INTEGRAL's SPI/ACS sensitivity to detect Galactic supernovae
The early gamma-ray emission of type Ia supernovae (SNIa) is controlled
by the amount of freshly synthesized 56Ni nuclei and the
changing opacity of the expanding ejecta. The spectral analysis of this
emission provides unique information on the explosive nucleo-synthesis
and on the properties of SNIa ejecta at early stages. Therefore an early
detection of a Galactic SNIa would be the ideal candidate for such studies.
The anti-coincidence system (ACS) of SPI onboard INTEGRAL can be used
not only to reduce the background but also to monitor the sky for
possible sources, thanks to its omni-directional view and its
spectroscopic capabilities (see
INTEGRAL POM May 2017 ). A study of the
sensitivity of the ACS suggests that the ACS is able to detect a SN in
our Galaxy during its early stages at about 6 - 12 days after the
explosion. The time of detection depends on the distance of the source,
not on its Galactic longitude.
To simulate the detection of a SN signal one can start by calculating
the response matrix of the ACS with Geant4 using the INTEGRAL mass
model (see left image). Then, the response to a theoretical gamma-ray
flux as predicted by SNIa explosion models can be computed. The
detection method consists of fitting a simple mathematical model to the
background ACS rate, as a function of the rate of high-energy events
(>150 MeV) in the Bismuth Germanate (BGO) blocks, then modeling again
the combined rate of the background and the SN counts. One considers the
SN signal to have been detected if the combined rate exceeds the modeled
ACS rate by three standard-deviations, and the satellite is out of the
radiation belts by at least half a day. An illustration of this process
is displayed in the right image. For this example, the rising of the SN
rate over the background can be seen and allowed the detection time to
be at 6.73 days. Additionally, by using the real ACS rates acquired in
1868 revolutions, a check was made whether no Galactic SNIa was missed
since shortly after launch up to March 2019.
