XMM RPS Users' Manual

   
Example observation of a bright extended source

Consider as an example for a bright extended X-ray source a relatively compact supernova remnant (SNR). In addition to trivial input like, e.g., target name, catalogued position, etc., some more input parameters must be chosen:

Choice of prime instrument

There might be a bright knot near the desired field centre. If moderate resolution spectroscopy is most important for the proposed science, the observer might want it to be imaged in the 'aim point' of the EPIC pn camera. Alternatively, if high-resolution spectroscopy is intended, (s)he may want to centre it on the RGS-1 instrument (which is the unit with the highest energy resolution). Let us, for the time being, assume that RGS-1 is prime.

Science mode of the prime instrument

RGS can probably be used in its standard SPECTROSCOPY mode. In case of doubt, i.e., if the source has prominent emission lines, SciSim should be used to model the spectrum and check for potential pile-up problems.

Total integration time requirement

Based on PIMMS observers can convert ROSAT, ASCA or other known flux and band data (if known), and enter these, together with, e.g., X-ray spectral model = thermal bremsstrahlung, kT = 5 keV, N(H) = 1e20 cm^-2 and the lower and upper limit of the energy band over which the X-ray flux was observed.

Length of observation vs. visibility constraints

Users must check that the requested observation fits into a continuous visibility period of the XMM orbit, using the XMM Target Visibility Tool. In case the required total integration time is longer than the longest possible visibility window, the observation must be split into an adequate number of individual observations.

Pointing coordinates

Assuming that the bright knot is not located at the centre of the SNR, its coordinates must be entered into the boresight fields. This is the position on which the prime instrument will be centred. The best data quality will be achieved in the aim point of the prime instrument.

Avoidance of nearby bright sources

Optical and X-ray catalogues should be searched for nearby bright sources which might lead to contamination of either the X-ray (e.g., RGS spectral overlaps) and/or optical/UV observations. If any such sources exist and must be avoided, this might lead to a position angle constraint (which in turn makes the observation ``fixed'' in time).

Science modes of the other instruments

The science modes of the instruments will mostly be determined by the level of photon pile-up to be expected. As mentioned above (in § 5.3.2), EPIC pile-up calculations should be oriented at the brightest emission region's count rate. In the case of a bright knot in a known SNR, this could be done by estimating from (for example) ROSAT images the brightness within one XMM Point-Spread Function (PSF; see UHB section on XMM X-ray PSF ). For the PSF a Full Width at Half Maximum (FWHM) of 6'' can be assumed.

For the pn camera we compare the merits of three modes: Full Frame Mode could be used if the pile-up constraints are acceptable for the science goals. Extended Full Frame could be used if there is a high dynamic range in brightness and if image smearing of order 10% of photons is unacceptable. Large Window Mode can be used for sources up to 15' extent, and where exterior to this range there are no bright regions that would smear onto the desired image.

For MOS we compare the merits of three modes as well: Full Frame Mode could be used if the pile-up constraints are acceptable for science goals. The Large Window ( $300\times300$ pixels) can be used for sources up to 6' extent, Refreshed Framestore mode can be used for a full central CCD extent of 11' and pile-up is high with the normal mode, accepting that a low duty cycle obtains.

The expected RGS count rates are lower than those for EPIC. Therefore, RGS can in most cases be operated in its SPECTROSCOPY mode.

If no high time resolution is required for the OM observations, an imaging mode default configuration should be chosen (in this case, since RGS-1 is prime instrument, RGS 1 IMG). The optical surface brightness of the brightest region of the target must be compared with the OM brightness limits (UHB Table 19 ).

EPIC filters

Using the instructions provided in the UHB on EPIC filters , the user must decide which optical blocking filter suppresses optical loading in the soft part of the X-ray passband sufficiently and at the same time has minimal impact on the proposed science. Observations without any filter, especially of bright sources, should not be conducted, because they will be contaminated by optical radiation compromising the accuracy of the EPIC energy calibration.

RGS readout sequence

In case of standard spectroscopy observations there is no need to change anything in the RGS readout sequence. CCDs 1-9 will then be read out sequentially. However, there might be strong emission lines, which the user wants read out faster than the rest of the spectrum. One way of doing this would be to use the RGS SPECTROSCOPY mode and a readout sequence of [3 1 2 3 4 5 3 6 7 3 8 9], in which CCD#3 is read out 4 times more often than all others (assuming that the bright line would be registered on chip 3; see the UHB section on RFC arrays ).

OM brightness limit

Before planning details of OM observations, users should check for the presence of bright optical/UV sources within the OM's FOV. There should be no source in the FOV that violates the brightness constraints tabulated in UHB Table 19 . If any such source should exist (and it cannot be avoided by, e.g., reducing the OM FOV by using the magnifier as optical element), the OM MUST be put in the ``GO-OFF'' mode (which corresponds to the blocked filter position) to protect it from radiation damage.

OM filters

If the observer has no particular interest in specific wavelength ranges, the recommended OM filters (as listed in the UHB section on OM default configurations ) should be chosen.

Length of exposures

Targets with extremely high count rates might lead to very high telemetry rates. The limits listed in the UHB section on EPIC modes apply. If these are not exceeded, X-ray observations can be obtained in a single exposure covering the entire duration of the observation.

The OM exposure times should be chosen according to the explanations in § 5.3.3.5 and the OM chapter of the UHB. For the OM imaging mode each exposure must have a length of 750-5000 s. An example for how OM default configurations work is shown above, in § 3.3.