INTEGRAL observes the transient X-ray pulsar RX J0440.9+4431 in the supercritical accretion regime
Be X-ray binaries consist of an accreting rotating neutron star (an X-ray pulsar) and
a main sequence Be-star that provides matter for accretion. At moderate accretion rates,
a pulsar accretes matter to within compact regions near its magnetic poles, which results
in X-ray emission. During giant outbursts, the pressure of this radiation can be so great
that it stops the infalling matter, forming a radiation-dominated shock. Such accretion
regimes are called subcritical and supercritical, respectively, and the boundary luminosity
is called critical.
For a long time, RX J0440.9+4431 (hereafter J0440) was considered to be a persistent
low-luminosity (about 1034 erg/s) Be X-ray binary. However, the transient nature
of J0440 was revealed after a series of outbursts, during which its luminosity increased by
hundreds of times.
J0440's first giant outburst occurred in 2023 (see image, top panel). During this outburst,
the luminosity was ten times higher than during other outbursts, and the pulsar transitioned
from the subcritical accretion regime to the supercritical one, which was accompanied by a
sharp change in its observed properties. INTEGRAL's broadband observations played a very
important role: together with the data from NuSTAR, it was possible to observe changes in
the pulsar's unusual spectral shape during the switch between accretion regimes (see image,
bottom left panel), and also to study the temporal properties of the pulsar in hard X-rays
up to 120 keV (see image, bottom right panel). Previous estimates of the pulsar's magnetic
field were revised as part of the study.
The top panel of the image shows the light curve of J0440 based on NICER data. Vertical
stripes represent intervals of the broadband observations, including INTEGRAL and NuSTAR.
The colours of the stripes match with the spectra displayed in the bottom left panel. The
bottom left panel shows the unfolded spectra of J0440 during outbursts in 2023, as well as
in 2010, and its approximation with a two-hump continuum model (solid lines; dashed lines
represent the separate hump components). The bottom right panel shows the pulse profiles of
J0440 as a function of the energy based on the INTEGRAL data. Two periods are shown. The pulse
profiles are spaced along the y-axis for better visualization.The fluxes in each profile were
normalized to their average. The phase of maximal flux was taken as phase zero.
Credits:
"RX J0440.9+4431: another supercritical X-ray pulsar",
Alexander Salganik, Sergey S. Tsygankov, Victor Doroshenko, Sergey V. Molkov, Alexander A. Lutovinov,
Alexander A. Mushtukov & Juri Poutanen,
2023, MNRAS, submitted, https://arxiv.org/abs/2304.14881
