INTEGRAL constraints on primordial black holes and particle dark matter
Primordial black holes (PBHs) are a well-motivated dark-matter (DM)
candidate. In particular, PBHs with masses between about 1017
g and 1023 g can compose the entirety of the DM. The most
promising way to discover low-mass PBHs is via their Hawking radiation.
Gamma-ray experiments can discover the PBH DM by comparing the flux of
gamma-rays produced via Hawking radiation to the measured flux. The
higher the PBH mass, the lower is its temperature (TBH). The
spectrum of photons emitted by BHs through Hawking radiation peaks at
about 6 TBH, which makes INTEGRAL the ideal observatory to
search for the emission from PBHs with masses in between
~1016 g to 1017 g.
The INTEGRAL/SPI measurements of the gamma-ray spectrum of our Galaxy in
the energy range 27 keV to 1.8 MeV were presented in Bouchet
et al. 2011, ApJ 739, 29. Here, conservative limits are obtained by
comparing the total emission expected from PBHs with the gamma-ray
measurements of Bouchet et al. (without subtracting any astrophysical
backgrounds). Also, constraints on particle DM that decays or
annihilates to photons using the same dataset are obtained, resulting in
the strongest bounds in the relevant DM mass range.
At the left, the latitudinal variation (integrated over the longitude
range -23.1 degrees to +23.1 degrees) of the soft gamma-ray spectrum of
the Milky Way by INTEGRAL in the energy range 0.2 to 0.6 MeV is shown.
The data points measured by INTEGRAL/SPI are denoted by the crosses. The
red circles show the Hawking emission from a PBH (acting as the DM
candidate) with mass 1.5x1017 g. The blue diamonds correspond
to the emission by a DM particle of mass 0.5 MeV annihilating to photon
pairs with an annihilation cross section of 6x10-31
cm3 s-1. The arrows show the latitudinal bin where
the emission from these DM candidates exceeds the 1-sigma measured flux.
This illustrates how the conservative limits for PBHs, as well as
annihilating and decaying DM, are derived.
At the right, various upper limits on the fraction of DM composed of
PBHs (fPBH) as a function of the PBH mass (MPBH)
are displayed. Constraints derived from the measurements of cosmic-rays
by the Voyager-1 satellite, the CMB, and extra-galactic gamma-rays are
denoted by red, purple, and green lines, respectively. The constraint on
PBHs due to the measurement of the 511 keV line at the Galactic Center
by INTEGRAL/SPI is also denoted by the blue line. The conservative
constraint derived here, at 95% CL, is denoted by the black line.
Modelling the astrophysical emission and subtracting it, as well as
using more finely binned data, could yield tighter limits on PBH DM in
the future. Tentative constraints from an alternative INTEGRAL analysis,
with finer energy binning, are indicated by the dotted gray line;
however, this analysis did not account for the possibility of a DM
signal when extracting the Galactic emission profile, so at present this
constraint relies on the assumption that any DM signal must be
subdominant compared to the Galactic astrophysical emission.
In summary, the INTEGRAL/SPI measurements yield the strongest
constraints on ultra-light PBH DM, converging towards closing the window
on this mysterious DM candidate.
