# About the observational check of the mechanism of gamma radiation in Soft
Gamma Repeaters (SGR)

G.S. Bisnovatyi-Kogan 111Space research institute RAS, Moscow.

Abstract.

Soft gamma repeaters (SGR) are identified as single neutron stars (NS) inside
the Galaxy, or nearby galaxies, with sporadic transient gamma radiation. A
total number of discovered SGR, including relative Anomalous X-ray pulsars
(AXP), is few tens of objects. Many of them show periodic radiation, connected
with NS rotation, with periods 2-12 s. The slow rotation is accompanied by
small rate of loss of rotational energy, which is considerably smaller than
the observed sporadic gamma ray luminosity, and is many orders less that the
luminosity during giant bursts, observed in 4 SGR. Therefore the energy source
is usually connected with annihilation of very strong NS magnetic field.
Another model is based on release of a nuclear energy stored in the NS
nonequilibrium layer. We suggest here an observational test with could
distinguish between these two models.

Keywords: neutron stars; soft gamma repeaters; magnetic field annihilation;
nuclear reactions

##  1 Introduction

Soft gamma repeaters (SGR),are discovered observationally in the year 1979 [1,
2, 3] as a giant burst with recurrent gamma flared, and soon were connected
with the discovered short gamma ray bursts (GRB) [4] as a separate class of
GRB. They are identified now as single neutron stars (NS) inside the Galaxy,
or nearby galaxies, with sporadic transient gamma radiation. A total number of
discovered SGR, including relative Anomalous X-ray pulsars (AXP), is few tens
of objects [5]. Some of them show periodic radiation, connected with NS
rotation, with periods 2-12 s. It seems now that short GRB include two
different types of objects: merging of binary NS at cosmological distances,
and another class represented by giant bursts in SGR at neighbouring galaxies
at distances, when recurrent sporadic SGR radiation is too faint for
registration at the Earth. Such events had been discussed in [6], and were
discovered in the galaxies M31 (Andromeda) [7], and a galaxy in the M81 group
of galaxies [8], see also [9, 10].

The slow rotation is accompanied by small rate of loss of rotational energy,
which is considerably smaller than the observed sporadic gamma ray luminosity,
and is many orders less that the luminosity during giant bursts, observed in 4
SGR [2, 11, 12, 13]. Therefore the energy source is usually connected with
annihilation of very strong NS magnetic field [14, 15]. Another model is based
on release of a nuclear energy stored in the NS nonequilibrium layer [16, 17,
19, 20]. We suggest here an observational test with could distinguish between
these two models.

##  2 Periodical pulses

First discovery of FXP 0520-66 by Konus experiment [1, 2] was accompanied by
observation of periodic pulsation in this source with the period $P\approx 8$
s. in the afterglow radiation of the giant burst, lasting about 200 s. Similar
pulsations had been observed in the afterglows of giant bursts it the objects
$SGR1900+14$ with the period $P\approx 5.16$ s. [11, 21, 22], and SGR 1806-20
with the period $P\approx 7.47$ s. [12, 24]. The periodicity in the radiation
of this source was found earlier [25] in one of the recurrent outbursts,
indications to such periodicity had been found in [26], and first discovery of
this source in the year 1986 was reported in [27, 28]. The source SGR 1806-20
was probably the first SGR, in which periodic pulsations had been discovered
in one of the recurrent outbursts, which period was later confirmed in the
afterglow of the giant burst 27 December 2004, which was the most powerful
from four observed giant bursts. Periodic pulsations had been found in the
recurrent outbursts of several other SGR [5, 29, 30, 31].

Connection of SGR with highly magnetized slowly rotating NS (magnetars) meets
several problems. They are connected with observation of low magnetic field
SGR [29, 30]. Observations of several, quite normal radio pulsars without
traces of recurrent gamma outbursts, having magnetar-like periods and magnetic
fields [32] are not consistent with this model. These problems are discussed
in details in [33, 34]. It is evident, that magnetic origin of gamma ray
outbursts in SGR implies a different behaviour of pulsations, in comparison
with nuclear explosion origin [18, 20, 35].We suggest that this difference
should be visible in the behaviour of pulsation phases, what could be derived
from observations.

##  3 Phase analysis

Phase analysis of pulsations in rotating magnetized NS, observed as pulsars
(radio, optical, X-ray, gamma-ray) is a very powerful tool, permitting to
clarify the properties of the sources. In the binary millisecond pulsars with
extremely precise time stability of pulses the timing analysis of pulses gave
the best precision results in favour of confirmation of General Relativity
(GR) as a true theory of gravitation (see e.g. [36]). This stability is
connected with a very stable NS rotation, with practically fixed encoring of
the magnetic field to the NS body. Such properties permit to exclude the
changes connecting with a motion of magnetic field over NS surface, and
interpret the visible period variations in timing data to the pulsar motion in
the binary system, and different GR effects.

In SGR the precision of observation is incomparably worse than in
radiopulsars, so only sufficiently strong phase shifting in observations could
be noticed and measured. NS in SGR is considered as single object, so no
motion in binary, and very small GR effects are not be visible in their timing
data. The reason for strong phase shift in the SGR pulsations could be
expected due to motion of the the burst location over the NS surface from one
burst to another one. Such motion is unlikely to be possible if the transient
bursts are connected with magnetic events. Extremely high conductivity of the
pulsar matter prevents any magnetic field changes in static situation, and
does not permit changes in the large scale magnetic field configuration formed
by electrical currents situated deep inside, it quiet region of the star. The
large scale of the magnetic field should determine the position of the
bursting spot in all cases, not permitting for its motion over the star
surface. The shift of the explosion spot over the surface of the NS is
possible in the case when the explosion mechanism is not connected directly to
the magnetic field large scale structure. Such mechanism of explosion,
connected with processes in the NS non-equilibrium layer [16] was considered
in [20, 35, 33, 34]. Earlier it was suggested as a mechanism for formation of
gamma ray bursts (GRB) inside the Galaxy [17, 37]. Discovery of cosmological
origin of GRB exclude this mechanism for GRB model, but it remains for
application to SRG bursts. A considerable phase shift in the gamma ray pulsar
timing, between the observed transient bursts, would be a strong argument in
favour of the nuclear explosion SGR burst mechanism.

##  4 Phase shift observations in SGR

The pulsed radiation of SGR was first discovered in the famous 5 March 1979
burst [1, 2]. Subsequently it was visible very distinctly in the afterglows of
two other giant bursts SGR 1900+14 and SGR 1900+14. Analysis of many recurrent
bursts in the last two objects had shown the periodic component in their
radiation, with almost the same, slightly growing period.

###  4.1 SGR 1900+14

The pulsations with a period 5.16 s. have been observed in the afterglow of
the giant burst during several hundred of seconds. [11, 21]. Search for
periodic pulsations had been done in recurrent bursts and persistent radiation
of this object in subsequent years in the bands from radio [38] to optics and
hard X-rays [39, 40, 41, 42, 43, 44, 45]. The pulsation with a corresponding
period have been observed in all bands, except radio.

###  4.2 SGR 1806-20

The pulsed signals with the period 7.47 s. had been discovered in the year
1998 [26, 25], 6 years before the giant burst in this SGR. After observations
of the giant burst [12, 24], periodic pulsations in this SGR had been visible
in its radiation during subsequent observations in several X-ray bands [46,
47].

###  4.3 SGR J1935+2154

SGR J1935+2154 was discovered in 2014 [48], and has a spin period $P\sim 3.25$
s. It is one of the most active magnetars, showing outbursts almost every year
[49, 50]. Its reactivation date in 2020 April 27, was observed by several
X-ray and gamma-ray instruments when a burst storm and an increase of the
persistent X-ray flux were detected [51, 52]. Subsequent activity of this GRB
during 7 months was followed by SMM and Chandra missions [53].

##  5 Conclusions

The three above considered objects were observed during a long time, when
several strong outbursts happen. They looks out as best candidate for making
phase analysis, which should show the behaviour of the peak activity spot on
the NS surface. If it does not change its position on the NS during all the
time, than it is most probably connected with the large scale structure of the
magnetic field, and witnesses in favour of the magnetar model. If its position
changes after strong outburst that its activity is presumably determined by
nuclear explosions, connected with existence of non equilibrium layer in NS
envelope.

## Acknowledgements

The author is grateful A.S Pozanenko for discussions, and N.R. Ikhsanov for
useful comments. This work is partially supported by RFBR grant 20-02-00455.

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