  Context: Hard X-rays from solar flares are an important diagnostic of
particle acceleration and transport in the solar atmosphere. Any observed X-ray
flux from on-disc sources is composed of direct emission plus Compton
backscattered photons (albedo). This affects both the observed spectra and
images as well as the physical quantities derived from them such as the spatial
and spectral distributions of accelerated electrons or characteristics of the
solar atmosphere. Aims: We propose a new indirect method to measure albedo and
to infer the directivity of X-rays in imaging using RHESSI data. Methods:
Visibility forward fitting is used to determine the size of a disc event
observed by RHESSI as a function of energy. This is compared to the sizes of
simulated sources from a Monte Carlo simulation code of photon transport in the
chromosphere for different degrees of downward directivity and true source
sizes to find limits on the true source size and the directivity. Results: The
observed full width half maximum of the source varies in size between 7.4
arcsec and 9.1 arcsec with the maximum between 30 and 40 keV. Such behaviour is
expected in the presence of albedo and is found in the simulations. A source
size smaller than 6 arcsec is improbable for modest directivities and the true
source size is likely to be around 7 arcsec for small directivities.
Conclusions: While it is difficult to image the albedo patch directly, the
effect of backscattered photons on the observed source size can be estimated.
The increase in source size caused by albedo has to be accounted for when
computing physical quantities that include the size as a parameter such as
flare energetics. At the same time, the study of the albedo signature provides
vital information about the directivity of X-rays and related electrons.
