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INTEGRAL ToO observation of IGR J00291+5934 during its July/August 2015 outburst: spin and orbital ephemerides update for the July/August 2015 epoch

ATel #7949; L. Kuiper (SRON), M. Falanga (ISSI), V. De Falco (ISSI), W. Hermsen (SRON; UvA), E. Bozzo (ISDC), C. Ferrigno (ISDC)
on 25 Aug 2015; 14:42 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

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

INTEGRAL carried out a target of opportunity (ToO) observation of the accreting millisecond X-ray pulsar IGR J00291+5934 on July 27-29, 2015 for about 170 ks (satellite revolution 1569 in hexagonal dither mode), following the discovery of a new outburst from the source (ATEL #7835, #7836, #7837, #7849 and #7852; and GCN Circ. #18051 and #18061).

We performed an imaging analysis using near-real time data from the Soft-Gamma-Ray imager ISGRI onboard INTEGRAL in 10 logarithmically binned energy intervals spanning the 20-300 keV band. IGR J00291+5934 was significantly detected up to the 150-212.5 keV band, and the time-averaged spectrum could be satisfactorily described by a power-law model with photon index -1.78 +/- 0.04. The estimated X-ray flux from the spectral fit was (4.26 +/- 0.07)E-10 erg/cm^2 s (20-100 keV). During the ToO observation the 20-100 keV count rate dropped smoothly from about 8 cnts/s to 4 cnts/s (roughly from 35 to 18 mCrab).

Using the ISGRI Solar-System barycentered timing stamps, adopting the optical position for IGR J00291+5934 given in Torres et al. (2008, ApJ 672,1079), we updated the spin and orbital ephemerides of IGR J00291+5934 for current July/August 2015 epoch. Starting from the ephemerides of the Dec. 2004, Aug./Sept. 2008 outbursts reported previously by Papitto et al. (2011, A&A 528, A55), we applied an optimalization code on the Z_1^2-statistics in which the source spin frequency and the time of the ascending node (Tasc) are treated as free parameters. We detected pulsed emission up to 150 keV at a 5.5 sigma level.

The best determined spin frequency is 598.8921299(8) Hz at epoch 57231.0 MJD (TDB). The updated time of ascending node is 57231.847035(25) MJD (TDB). Uncertainties are all given at 1 sigma confidence level. The spin frequency measured from the INTEGRAL data is consistent with the spin-down trend of -4.1E-15 Hz/s given in Papitto et al. (2011). The updated value of Tasc makes a refinement of the orbital period possible. By fitting the Tasc values measured during the Dec. 2004, Aug/Sept. 2008 outbursts (see Table 1 of Papitto et al. (2011)) along with the upgraded July/Aug. 2015 value we obtained an improved orbital period Porb of 8844.07672(2) s (cf. the Dec. 2004 outburst value of 8844.079(1)). We could not find any evidence for an orbital period derivative.

We thank the staff at ISOC for promptly implementing the INTEGRAL ToO observation and the ISDC for making the near real time data directly available to us.