7.3.2. Multi-Instrument Analysis

Thanks to the properties of the 3ML framework, data from different instruments can be combined. So far, this feature was used only to combine the POLAR data with that from *Fermi*-GBM and *Swift*-BAT. This allowed to improve the spectral fits, as the error on the spectrum adds to the systematic error on the polarization measurement, which in turn led to more precise polarization measurements. The 3ML framework additionally allows to fit physical models directly to the data, rather than fitting the data with empirical models and subsequently comparing the results with a parameterized outcome of a theoretical prediction. Although easier, the latter method has, especially in the field of gamma-ray spectrometry, been found to result in over-interpretation of data analysis results and to inconsistent conclusions (see discussion in Section 2.3.1). The fitting of physical models directly to data is especially desirable in the field of polarimetry as it allows to fit these models, potentially unbinned in time and energy, directly both to spectral and polarization data at the same time.

Apart from combining spectral and polarization data in the analysis, in theory, the same can be done using data from two polarimeters in case two different polarimeters observed the same GRB. In fact, several GRBs were observed by both Astrosat and PO-

LAR [187,321]. It would therefore be highly desirable to perform combined analysis of the Astrosat and POLAR data for such GRBs as it would, firstly, allow to study the cause of the likely discrepancy between the results from both instruments. Secondly, it can allow for more detailed measurements of the polarization of these GRBs.

With upcoming instruments sensitive in different energy ranges, such analysis tools will in the future allow to fit physical models to both spectral and polarization data over a broad range in energy by, for example, combining the data of the LPD, LEAP, POLAR-2, and AMEGO or COSI. Whereas with the current level of polarimetry analysis tools, the data has to be studied separately, leaving the full potential of the data unexploited.

As the polarization tool in the 3ML framework discussed here is new and has not been used for the polarization analysis of other instruments, it is to be seen if it will be used by the wider community. However, with the potential of two large-scale polarimeters in LEAP and POLAR-2 launching in the coming years, as well as polarimeters sensitive at keV and at MeV energies, there is a clear need for a collaborative effort between the groups to either further develop this tool or construct a completely new one.
