Physics Letters B 789 (2019) 538–544
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Positrons from primordial black hole microquasars and gamma-ray
bursts
Volodymyr Takhistov
Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, CA 90095, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
7 July 2018
Received
in revised form 19 December 2018
Accepted
19 December 2018
Available
online 28 December 2018
Editor:
H. Peiris
We propose several novel scenarios how capture of small sublunar-mass primordial black holes (PBHs)
by compact stars, white dwarfs or neutron stars, can lead to distinct short gamma-ray bursts (sGRBs) as
well as microquasars (MQs). In addition to providing new signatures, relativistic jets from these systems
will accelerate positrons to high energies. We find that if PBHs constitute a sizable fraction of DM, they
can significantly contribute to the excess observed in the positron flux by the Pamela, the AMS-02 and
the Fermi-LAT experiments. Our proposal combines the beneficial features of astrophysical sources and
dark matter.
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Primordial black holes (PBHs) can appear from early Universe
dynamics and account for all or part of the dark matter (DM)
[1–13]. Aside from being a theoretical curiosity, null search results
for conventional DM particle candidates [14,15]as well as the pos-
sible
implications [16–23]for the newly-opened field of gravity-
wave
astronomy [24–26]further elevate the interest in PBH-related
studies. Recent investigations have shown that PBHs could shed
light on a variety of outstanding astronomical puzzles, such as the
origin of r-process nucleosynthesis material [27].
Observations
by several experiments, including PAMELA [28],
Fermi-LAT [29] and AMS-02 [30], have identified a rise in positron
cosmic ray flux above ∼ 10 GeV. The origin of this phenomenon re-
mains
elusive. A multitude of proposals have been put forward to
address it, which can be generally grouped together as those based
on astrophysical sources (e.g. pulsars [31–36], supernova remnants
[37–41], microquasars [42]), particle dark matter annihilations/de-
cays
[43–46]as well as those based on cosmic ray propagation
effects [47–49].
In
this Letter we discuss how PBHs interacting with compact
stars can incite distinct gamma-ray burst (GRBs) and microquasar
(MQs) sources, which can accelerate particles to high energies and
contribute to the positron excess. Heuristically, if a small PBH with
sublunar mass of 10
−16
M
M
PBH
10
−7
M
is captured by a
compact star [50], a white dwarf (WD) or a neutron star (NS), it
will eventually consume the host and result in a stellar-mass BH.
E-mail address: vtakhist@physics.ucla.edu.
The system’s energy, released on dynamical time-scales, is suffi-
cient
to power a short GRB. In a different scheme, the resulting
stellar-mass BH could steadily accrete matter if the considered star
system is a binary, powering a microquasar jet. While a GRB ex-
plosion
provides a singular energy injection, a microquasar jet is
a continuous injection source. Relativistic positrons, accelerated ei-
ther
through a burst or a continuous jet, will diffuse and are ob-
servable.
2. Black hole capture
A small PBH can become gravitationally captured by a NS or
a WD if it loses sufficient energy through dynamical friction and
accretion as it passes through the star. We briefly review the main
capture ingredients, following [23,50]. The full capture rate is given
by F = (
PBH
/
DM
)F
0
, where
PBH
is the PBH contribution to the
overall DM abundance
DM
. The base Galactic capture rate F
0
is
F
0
=
√
6π
ρ
DM
M
PBH
R
NS
R
s
v(1 − R
s
/R
NS
)
1 −e
−E
loss
/E
b
,
(1)
where ρ
DM
is the DM density, M
PBH
is the PBH mass, v is the
DM velocity dispersion (assumed to have Maxwellian distribu-
tion),
E
b
= M
PBH
v
2
/3, R
NS
is the radius of the NS with mass
M
NS
and Schwarzschild radius R
S
= 2GM
NS
. Convention c = 1is
used throughout. If the interaction energy loss E
loss
exceeds ki-
netic
energy of the PBH, then it will be captured. The average en-
ergy
loss for a NS is E
loss
58.8 G
2
M
2
PBH
M
NS
/R
2
NS
. Throughout this
work we consider a typical NS to have radius R
NS
∼ 12 km, mass
M
NS
∼ 1.5M
and spinning with a milli-second period P ∼ 1ms
https://doi.org/10.1016/j.physletb.2018.12.043
0370-2693/
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.