INTEGRAL Picture Of the Month
April 2012

INTEGRAL POM
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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.

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