INTEGRAL catches the birth of a symbiotic X-ray binary: IGR J17329-2731
Symbiotic X-ray binaries comprise a compact object, typically a neutron star, accreting
from the stellar wind of a red giant companion. It is the accretion of this wind onto the
compact object, that powers the X-ray emission we see from these systems.
On 13 August 2017, INTEGRAL revealed for the first time high energy radiation coming from
the direction of a new source it had dicovered, IGR J17329-2731. Follow-up observations
revealed that this source hosted a late M red-giant companion, and a slowly rotating neutron
star spinning with a period of 6680 sec. The discovery of an absorption feature in the X-ray
spectrum of the source, interpreted as a cyclotron resonant scattering feature, unveiled
that the source was also strongly magnetized (the estimated magnetic field of the object is
of the order of 2.4E12 G).
As the neutron star magnetic field has been long thought to decay with time, on a time
scale of the order of few million years, the discovery of a strongly magnetized neutron
star in a binary with a late M giant makes the evolutionary scenarios for the formation
of such system puzzling. The well-known evolution of a star up to the red giant phase takes
up to several billion years, thus indicating that the magnetic field of its neutron star
companion might not have substantially decayed with time. And thus it challenges all known
theories of the magnetic field decay.
An alternative possibility is that the neutron star formed in a late phase of the binary
system evolution, and specifically from the collapse of an aged white dwarf having undergone
a long phase of accretion from the red giant companion. The process of neutron star formation
from the accretion induced collapse of a white dwarf is still poorly understood and the
discovery of IGR J17329-2731 might as well demonstrate that this is truly possible.
As no X-ray emission from IGR J17329-2731 was ever recorded before August 2017 by INTEGRAL
during 15 years of operation in space, it is likely that the satellite caught the very
first moment in which the source shined as a symbiotic X-ray binary. This could be due to
the wind of the evolved red giant just becoming low and dense enough to trigger a significant
accretion or to an isolated episode of stronger mass loss rate from the red giant leading
to a detectable high energy emission from the neutron star companion.
"IGR J17329-2731: The birth of a symbiotic X-ray binary",
E. Bozzo, A. Bahramian, C. Ferrigno, A. Sanna, J. Strader, F. Lewis, D. M. Russell,
T. di Salvo, L. Burderi, A. Riggio, A. Papitto, P. Gandhi, P. Romano
A&A, Forthcoming article,
doi:10.1051/0004-6361/201832588
