HOP 0145

abstract of observational proposal

SCIENCE CASE: Validating the XRT ground calibration and its monitoring in-flight is obviously an issue of paramount importance, for most scientific applications of the XRT data.

Recent literature shows various discrepancies between observations with XRT and with other instruments. For example, O'Dwyer et al. (2009, A&A, in press) found a significant difference between the DEM obtained with the XRT filters
with that one obtained with emission lines from EIS. The XRT filters are broad-band, but are normally dominated by Fe XVII emission in (quiescent) active regions, with some contribution from lines formed at lower and higher temperatures, and the continuum.

The EIS spectrometer observes lines emitted over a broad temperature range. Hot lines, formed at the same temperatures as those of the lines contributing to the XRT bands are also observed (see details in Del Zanna A&A 2008, 481, 69).
In particular, Fe XVII lines are observed, which allows a direct cross-calibration between EIS and XRT (for active regions), using the atomic data for this ion. After a few years of work, many new identifications of Fe XVII lines in the EIS spectra have been found, and new atomic data assessed
(Del Zanna & Ishikawa 2009, A&A in press), so we now have a fairly good understanding of the Fe XVII lines which can be used to cross-calibrate XRT and EIS. The drawback is that the EIS Fe XVII lines are normally weak, so long exposures are needed. A few problems with the intensities of some of the EIS lines are also present and need further investigations/observations.

The plan is to obtain long exposures in a wide range of EIS lines of some key elements, to obtain a DEM(T) distribution and assess the XRT calibration with forward-modeling. The EIS spectrometer does not have an in-flight radiometric calibration, however preliminary comparisons following on the O'Dwyer et al. (2009) study are very encouraging. In any case, a relative EIS/XRT calibration
valid for active region observations can be achieved. As a reference, it will be useful to run the study on the quiet Sun at the limb.

Simultaneous observations in Fe XIX with the SOHO/CDS spectrometer will be very useful.

request to EIS

A specfic EIS study, named GDZ_AR_HOT_1_280X280_SLIT2_50S was designed to include the main lines formed at high temperatures, in particular Fe XVII.
They are all weak lines (unless the AR flares) hence the use of 50s exposures, which will saturate the Fe XII 195 line. The study uses a single raster with the 2" slit, a step size of 4" and with 70 exposures covers about 280" in solar X. The extent of the FOV along the slit is approximately the same.

Total duration of the raster: 1h 3m

Total telemetry volume (estimated): 108 Mbits.

------------------------------------------
Details for EIS study definition:
ACRONYM: GDZ_AR_HOT_1_280X280
OBJECTIVE: Perform a raster over an active region for cross-calibration
with Hinode XRT. Scan a 280x280 area with the 2" slit and a sparse raster,
with long exposures (50s) and with all the brightest lines
formed at high temperatures, in particular Fe XVII.

TARGET : Active region at the limb.
EXPOSURE TIME : 50s
RASTER : Scanning
SLIT : 2"
STEP_SIZE : 4"
FOV : 280"x280"
SPECTRAL WIND.: 22
DATA COMP. : DCPM
RASTER DURATON: 1h 3m
DATA volume : 108 Mbits
LINES : Fe VIII, IX, X,XI,XII,XIII,XIV,XV,XVI,XVII, XXIII
Ni XVII, O IV, V, VI, Mg VI, Si VII, Ca XIV, XV, XVI, XVII

remarks

TARGET

Must be a quiescent active region observed at the solar limb. Ideally, the study should be repeated twice. Once, when the AR is visible on the disc, and once when it is slightly beyond the limb.

The study should be repeated at the limb on the quiet Sun, with no active regions present.

Fixed pointing is desirable.

OBSERVING PERIOD:

This is a ToO. The study should be repeated three times, for a total of 3h. Possibly, repeated 1-2 days before/after; preferably during SAA-free time period.

POINTING: a quiescent active region observed at the solar limb.

INSTRUCTION TO PLANNERS: Make sure that the XRT images and the EIS rasters are taken at the same time.

Study must be repeated at least 3 times.

WARNING:
The 50s exposure will likely saturate the srongest line.