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Mapping the inner flow around accreting black holes

Periodic Reporting for period 1 - BHmapping (Mapping the inner flow around accreting black holes)

Reporting period: 2019-02-01 to 2021-01-31

Astrophysical black holes (BH) are extremely simple objects: they are fully described by their mass and spin. However, the way they are fed and how they influence their surroundings is an outstanding and complex problem of high energy astrophysics. Accretion is the physical process that makes BH “visible” to our detectors, thus allowing us to see close to the BH and offering the opportunity to measure its parameters. On the other hand, the strong magnetic and radiation fields close to the BH can result in the production of jets and winds of expelled plasma which influence the host galaxy. In addition, the observed phenomenology changes significantly throughout the life cycle of a BH system. Understanding how accretion and ejection of plasma around BHs operate is of the utmost importance in order to understand how BHs work.

The inner regions of BH-accreting systems (from BH X-ray binaries - BHXRB - to active galactic nuclei - AGN) are key to study these mechanisms. Nonetheless, the distribution of the accreted gas orbiting close to the BH, its dependence on the accretion state, and the way this leads to the formation of outflows is not well understood. The BHmapping project addressed these open questions by studying the geometry of the gas around BHs of different mass and in different accretion states. The project relied on the use of advanced spectral-timing analysis techniques, which were applied to new X-ray data from the highest throughput and highest time resolution detectors, as well as state-of-the-art and newly developed X-ray spectral-timing models to interpret the results.
In order to resolve phenomena occurring close to the BH we need to explore a high energy (X-rays) and short timescales of variability (down to a few seconds for AGN, and down to fractions of a msec for BHXRBs) regime. The main analysis approach used in the project combines temporal and spectral information associated with the observed X-ray emission. Compared to more standard techniques that keep these information separated, this approach greatly increases the sensitivity to the shortest variability timescales, thus being able to map regions very close to the BH. In order to fully exploit this potentiality, the BHmapping project made use of the best X-ray data from high sensitivity detectors and long monitoring campaigns.

Scientific achievements:

X-ray spectral-timing techniques were used for the first time to study outflowing X-ray obscuring gas in AGN. X-ray obscurers have been recently discovered to temporarily and recurrently block the direct X-ray emission from the bright nucleus. The researcher studied the AGN NGC 3783 and measured, in an independent way, the density and location of the obscurer. Spectral-timing techniques allowed resolving variations of the obscurer on very short timescales (between half an hour and ten hours). To understand the origin of these variations, the researcher built a photo-ionization spectral-timing model, which showed the occurrence of quick changes in the ionization state of the gas in response to fast variations of the ionizing hard X-ray flux. The response timescale allowed locating the obscurer within the Broad Line Region.

The BHmapping project also focused on the problem of constraining the geometry of the inner accretion flow as a function of the accretion state in BHXRBs. To this aim, the researcher and the supervisor of the project developed two new methods to constrain the inner radius of the accretion disc. These methods are based on considering the effects of hard X-ray irradiation of the inner accretion disc. When applied to the time-averaged spectra of BHXRB systems in the hard state, they yield results consistent with significant disc truncation. Following these indications and using spectral-timing methods, the researcher studied the variations of the geometry of the inner accretion flow as a function of the accretion state throughout an entire outburst of the BHXRB MAXI J1820+070. The availability of a long monitoring of the source in the hard state allowed the researcher and her PhD students to additionally constrain the structure of the primary X-ray source as a function of distance from the BH. The same methods were used to constrain the effects of the stellar wind on the timing properties of the BHXRB Cyg X-1.

Finally, within the BHmapping project, the researcher contributed to an ongoing collaboration aimed at investigating the coupling between the fast variability properties of the accretion flow and the jet in BHXRBs. Within this collaboration, spectral-timing techniques were applied to multiwavelength data. This study showed that the timing properties of the jet evolve more slowly as a function of the accretion state than those of the inner accretion flow. This finding represents a test for current disc-jet models.

The techniques and the main results of the BHmapping project were used as the base for new research projects and observing proposals. The researcher presented the results in 4 invited and 1 contributed talks at international conferences and workshops, and in 5 invited seminars at international and local research institutes and universities. The researcher was part of the scientific organizing committee of 3 conferences and workshops and participated in the outreach activity “Astronomy on Tap”, with a public talk on Black Holes.
The increasing availability of good quality broad-band X-ray spectral data has been proving essential for constraining the properties of the innermost accretion flow in BH-accreting systems. However, current measurements are mostly based on the use of a single technique, mostly the fit of reflection models to time-averaged X-ray spectra. While relevant, the self-consistency of the obtained results is rarely checked with independent methods.
Within the BHmapping project this problem was addressed in two ways. On the one hand, two spectral methods were developed to obtain independent estimates of the inner radius of the disc. The application of these methods reveals inconsistencies in several measurements reported in the literature. On the other hand, X-ray spectral-timing techniques were used to independently constrain the geometry of the inner accretion flow in BHXRBs. This allows accounting for the strong spectral variability of the source, an important aspect neglected in time-averaged spectral analysis. Overall, the results obtained are consistent with a scenario where strong evolution of the disc truncation radius occurs throughout an outburst, with the inner X-ray source showing spectral inhomogeneities usually not accounted for in time-averaged spectral fits.

In the BHmapping project, X-ray spectral-timing techniques were also used to independently constrain the properties of outflows. In particular, these were applied, for the first time, to the newly discovered X-ray obscurers in AGN. This allowed investigating variability timescales normally inaccessible to standard techniques. Most notably, this study proved the great potential of such approach, especially in view of the upcoming generation of high collective area and high spectral resolution calorimeters onboard XRISM and Athena. Finally, these techniques proved powerful in constraining jet-accretion flow coupling mechanisms, demonstrating the importance of simultaneous multi wavelength, high-time resolution monitorings of BHXRBs.