INTEGRAL Picture Of the Month
September 2022

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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.


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