Discovery of a cyclotron absorption line in the persistent Be/X-ray pulsar RX J0440.9+4431
Before 2010, the X-ray pulsar RX J0440.9+4431 belonged to the few
persistent low-luminosity binaries with Be companions which harbor
a slowly rotating neutron star (pulse period ~202.5 s). The first
evidence of the pulsar outburst activity in X-rays was found in
March 2010 by the MAXI all-sky monitor. During the following
outburst, in September 2010, INTEGRAL was used to investigate the
properties of RXJ0440.9+4431 in hard X-rays for the first time.
Another outburst was detected by Swift in January 2011.
Taking into account that the peak luminosities of all three
outbursts were relatively low (allowing to consider them as Type I
outbursts), and that all outbursts were roughly equally spaced in
time, the orbital period of RXJ0440.9+4431 was estimated to ~155 days.
This value is in good agreement with the period of about 150 days
derived from the Corbet Pspsin - Porbit
correlation.
Spectral analysis of data obtained with INTEGRAL during the
2010 September outburst revealed a ~32 keV cyclotron resonant
scattering feature in the source spectrum, that allowed to estimate
the magnetic field strength of the neutron star as
B = 3.2 x 1012 G . It makes RX J0440.9+4431 only the
second persistent Be/XRP system (after X Persei) with known magnetic
field strength and orbital period.
The image shows in the top panel the lightcurves obtained by Swift
(black) and INTEGRAL (red). Below the composite spectrum is shown
using (a) data from Swift (green), INTEGRAL/JEM-X (blue) and
INTEGRAL/ISGRI (red). The residuals to the fit in units of sigma
are shown without (b) and with (c) a 32 keV cyclotron absorption
line.
More details and further information:
"Broad-band observations of the Be/X-ray binary pulsar RX J0440.9+4431:
discovery of a cyclotron absorption line", MNRAS, in press (2012)
S.Tsygankov, R.Krivonos, A.Lutovinov
http://dx.doi.org/10.1111/j.1365-2966.2012.20475.x