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Orbital parameters and spin evolution of RX J0520.5-6932

ATel #5856; M. Kuehnel (Remeis-Observatory & ECAP, Univ. Erlangen), M. H. Finger (USRA), F. Fuerst (Caltech-SRL), K. Pottschmidt (CRESST-UMBC/NASA-GSFC), F. Haberl (MPE, Garching), and J. Wilms (Remeis-Observatory & ECAP, Univ. Erlangen)
on 6 Feb 2014; 17:59 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

The LMC X-ray binary RX J0520.5-6932 is currently undergoing a luminous type II X-ray outburst since December 2013 (see ATel #5760). On 9 January 2014 the source reached the Eddington luminosity (ATel #5760) and hard X-rays were detected in Swift/BAT (ATel #5675) and Fermi/GBM (ATel #5719).

The source shows X-ray pulsations around 8.034(5) s (Vasilopoulos et al.; ATel #5673). In the period evolution of the last months measured by the GBM Pulsar Project a modulation with a period of around 25 days is visible. This period is in agreement with findings by Coe et al. (2001), who suggested an orbital period around 24.45 days based on the OGLE data of the optical companion star.

We have analyzed the pulse period evolution provided by the GBM pulsar project to confirm this orbital period and to derive the orbital parameters of the binary. To account for the strong spin-up of the neutron star due to accretion torques, we modelled the change of the spin-period \dot{P(t)} based on the luminosity L(t) of the source as suggested by Ghosh & Lamb (1979):

\dot{P(t)} = - b * (P(t)/P(t0))^2 (L(t)/L(t0))^\alpha

Here P(t) is the spin-period of the neutron star at the time t and b the torque strength. The normalization constants P(t0) and L(t0) at the reference time t0 are used to express b in units of s/s. After having found P(t) the spin periods are modified by applying the Doppler shift of the orbital motion.

We assume the source luminosity, L(t), to be proportional to the 15-50 keV count rate as measured by Swift/BAT and alpha to be 6/7 due to accretion from a disk rather than from a dense stellar wind (alpha = 1; Ghosh & Lamb, 1979). A fit of this model to the GBM-data results in a good description of the data (chi-square = 14/15dof) with the following preliminary orbital parameters (uncertainties are at the 90% confidence level):

Porb = 23.93(7) d
T90 = MJD 56666.41(3) d where T90 is the time of a mean longitude of 90 degrees
asini = 107.6(8) lt-sec
g = e*sin(omega) = -0.023(7)
h = e*cos(omega) = -0.017(7)
The parameters of the pulse ephemeris are:
P(t0) = 8.034353(13) s at t0 = MJD 56669
b = 2.998(19) x 10^-9 s/s with L(t0) = 0.0066 cts/s/cm^2

A precise orbital solution will only be obtainable after the large outburst in order to understand the intrinsic spin-up and to break the parameter degeneracy introduced by the small number of data points. Nevertheless, this orbital ephemeris should allow for pulse phase resolved spectroscopy and we encourage further observation with hard X-ray telescopes.