Deep INTEGRAL, NuSTAR and Swift ToO observations of accretion-powered millisecond pulsar IGR J17591-2342: a peculiar outburst profile showing a re-brightening onset on Aug. 18, 2018

ATel #12004; Lucien Kuiper (SRON, The Netherlands), Sergey Tsygankov (Univ. of Turku, Finland), Maurizio Falanga (ISSI, Switzerland), Duncan Galloway (Monash Univ., Australia), Juri Poutanen (Univ. of Turku, Finland), Ilya Mereminskiy (Space Research Institute, Moscow, Russia)
on 3 Sep 2018; 15:05 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

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We report on coordinated target-of-opportunity observations of AMXP IGR J17591-2342 during its July/August/Sept 2018 outburst (see ATEL #11941, #11954, #11957, #11970, #11981, #11988, #12000) by INTEGRAL and NuSTAR at hard X-rays/soft gamma-rays, accompanied by monitoring observations at soft X-rays by the XRT aboard The Neil Gehrels Swift Observatory. The dedicated INTEGRAL ToO (PI. S. Tsygankov) observations have been performed during Aug. 17 14:03:53 to Aug. 19 14:08:39, 2018 for an exposure time of 163.1 ks (INT. ToO-I, INTEGRAL Revolution-1989) and Aug. 25 17:30:47 to Aug. 27 17:34:10 (INT. ToO-II, 163.1 ks, Rev-1992). The first INTEGRAL observation was coordinated with a 29.2 ks NuSTAR ToO observation (NU-ToO-II; Observation id. 80301311002) taken about 4 days later than a public NuSTAR ToO (exposure 27.1 ks; NU-ToO-I; Obs. id. 90401331002). Swift monitored IGR J17591-2342 for 0.47 ks (Obs. id. 00010804004) and 3.94 ks (Obs. ids. 00010804006/7) during INT. ToO-I and INT. ToO-II, respectively.

Spectral analysis of the ISGRI 20-300 keV near-real time data of INT. ToO-I adopting a power-law model yielded a 20-60 keV flux of 9.1 +/- 0.7 mCrab and a photon index of 2.02 +/- 0.10. The second NuSTAR ToO overlapping with INT. ToO-I showed a photon-index of 1.87 +/- 0.01 and a 20-60 keV flux of 6.81 +/- 0.08 mCrab (combined Swift XRT / NuSTAR fit), slightly lower, but consistent with the ISGRI flux, because we observed an increasing count-rate trend during this second NuSTAR ToO, contrary to the first NuSTAR observation which showed a decaying count rate trend (20-60 keV flux: 15.7 +/- 0.08 mCrab; photon-index 1.84 +/- 0.01).

Spectral analysis of INT. ToO-II 20-300 keV data yielded a 20-60 keV flux of 18.2 +/- 0.7 mCrab, twice the flux as observed during the INT. ToO-I, and a photon-index of 1.92 +/- 0.05. Moreover, the source is now significantly detected up to ~150 keV in line with the assumed power-law model. Apparently, a re-brightening already set in during the beginning of the first INTEGRAL ToO on Aug. 18, 2018 (MJD 58348). Analysis of ISGRI data taken during (public) Galactic Bulge observations (PI E. Kuulkers) of Rev-1994 and 1995 (end Sept. 2, 2018; 16:42:46) showed that the source is still bright with 20-60 keV fluxes in the range 25 - 38 mCrab (see http://www.sron.nl/~kuiper/ATEL/IGRJ17591-2342/ for the preliminary outburst profile for the 20-60 keV band; NuSTAR data points are indicated by filled squares).

We also performed a timing analysis of the 20-300 keV ISGRI data of INTEGRAL Revolutions 1989, 1992, 1994 (Gal. Bulge) and 1995 (Gal. Bulge) adopting the NICER timing model given in Table 1 of Sanna et al. (2018) (arXiv:1808.10195). We clearly detected the 1.9 ms pulsed signal in the 20-150 keV band at 5.2 sigma confidence level with a profile shape similar to that observed by NuSTAR during NU-TOO-I.

Finally, we also searched the NuSTAR, INTEGRAL JEM-X (3-20 keV) and XRT data streams for X-ray bursts from IGR J17591-2342, and found none. Because of the peculiar outburst behaviour of the source with a re-brightening onset at Aug. 18, 2018 we decided to apply for an XMM-Newton ToO, which has been granted. Further low/high-energy observations of this enigmatic source are strongly encouraged.

We thank the INTEGRAL, NuSTAR and Swift teams for scheduling our ToO observations.