A Sudden State Change in 1E 1740.7-2942

ATel #69; D. M. Smith, Space Sciences Laboratory, U. C. Berkeley; W. A. Heindl, Center for Astrophysics and Space Science, U. C. San Diego; C. B. Markwardt and J. H. Swank, NASA Goddard Space Flight Center
on 21 May 2001; 20:59 UT
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

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The black hole candidate 1E 1740.7-2942 has made the most abrupt spectral transition since the beginning of an RXTE/PCA monitoring campaign in Feb. 1996. It transitioned from a soft power law of index -2.2 on May 12 to a hard power law of index -1.5 on May 14. It had reached the -2.2 index via two months of gradual softening. The total photon flux, corrected for interstellar absorption and extrapolated to 2-100 keV, remained virtually unchanged through the gradual softening and abrupt hardening. This is shown in the figure available at the URL below. The result of the hardening is that the flux at 15 keV has approximately doubled in two days, which is unusually dramatic for this source. Both theory and recent observations suggest that a sudden onset or increase in hard- state emission might be related to an ejection/brightening of the radio jets, so radio observations are strongly encouraged.

1E 1740.7-2942 and GRS 1758-258, another black-hole candidate in the Galactic bulge with radio jets, differ from Cyg X-1 in that their power-law index and luminosity do not vary simultaneously, but show hysteresis. In a paper submitted to The Astrophysical Journal (astro-ph/0103304) we show that the patterns of variation are consistent with a two-accretion-flow model: a hot halo and thin disk are fed independently, the hot halo showing changes in the accretion rate immediately while the changes have to propagate at the viscous timescale (weeks) in the thin disk. To first order, the total photon flux is proportional to the inner disk accretion rate, and the power-law index is inversely related to the halo flow. In Cyg X-1, the spectra (and therefore the flows) change simultaneously because the disk is small.

When the power law flux in GRS 1758-258 recently dropped abruptly, we deduced that the accretion, and therefore the halo flow, had stopped, and we predicted (Astronomer's Telegram #66) that the remaining thermal disk spectrum would decay with a timescale of about a month. This prediction was fully confirmed (astro-ph/0103381, accepted for The Astrophysical Journal Letters). The corresponding explanation for the current event in 1E 1740.7-2942 would be that the halo flow has been decreasing gradually and now has suddenly jumped up. The prediction is that we will see a delayed (and perhaps slightly blurred) increase in the absorption-corrected photon flux (panel 2 of the figure at the URL below) once the change in accretion rate has propagated to the inner disk (in 3-6 weeks).

This directory contains .eps and .pdf versions of the figure