X-ray polarimetry with XL-Calibur

The aim of the XL-Calibur mission is to provide new insights on celestial compact objects - mass-accreting neutron stars and black holes, as well as pulsars – using X-ray polarimetry.

The angular extent of sources means that direct imaging will not be possible at any wavelength for the foreseeable future.

Knowledge on source high-energy emission mechanisms and geometry stems instead from the established techniques of spectroscopy and timing. Results are often model-dependent with interpretation subject to degeneracies which cannot be resolved.

Spectropolarimetric observations are conducted by XL-Calibur in the 15-80 keV energy band from a stabilised balloon-borne platform in the stratosphere. XL-Calibur has been developed by groups from USA, Japan, and the group at KTH.

The mission derives from a flight-proven predecessor mission, X-Calibur, in which the KTH group participated with SNSA support.

As a second-generation balloon mission, XL-Calibur will provide precision measurements thanks to the several-fold increase in signal rate together with an order-of-magnitude improvement in the signal-to-background ratio.

Thanks to previous SNSA funding, KTH now has a central role in XL-Calibur, through (i) development and operation of the anticoincidence shield, which encapsulates the polarimeter to mitigate measurement backgrounds, (ii) participation in integration and testing activities, (iii) data reduction, analysis and simulation work required to provide scientific results from observations.

The maiden flight of XL-Calibur is scheduled for summer 2022 from Esrange Space Centre. During the week-long flight to Canada, observations of the Crab pulsar and the black-hole binary Cyg X-1 will be made.

XL-Calibur is scheduled to fly a second time from McMurdo base on Antarctica in 2023 for observations of the accreting neutron stars GX301-2 and Vela X-1. Especially during the Antarctic flight, observations of other bright transient events are also expected.

As well as providing results of unprecedented precision, there is valuable synergy with the NASA/ASI IXPE mission which is currently conducting imaging polarimetric studies in-orbit in a complementary energy band (2-8 keV).

Broadband observations are important for polarisation studies since the energy-dependence of polarisation parameters, rather than the absolute values at specific energies, often reveal the geometry and physical properties of the emission region.

Funding is primarily requested for personnel costs, rental of laboratory premises at KTH, travel for integration/test and collaboration activities, and participation in the Antarctica campaign.