Iron Kα line of Proca stars

Abstract

X-ray reflection spectroscopy can be a powerful tool to test the nature of astrophysical black holes. Extending previous work on Kerr black holes with scalar hair [1] and on boson stars [2], here we study whether astrophysical black hole candidates may be horizonless, self-gravitating, vector Bose-Einstein condensates, known as Proca stars [3]. We find that observations with current X-ray missions can only provide weak constraints and rule out solely Proca stars with low compactness. There are two reasons. First, at the moment we do not know the geometry of the corona, and therefore the uncertainty in the emissivity profile limits the ability to constrain the background metric. Second, the photon number count is low even in the case of a bright black hole binary, and we cannot have a precise measurement of the spectrum.

relxill_nk: A Relativistic Reflection Model for Testing Einstein’s Gravity

“…The method was originally proposed and developed for measuring black hole spins under the assumption that the metric around astrophysical black holes is described by the Kerr solution [36,37]. More recently, the technique has been proposed for testing Einstein's gravity in the strong field regime [38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Note that spin measurements (if we assume the Kerr metric) or tests of the Kerr metric require fitting the whole reflection spectrum, not just the iron line, even if often (but not always) the iron line is the feature that primarily determines the measurement of the parameters of the background metric in the strong gravity region.…”

Testing Einstein-dilaton-Gauss-Bonnet gravity with the reflection spectrum of accreting black holes

“…[48][49][50][51][52][53] to study the possibility of testing a number of non-Kerr metrics. We simulate an observation with a specific instrument employing the iron line calculated in the non-Kerr metric, and we fit the simulated data with the iron line of a Kerr model.…”

Iron Kα line of Kerr black holes with Proca hair

“…In this paper, we continue our explorative study to understand the capabilities of present and future x-ray missions to test BH solutions in 4D Einstein's gravity in the presence of matter using x-ray reflection spectroscopy, the so-called iron line method [31][32][33][34]. We extend previous work on Kerr BHs with scalar hair [2], boson stars [35], and Proca stars [36]. Other phenomenological studies of BHs with synchronized hair include their shadows [37][38][39][40], quasiperiodic oscillations [41] and some brief analyses of their quadrupoles and orbital frequency at the innermost stable circular orbit [23,24].…”

“…As already stressed in previous work, see in particular Ref. [36], accurate tests of the metric around astrophysical BHs will only be possible in the presence of the correct astrophysical model for the corona. Phenomenological metrics, like a power-law or a broken power-law, are approximations unsuitable to get strong and reliable constraints on the spacetime metric with the high quality data expected with the next generation of x-ray facilities.…”

“…We employ the strategy already used in our studies of Kerr BHs with scalar hair [2], boson stars [35], and Proca stars [36], as well as to the study of other metrics [58,59]. We simulate observations of our metrics and we then fit the data with a Kerr model.…”