Frequently asked questions (FAQ)

Why is eSASS called eSASS?

eSASS derives its heritage mainly from the ROSAT and XMM-Newton data analysis packages. This is also the case for its name:

Drawing from the above acronyms eSASS might thus stand for:

In another line of reasoning, we started out to refer to the eROSITA data analysis software as eROSITA-SASS (with SASS referring to the ROSAT term), which eventually was shortened to eSASS. This also nicely fits with eRASS, another eROSITA acronym, which stands for eROSITA All-Sky Survey, referring back to the RASS (ROSAT All-Sky Survey). It is important to note that while ROSAT provided two different software packages for the pipeline (SASS) and interactive (EXSAS) data analysis, both functions are combined in one package in eSASS. The "Standard" for the first "S" in SASS is not appropriate for eSASS and it was replaced with "Science". As the "e" in this case refer to eROSITA, it's probably best to think of eSASS as standing for:

How do I install eSASS?

Go to this page for for details.

How do I run eSASS tasks?

You will be able to run eSASS tasks at the command prompt after you executed one of the following setup scripts:

$ source /<my eSASS path>/eSASS4EDR/bin/esass-init.sh

or

$ source /<my eSASS path>/eSASS4EDR/bin/esass-init.csh

How does eSASS access calibration data?

eSASS make use of the HEASARC CALDB framework to manage the eROSITA calibration files. This is handled automatically by the eSASS setup script.

Why do I keep getting the error message: "**STOP** Error initializing PIL" when I run eSASS task X?

This is a relatively frequent error message which occurs in the following situations:

  1. Your command line call is not correct. Please make sure you entered all required command line parameters in the correct spelling. Array parameters require string quotes. Use FTOOLS command plist for a list of all task parameters.
  2. You recently switched to a different eSASS release version. It is possible that the command line of the task in question has changed. Please adjust your scripts and delete the .par file of the task in question from the pfiles directory in your home directory.
  3. For whatever reason, the .par file of the task in question in the pfiles directory in your home directory may be garbled. Please delete it. It will be automatically recreated once you call the task again.
  4. The contents of the PFILES environment variable may not be correct (for whatever reason). In this case, the PIL error will occur with each eSASS task you call. The PFILES variable needs to start with the full path of the pfiles directory in your home directory followed by a semicolon. The .par directory of the eSASS release you are using as well as the pfiles directories of any other FTOOLS based tools you may have installed should be listed after the semicolon. The PFILES environment variable should normally be set correctly by the eSASS setup script.

Why do I keep getting the error message: "**ERROR2** YY //YY" when I run eSASS task X?

This error is harmless and it has no impact on the task processing.

Why is the size of the images created with evtool so big?

Most of the image's size is comprised of the other extensions in the image file (approximately 63 different tables). Please have a look at the evtool task documentation to disable such behaviour.

How can I download eROSITA data?

Please refer to this page for further information.

Do I need to acknowledge the use of public eROSITA data in my publication?

Please refer to this page for further information.

What is the chopper and how can it affect my observation?

eROSITA generates a CCD frame about every 50 ms. The chopper parameter can be used to specify that only the n-th frame will be read out, stored and sent back to Earth. For example, in order to reduce the telemetry due to bright sources, one can use chopper values > 1. The chopper value can be commanded individually for each of the seven eROSITA cameras. If a chopper value equal to n (>1) is commanded for a camera, then the effective exposure time of that camera of the observation will be reduced by a factor n.

Some of the eROSITA Calibration and Performance Verification (Cal-PV) observations were performed with chopper values different from 1. These observations include Vela SNR, Puppis A, LMC N132D, and a few more. The validation report that comes with each Cal-PV observation specifies if the observation you are interested in was performed with a chopper value different from 1.

An important thing to consider is that the eSASS software version that was used to produce these event lists and that is released in this site, does not account properly for chopper values > 1. The software performs 'double accounting', i.e. for chopper=n, it not only divides the dead time fraction (DEADC) by n, but also cuts away good time intervals (GTI) at the same cadence. Therefore the exposure produced is 1/n the value it should have.

For example, for chopper=2, DEADC will be 0.5, and the GTI fraction will also be 0.5. Therefore the exposure will be 0.25, i.e 0.5x0.5, where it should be 0.5. Therefore, it needs to be multiplied by 2 to bring it to the correct value.