Abstract
We report on an observation of the Galactic black hole candidate GRS 1739-278
during its 2014 outburst, obtained with NuSTAR. The source was captured at the
peak of a rising "low/hard" state, at a flux of \~0.3 Crab. A broad, skewed iron
line and disk reflection spectrum are revealed. Fits to the sensitive NuSTAR
spectra with a number of relativistically blurred disk reflection models yield
strong geometrical constraints on the disk and hard X-ray "corona". Two models
that explicitly assume a "lamppost" corona find its base to have a vertical
height above the black hole of h = 5 (+7, -2) GM/c^2 and h = 18 +/-4 GM/c^2
(90\% confidence errors); models that do not assume a "lamppost" return
emissivity profiles that are broadly consistent with coronae of this size.
Given that X-ray microlensing studies of quasars and reverberation lags in
Seyferts find similarly compact coronae, observations may now signal that
compact coronae are fundamental across the black hole mass scale. All of the
models fit to GRS 1739-278 find that the accretion disk extends very close to
the black hole - the least stringent constraint is r = 5 (+3,-4) GM/c^2. Only
two of the models deliver meaningful spin constraints, but a = 0.8 +/-0.2 is
consistent with all of the fits. Overall, the data provide especially
compelling evidence of an association between compact hard X-ray coronae and
the base of relativistic radio jets in black holes.
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