The first 56Ni & 56Co radioactive decay gamma-ray lines seen from a Type Ia supernova
Supernovae (SNe) of Type Ia are used as "standard candles" in cosmology. A
physical understanding of this type of explosion is not yet obtained. Several
models are discussed for SNIa, all involving white dwarf stars. In all cases,
nuclear fusion of carbon to heavy nuclei create a large mass of radio-active
56Ni. The decay of these radioactive nuclei create the energy that
makes the SN shine for many months. The nuclear decay also produces gamma-ray
lines, from the 56Ni decay chain through 56Co to 56Fe.
A new SN was detected in M82, the Cigar Galaxy, on January 21 (see top B/W image).
It was identified as a type Ia SN and is referred to as SN2014J. At the distance
of about 3.5 Mpc, this is the closest type-Ia SN discovered in the past 4 decades.
Because of its proximity it is a unique event. INTEGRAL, the only observatory
currently capable of doing high-resolution gamma-ray spectroscopy observed this
SN almost exclusively from end of January to end of June, i.e., during the whole
bright SN phase.
For the first time, INTEGRAL detected the 56Ni decay lines at 158 and
812 keV, some two weeks after the explosion, see the SPI hard X-ray/gamma-ray
spectrum shown in lower left figure. The origin of the signal agrees within the
measurement error with the position of the SN (indicated by the cross in the same
figure). The early appearance of these gamma-ray lines has interesting implications
for the early explosion stages.
The lower right figure shows the gamma-ray spectrum of the SN obtained by SPI
(red points) over the period 50 - 100 days after the explosion. Clearly, also for
the first time, the 56Co decay gamma-ray lines at 847 and 1238 keV are
seen. Blue points show ISGRI/IBIS data for the same period. The flux below ~60 keV
is dominated by the emission of M82 itself (as seen in 2013 during M82 observations
with INTEGRAL, see the
INTEGRAL POM July 2014).
The black curve shows a fiducial model of the SN spectrum for day 75 after the
explosion.
INTEGRAL thus, for the first time, confirms by direct measurement of the primary
gamma-ray lines the 56Ni origins of SN light. The INTEGRAL measurements
of this sufficiently-nearby SN provide a unique opportunity to compare the direct
gamma-rays from the SN's energy source with the more-indirect other radiation. This
will help astrophysicists to refine their models on how in fact these explosions do
occur, because the explosion details affect how much new nuclei are created, and how
they move and interact with the remainder of the exploding star. These observations
constitute a reference in SNIa science, and thus an important scientific legacy for
years to come.
