Accretion is the most efficient way to produce energy and it powers a wide range of astrophysical objects at all ages of the Universe, ranging from distant gamma-ray bursts to close-by planet forming systems. The most violent and rapid manifestations of accretion phenomena are associated with black holes and neutron stars. More than fifty years of multi-wavelength observations of accreting objects have shown that relativistic ejections are seemingly associated with specific phases of accretion through mechanisms that are yet to be understood. In addition, studies of accretion in highly magnetized, young neutron star will probe the behavior of matter in extreme magnetic fields.
Accretion and ejection physics in black holes on all mass scales
In AGN and X-ray binaries (XRB), the bulk of the emission is released in the X-ray and soft gamma-ray bands, i.e, typically in the range of ∼1 keV to a few MeV. The spectral window of PHEMTO is thus ideal to probe the physics of accretion. It has, however, become clear over the past decade that the diagnostics enabled by spectroscopy alone are not sufficient to model theoretically the broad band emission in accreting sources [Nowak et al., 2011]. Polarimetry provides a new, independent and complementary window onto this physics, providing access to new parameters such as system geometry or magnetic field strength. The currently accepted contributors to the high-energy emission from stellar and supermassive black holes and neutron stars with a low magnetic field, are an accretion disk, a Comptonised medium (referred to as ‘corona’, whose origin, geometry and global properties are still not completely known), and possibly a jet. They contribute to the overall emission in different proportion, resulting in different emission states. X-ray polarimetry, as already demonstrated by IXPE, is a unique tool to study accretion into a deep gravitational well and measure relativistic aberration and beaming, gravitational lensing, and gravito-magnetic frame-dragging. By bridging the spectral gap between IXPE and INTEGRAL (and improving the sensitivity of the latter), PHEMTO will disentangle different polarized emission from these sources in different states. The innovative potential of X-ray/soft gamma-ray polarimetry has also been demonstrated by the INTEGRAL polarimetric studies of various sources such as the Crab pulsar/nebula, GRBs and XRBs showing highly polarized emission above 400 keV [Goetz et al., 2013, Rodriguez et al., 2015, Cangemi et al., 2023, Bouchet et al., 2024], probably associated with synchrotron emission from the jet. PHEMTO will open the 50-400 keV range to sensitive spectro-polarimetric studies of accretion/ejection for the first time.
Accretion in highly magnetized neutron stars
An important subclass of X-ray binaries comprises a high mass star and a neutron star as accretor: these are know as neutron-star high-mass X-ray binaries (NS-HMXB). Since the neutron star is relatively young, it is still endowed in a strong magnetic field of the order of 1012 G which might imprint characteristic signatures in the hard X-ray spectrum, known as cyclotron line scattering features [see Staubert et al., 2019, for a review]. These features appear as absorption lines with a large width of a few keV at an energy between 10 and 100 keV on the exponentially decaying part of the spectrum. They constitute the only direct measurement of the neutron star magnetic field in its immediate vicinity. Despite this phenomenon being known since 1976, a lively debate is still present on the location of the line-forming region and on the actual geometry of the accretion flow producing the X-ray emission. It has been shown [Meszaros et al., 1988] that, due to the birefringence of plasma embedded in a magnetic field, polarization properties of X-ray pulsar are energy-dependent and change dramatically when the photon energy crosses the cyclotron energy. IXPE indicated a low degree of polarization (< 10%) for some of these targets [Tsygankov et al., 2022, Forsblom et al., 2023], but we are missing the polarimetric coverage of the cyclotron lines energy band. PHEMTO has the unique potential to explore the cyclotron scattering features through polarization analysis and understand the behavior of matter in extreme magnetic fields.