XMM-Newton Science Analysis System


heliocentriccorr (barycen-1.22.1) [xmmsas_20230412_1735-21.0.0]

Introduction

Due to the motion of the Earth around the Sun and the motion of the spacecraft around the Earth, the observed wavelength of a line emitted by a celestial object is affected by the Doppler effect induced by the spacecraft velocity component in the direction of the target. Spectral lines measured from a high-resolution spectroscopy instrument, such as RGS, must then be corrected for the projected component of the satellite's velocity.

heliocentriccorr task calculates, for a given time, the observed radial velocity of the spacecraft with respect to the Sun in the target direction, taking into account the spacecraft attitude and the Earth's motion around the Sun. This value is stored in the VHELIOCOR keyword in the SPECTRUM header of the input spectrum.

In this version, heliocentricorr does not apply the Heliocentric velocity correction to wavelength values in the spectrum file. It only writes in a keyword (VHELIOCOR) with the Heliocentric velocity.

heliocentriccorr can be used in two ways:

• Introducing as inputs an RGS spectrum set + RGS source list file:
  In this case, heliocentriccorr gets the source coordinates from the source list file to calculate the spacecraft attitude. Then, it gets the time stamp corresponding to the mid point of the observation. Finally, the Heliocentric velocity is calculated and written into the VHELIOCOR keyword.
  
• Introducing as inputs the RA and DEC of the source + time stamp:
  With these three values, heliocentriccorr calculates the Heliocentric velocity and writes the result on the screen.