Speaker
Description
The advent of X-ray polarimetry has marked a major advancement in high-energy astrophysics, driven by the successful launch of IXPE and its groundbreaking measurements in the soft X-ray band (2–8 keV). Currently, several efforts are underway for getting more significant results in hard X-rays provided by previous missions like PoGo+. XL-Calibur is a balloon-borne hard X-ray polarimeter operating in the 19–64 keV band, employing a Be scatterer surrounded by CZT detectors enclosed inside an active BGO shield. We will present the results from the most recent XL-Calibur flight in July 2024. During an approximately week-long campaign, observations of the Crab Nebula and Cygnus X-1 were performed, yielding high-significance polarization measurements for both sources. For the Crab pulsar and Nebula, we measure phase-integrated polarization degree (PD = 25.1 ± 2.9) % with a polarization angle (PA= 129.8 ± 3.2)∘. Combining the data from XL-Calibur Crab observation with IXPE results reveals that PA approaches the pulsar spin axis at higher energies. Owing to the high sensitivity of XL-Calibur, we report the most precise pulse-phase–resolved hard X-ray polarization measurements of the Crab. Cygnus X-1 exhibits a low but significant polarization degree (PD = 5.0± 3.0) %, unexpectedly high for a black hole binary. The measured polarization angle (PA=− 28 ± 17)∘ is aligned with both the parsec-scale radio jet and previous soft X-ray polarization measurements by IXPE. We will discuss the details of these measurements along with their implications in the context of theoretical models and some problems which remains unresolved. Further, we will present the plans for the next next XL-Calibur flight.