The Residual X-ray Emission in the Venus Shadow
G. Peres(1,2), M. Afshari(1), A.F. Gambino(1), F. Reale(1,2)
(1)Dip.
di Fisica e Chimica, Università di Palermo
di Palermo “G.S. Vaiana”
(2)INAF/Oss. Astron.
Rationale:
To understand planetary and exoplanetary atmospheres we have studied the X-ray and EUV residual
emission in the shadow of Venus, and its evolution, during its transit across the Sun disk (2012 June
5/6).
Fig 1 shows the relevant features of the transit.
Fig. 2 shows the limb of the Sun and Venus in the X-ray band (Hinode/XRT) just before the transit.
Reale et al. (2014, submitted) studied the radius of Venus disk in EUV.
The X-ray flux (Hinode/XRT)
Fig. 3 shows the X-ray flux along ten cuts across the Venus shadow in the N-S
directions, taken at different times. The lower flux (~ 7 DN/s) pertain to the shadow,
extending over ~ 60 arc sec, the sharp jumps on both sides mark the border of the
shadow; we included small parts of the surrounding region on both sides. The average
X-ray flux in the shadow should be, in principle, compatible with noise; in fact it is not,
being a large fraction of the flux in the surrounding regions. Fig. 4 shows ten
analogous profiles in the E-W direction. We want to show just the values of the flux
inside and outside the shadow so we did not align the borders of Venus shadow.
The evolution of the X-ray flux:
Here we show the evolution of the flux in the shadow during the transit and compare it
with that in two rings adjacent and concentric with Venus shadow; the smaller ring
extends from R to 2R, R being the Venus radius; the larger ring extends from R to 3R.
Fig, 5 shows a zoom of an X-ray image with the Venus shadow and the surrounding
smaller ring where we evaluated the average flux.
Fig. 6 shows the evolution of the average flux in the shadow and that in the two
surrounding concentric rings.
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
COMMENT ON THE X-RAY ANALYSIS
One may expect that the residual emission in the shadow is due to scatter inside the telescope, tied to the wings of the Point Spread Function (PSF) of
Hinode/XRT. However the known characteristics of the PSF (Golub, 2014, private communucation) cannot justify such large values and at such a large
angular distance from the original source.There is a similar rise and fall between the flux evolution in the shadow and that in the two rings but there is no
correlation of the two evolutions. For instance the flux in the shadow does not reflect the effect of the X-ray limb brightening (the large dip at the
beginning of the evolution of flux in the rings), present instead in the evolution of the flux in the rings. There also appear to be a sort of delay in the
emission from the shadow. A scatter in the telescope should show matching evolutions in the shadow and in the rings.
A different view: EUV and UV (SDO/AIA)
To verify if the effect is due to the instrument, we checked with a different satellite and telescope: SDO/
AIA. Fig. 7 shows a section, analogous to Fig.3, but in the UV. Figs. From 8 to 11 show the evolution of
the average flux inside the Venus shadow and in the surrounding rings, analogously to what is in Fig. 6, but
in four wavelengths: respectively 335 A (forming at 3.5 MK), 193 A (1.5 MK), 304 A (0.1 MK), 1700 A
(chromosphere) lines. Dips in Fig. 10 are due to incorrect exposure times, but follow the trend.
Fig.7
COMMENT ON THE EUV – UV ANALYSIS
There is some similarity with results in the X-ray band but the
UV flux in the shadow is proportionally much smaller than in the
surrounding regions. The 335 A line (the hottest, 3.5MK) shows,
similarly to X-ray, a difference relative to the surrounding rings.
As we go from coronal to chromospheric lines, we get a
behaviour more and more similar in the shadow and in the rings.
SDO/AIA telescope, being a normal incidence one, is expected to
have considerable less scatter, and a sharper PSF, than an X-ray
telescope.
Fig.8
Fig.10
Fig.9
Fig.11
Conclusions
The shadow flux at both 335 A and X-rays (both forming in hot corona) coarsely follow the trend of the surrounding corona but with significant and
inexplicable differences and some delay. As we consider a more and more chromospheric emission the evolution is more and more similar.
The X-ray coronal emission may be scattered by droplets in the upper Venus atmosphere forming a halo (see e.g. Draine 2003, The ApJ, 598, 1026).
Future steps:
comparing these results with those of solar eclipses (any effect would not be due to the atmosphere)
determining the scatter of X-rays in the upper atmosphere of Venus according to Draine’s work.