Using X-Ray Polarimetry to Probe the Physics of Black Holes and Neutron Stars

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources. Introduction: The recent developments of broadband X-ray focusing mirrors with excellent angular resolution and matched broadband X-ray polarimeters make it possible to design and build sensitive X-ray polarimetry missions. Recognizing the scientific opportunities afforded by X-ray polarimetry, NASA and ASI are currently developing the Imaging X-ray Polarimetry Explorer (IXPE) [70], a Small Explorer (SMEX) mission to be launched in 2021, which is expected to measure the polarization in the 2–8 keV energy range. In this white paper, we advocate for the science opportunities afforded by an IXPE follow-up mission, called the X-ray Polarization Probe (XPP) which promises a sensitivity improvement by a factor of 5–10 over IXPE, an energy bandpass broadened from 2–8 keV to 0.1–60 keV, and an angular resolution improvement from 30′′ to 5′′-10′′ half power diameter (HPD). Such a mission will enable physics-type experiments with statistical samples of the most extreme objects in the Universe: black holes and neutron stars (see Table 1), and would allow us to probe matter, fields, and fundamental laws in extreme conditions. Black holes and neutron stars emit a substantial, in most cases even dominant, fraction of their energy in the X-ray band, making this the preferred band for their study. A series of X-ray imaging, spectroscopy and timing missions led to spectacular insights about the nature of these (and other) cosmic X-ray sources, as recognized by Riccardo Giacconi’s share of the 2002 Nobel Prize in Physics. X-ray polarimetry is expected to give another boost to the field by adding two more observables, the polarization fraction and angle, which encode vital information on the geometry of the systems. Importantly, the polarization constrains not only the geometry of the source on femto-arcsecond scales but also the structure of the magnetic and gravitational fields. The measurement of the X-ray polarization will allow us to test our models of the X-ray emission mechanisms, the propagation of the X-rays through the curved spacetime in the vicinity of the compact objects, the competition of plasma and vacuum birefringence, and the nature of X-ray scattering and selective, polarization dependent X-ray absorption [e.g., see the reviews of 50, 44, 69, 6, 40]. IXPE, to be launched in 2021, will be the first dedicated X-ray polarimetry mission following the pioneering OSO-8 mission [e.g. 68], various satellite-borne mission with some polarization sensitivity [i.e. INTEGRAL, ASTROSAT, and Hitomi 18, 67, 35] and purpose-built balloon-borne hard X-ray missions [i.e. PoGO+ and X-Calibur 25, 39]. IXPE will deliver high signal-to-noise polarimetric measurements of the brightest Galactic sources and first results for a few extragalactic sources. In the following, we will highlight several exceptionally promising science opportunities afforded by a XPP-type follow-up mission. Table 1: Summary of source classes (column 1), the fraction and number of sources per source class that can be observed with IXPE (column 2) and XPP (column 3) in 10 s exposures with a certain minimum detectable polarization fraction (<10% for magnetars and blazars,<5% for all other source classes, 99% confidence level) in a sample of bright RXTE sources, as well as prominent science drivers (column 4). Class IXPE XPP Science Drivers Stellar BHs 73% (27) 89% (33) Accretion Disk Dynamics, Role of B-field AGNs 0% (0) 52% (15) Black Hole Spin & Inclination, Corona Geometry Pulsars 41% (16) 95% (37) QED Birefringence Magnetars 0%(0) 57% (8) Stellar Surface, Plasma & QED Birefringence HMXBs 19% (8) 88% (37) Accretion, Plasma & QED Birefringence SNR+PWN 4.5% (1) 77% (17) Origin of Cosmic Rays, Rel. Particle Acc. Blazars 6% (1) 53% (9) Jet Structure, Particle Acceleration