Transient effects in Herschel/PACS spectroscopy
  The Ge:Ga detectors used in the PACS spectrograph onboard the Herschel space
telescope react to changes of the incident flux with a certain delay. This
generates transient effects on the resulting signal which can be important and
last for up to an hour. The paper presents a study of the effects of transients
on the detected signal and proposes methods to mitigate them especially in the
case of the "unchopped" mode. Since transients can arise from a variety of
causes, we classified them in three main categories: transients caused by
sudden variations of the continuum due to the observational mode used;
transients caused by cosmic ray impacts on the detectors; transients caused by
a continuous smooth variation of the continuum during a wavelength scan. We
propose a method to disentangle these effects and treat them separately. In
particular, we show that a linear combination of three exponential functions is
needed to fit the response variation of the detectors during a transient. An
algorithm to detect, fit, and correct transient effects is presented. The
solution proposed to correct the signal for the effects of transients
substantially improves the quality of the final reduction with respect to the
standard methods used for archival reduction in the case where transient
effects are most pronounced. The programs developed to implement the
corrections are offered through two new interactive data reduction pipelines in
the latest releases of the Herschel Interactive Processing Environment.
