resik weekly

In the display above (click to enlarge), the solar X-ray fluence seen by RESIK (black points) is plotted atop GOES red line. Only periods with good spectral measurements are indicated. The total number of good spectral measurements is given in the title line.

New mode:	'3rd order reflections' (ORD3) is used as stand-by mode. 3rd order spectra observations are indicated on the main catalogue page http://www.cbk.pan.wroc.pl/2002.htm (cf. green boxes).
Mode tests:	On 20 December 2002 (from 00:00 till 12:00) test of the new version of ORD1 mode has been made . In this test the HV on detector B was set to 'old' value 1419 V. The aim was to lessen the 'Notch' effects. Test went OK, and the ORD1 HV settings will be set to1419 V starting next week. See the spectrum below taken during the test.
Calibrations made:	No calibrations made
ADS Settings:

(ORD1) HV: Det. A - 1450 V, Det. B - 1389 V.				(ORD3) HV: Det. A - 1389 V, Det. B - 1328 V.
Channel		l Band	ADS	Channel		l Band	ADS
#1	#2	3.37 - 3.88 Å	35 - 85	#1	#2	1.16 - 1.29 Å	110 - 230
#2	#0	3.82 - 4.33 Å	70 - 120	#2	#0	1.27 - 1.44 Å	110 - 230
#3	#3	4.31 - 4.89 Å	55 - 110	#3	#3	1.44 - 1.63 Å	110 - 230
#4	#1	4.96 - 6.09 Å	90 - 160	#4	#1	1.65 - 2.03 Å	140 - 230
		software indexes				software indexes
	spectroscopic notation (papers)				spectroscopic notation (papers)

As the CORONAS-F satellite is pointed precisely towards the direction of the centre of solar disk (see Weekly_note_50), this direction moves arcoss the stellar reference frame along the ecliptic, as Earth circles the Sun. Therefore all X-ray sources which got chance to illuminate the RESIK crystals at the propper angles (fulfilling the Bragg law) may contribute to the measured spectra. In addition to this, a direct illumination of the detector Be windows is possible, since no special collimator has been used. As explained in the sketch below, the ‘viewing’ solid angle for the detectors placed inside RESIK is rather large, extending up to tens of degrees in certain directions (the viewing angle is limited in some areas of the sky due to to vignietting by a neigbour tall instruments within the CORONAS-F payload or by the RESIK instrument hardware (i.e. the EUV filters mount).

Figure 1. A scheme of X-ray beams within one section of RESIK Bragg spectrometer

It is natural, that the stellar sources ‘moves’ in respect with the frame related to the Sun and as a consequence the illumination condition for these sources is changing. The relative rate of this motion is about 1degree/day.

Figure 2. The trajectory (right ascension and declination) of the Sun across the constellation of bright
X-ray sources (blue stars) is drawn for October.

As a consequence, directly illuminated portion of the detector (bluish ray path area in Fig. 1) varies smoothly in time. We observed the position of sharp edges of the bumps seen on the detector changing this way.
By estimating an approximate illumination angles from the momentary position of bump’s edges, we defined the variarion of the postulated non-solar illumination angle, and the corresponding variations of the source angular distance to the Sun.
Corresponding plot is drawn in Fig. 3.

Figure 3. Predicted distance from the solar centre to a number of brighter non-solar X-ray sources in October (blue lines). Olive heavy line represents the observed variation deduced from changing position of the bump on the detector. It fits best with the predicted line drawn for the Sco X-1 source (marked).

The coincidence between the observed and predicted Sun-Sco X-1 distance variations, led us to make further attempt to locate the non-solar source with much better precision. In this respect, we used the occultation times of the non-solar source by the edge of the Earth atmosphere. It is easy to imagine that on its semi-Sun-synchonous orbit, the CORONAS-F usually enters Earth shadow each orbit. For the non-solar source, its eclipse by Earh shadow is somewhat delayed in time, in case the source is off-set by tens of degrees.In the short period after sunset and before the source eclipse detectors observe only the non-solar source. The source occultation time can be estimated quite precisely, to within few seconds. Knowing the geographic location of the CORONAS-F satellite at the time of the source occultation allows to draw the position of the Earth occulting edge projected on the sky map.

For each recorded source occultation event, we can plot respective occultation contour. An example is shown in Fig. 4.

Figure 4. The positions of Earth occulted areas (the inside of ovals plotted) as determined from occultation timing for three dates: Ocober 22, 27 and November 5, 2002. The ovals cross in two regions of the sky. In one of them is Sco X-1 present.

We noticed that the Sco X-1 source is placed in one of these regions as shown in Fig. 4. No other bright X-ray source share this location. After zooming the region of interest (see Fig. 5), we noticed however, that Sco X-1 is slightly off-set from the predicted common contour intersection point.

Figure 5. Enlargement of the map region where the occultation contours cross. Full colored lines representing occulting Earth edge are drawn assuming the Earth has ‘standard’ radius of 6378 km. Dashed lines are drawn for the radius increased by the approximate height of X-ray non-transparent atmospere (+85 km).

On the close-up, we noticed the source being off-set from the common crossing point by few degrees, the distance much larger than the expected uncertainty of the common crossing point location. Then we realised, that the radius of Earth for occultation of the X-ray sky should be increased by 70 – 100 km as the atmosphere is transparent to X-rays only above these heights. With this increased radius, the position of Sco X-1 matches the crossing point precisely. Given the uncertainty of the Earth radius, the occultation timing and the uncertainty of the CORONAS-F ephemeries, the observed overlap is remarkable. This led us to the conclusion that the bumps noticed on the RESIK spectra are due to direct illumination of detectors by Sco X-1 (http://chandra.harvard.edu/xray_sources/sco/sco.html).

We will further analyse the observed Sco X-1 emission in order to precisely calibrate RESIK sensitivity in relation to Chandra, Newton and Intergral X-ray experiments. Such comparison may also allow for determination of common astrophysical photometric scale. Comparison of the Sco X-1 signal observed in different sections of RESIK can be helpful in cross-calibrating the sensitivity of individual spectrometer channels. The X-ray flux of Sco X-1 is few orders of magnitude weaker than the solar one, therefore we do not expect to see its Bragg spectra.

The report presented has been done in "real time" and so it may contain jargon, blunders, or trivialities. We do not have also an English native speaker in our Wroclaw group! We would be happy to discuss problems mentioned above in more details if necessary.
RESIK data are in the open public domain and can be requested from: http://surfwww.mssl.ucl.ac.uk/surf/data_request.html.