Subject: Re: broad emission-like line feature in the JEM-X spectra (helpdesk#10467)


  Dear Ekaterina -

  If the Vela X-1 data you sent me is typical for the kind
of structure you are observing, then the "bump" is simply 
part of the remaining systematic uncertainty in the JEM-X
spectral extraction and response (for coded mask instruments
these two are not independent). I will send a plot in a 
separate mail. For me the systematic structure rather is
a broad shallow 'valley' from ~7 to ~15 keV. 

  The origin of such structures is not fully understood, 
though some of the effects mentioned by Carol Anne Oxborrow 
will surely play a role. Other possible effects include the
energy-dependent change of illumination pattern (more
smeared out at low and high energies), shadowing effects etc.

  When analyzing JEM-X spectra for a reasonably bright source
like Vela X-1 you must apply systematic errors (see the grppha 
ftool) - the same is in principle true for spectra from all 
instruments of all missions but frequently the systematic error 
is too small to be important. Unfortunately, largely for the 
reason of ongoing discussions about the 'real' values, there 
is no clear guideline in the current documentation how much you 
should assume (a few %, more below 5 and above 20 keV). I would 
recommend you discuss this again with silvia.martinez@uv.es
who in her PhD thesis has gathered a lot of experience.

  So in summary: apply systematic errors and accept a certain
systematic structure to the residuals. Don't try to fit any
physical model to such shallow structures.

  regards,
  Peter