INTEGRAL/SPI constraints on medium-weight primordial black holes
The interest in black holes as macroscopic dark matter (DM) candidates
has been revived in recent years in light of the detection of
gravitational waves and the strong constraints on the simplest DM
particle candidates imposed from collider searches, direct and indirect
detection. Many scenarios predict the formation of primordial black
holes (PBH) in the early universe and suggest that the present-day
fraction of dark matter made of PBHs (fpbh) can be close to 1.
While strong constraints exist on a light (<1015g) and heavy
(>1022g) PBH representing dark matter, the PBH mass range
1016-1021g remains the only weakly-explored window
in the PBH DM mass space. Interestingly, the characteristic temperature
of the Hawking radiation from 1016-1017g PBHs is
in the keV energy band and decreases for heavier black holes. The energy
spectrum of the expected radiation is shown with the red, green and blue
lines in the left panel of the Picture, for several PBHs' masses. The
strength of the expected signal from a certain direction in the sky is
proportional to the number of PBHs along the line of sight. A detection
of such a signal in X-ray observations could provide a unique
opportunity to detect PBH dark matter.
The INTEGRAL/SPI is the currently operational, and one of the
most-suited, hard X-ray instrument for searches of such emission. For
20-years in orbit INTEGRAL/SPI collected a wealth of data of Milky Way
(MW) observations. This data is used to put constraints on the PBH
Galactic dark matter (see also INTEGRAL
POM August 2020).
The dark matter density distribution in the MW is strongly peaked
towards the direction of the Galactic Center, which results in the
consequently stronger Hawking radiation signal from the inner MW regions
in comparison to the outer ones. Thus, for dark matter searches the
observations of outer MW regions could be used as background ones for
the observations of the inner MW regions. Such "ON-OFF" approach is
robust, background-model independent, and allows us to cancel most of
the time-variable instrumental and astrophysical backgrounds with an
accuracy better than 0.5%.
The residual emission could be interpreted as an unaccounted systematic
uncertainty and used to put constraints on the fraction of Galactic dark
matter consisting of PBHs. Depending on a type of systematic uncertainty
(strongly correlated versus uncorrelated ones) the derived limits are
shown in the right panel of the Picture as green and blue lines.
The left panel shows in black the Hawking radiation spectra versus
energy for the several PBH masses for the excluded values of
fpbh along with the residual ON-OFF emission.
Although the limits derived are weaker than those presented in, other,
recent papers; these make use of background measurements and do not rely
on background-template models. This allows the new study a clear
estimation of the impact of systematic uncertainty connected to the time
variations of the instrumental background.
Still, the derived constraints with the help of INTEGRAL/SPI
observations remain extremely strong in the PBH mass range of
1016-1017g, namely excluding such PBHs as the main
contributors to Galactic dark matter.
Only with the next-generation missions such as eXTP or THESEUS one can
expect substantial, by 1-2 orders of magnitude improvement of the
derived results along with a somewhat expanded range of potentially
probed PBHs masses.
Credits:
"Search for primordial black hole dark matter with X-ray spectroscopic
and imaging satellite experiments and prospects for future satellite missions",
Denys Malyshev, Emmanuel Moulin & Andrea Santangelo
Submitted to Phys. Rev. D
Preprint available at
https://arxiv.org/abs/2208.05705
