress, after which the reason for the ring behaviour can be better discussed in terms of the inner disc radius, mass transfer rate and the coronal (hot phase) temperature and density.

'What do they tell and what is the spell they cast? Some of them fall and seem to recall the past.' (Smoke Rings by Ned Washington and H. Eugene Gifford)

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