NICER and SWIFT/XRT detect a new outburst of the accreting millisecond X-ray pulsar SWIFT J1756.9-2508

ATel #12882; A. Sanna (Univ. of Cagliari, Italy), F. Pintore (INAF-IASF, Milan), A. Riggio, L. Burderi, A. F. Gambino (Univ. of Cagliari, Italy), K. C. Gendreau, Z. Arzoumanian, P. M. Bult (NASA/GSFC, USA), T. Di Salvo, R. Iaria (Univ. of Palermo, Italy), C. Ferrigno, E. Bozzo (ISDC, Switzerland), A. Papitto (INAF-OAR, Italy)
on 22 Jun 2019; 13:24 UT
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Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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Following a Swift/BAT count rate increase, from June 11 (MJD 58645), towards the direction of the accreting millisecond X-ray pulsar SWIFT J1756.9-2508, NICER and SWIFT/XRT performed pointed observations between 2019, June 19-22, collecting a total of 8.4 ks and 1 ks of exposure, respectively. Both NICER and SWIFT/XRT observations showed clear detection of a source at ~25 ct/s (1-10 keV) and ~2.5 ct/s (0.3-10 keV), respectively (background level in these bands are of the order of a few percents). The NICER light curve shows a decay in count rate of ~2.5 cts/s per day, suggesting that SWIFT J1756.9-2508 is slowly going back to a quiescence state.

Using the method of Evans et al. (2009, MNRAS, 397, 1177), we found a UVOT-enhanced position of RA = 269.23888 deg, DEC = -25.10780 deg (J2000), with an associated uncertainty of 2.3 arcsec (90% c.l.). This location is consistent, within errors, with the position of SWIFT J1756.9-2508 reported by Krimm et al. (2007), confirming that the accreting millisecond X-ray pulsar started a new outburst. This is the fourth outburst of the source observed since its discovery, the previous one started on 2018 April 1.

We barycentered the NICER photon arrival times adopting the source coordinates reported above and we corrected the delays of the photon time of arrivals caused by the X-ray pulsar orbital motion under the hypothesis of a circular orbit and assuming the latest orbital ephemerides reported in Sanna et al. 2018 (MNRAS 451, 1658-1666). Folding the data, we retrieved a pulsation detection (~10 sigmas) at a frequency F0=182.0658(3) Hz. We modeled the pulse profile with the superposition of two harmonically related sinusoids with a fractional amplitude of 3.6% for the fundamental and 2.4% for the first overtone, both measured in the 1-10 keV band. A more detailed analysis is underway.

The NICER X-ray spectrum is consistent with an absorbed comptonized continuum (Nthcomp in Xspec) giving a reduced chi^2 = 532.58 for 520 d.o.f. We measured an absorption column density of nH = 6.1(2)E22 cm^-2, a seed photons temperature of kT = 0.73(4) keV, and a power-law photon index of Gamma = 2.18(8). The electron temperature parameter was highly unconstrained and we fixed it to 75 keV (as in the previous outburst). There is no evidence of a Fe line feature at ~6.4-7 keV in the spectrum. The unabsorbed 0.3-10 keV flux is 3.78(8)e-10 erg/s/cm^2. These values suggest that the source is in a typical atoll-type island (hard) spectral state. The Swift/XRT PC-mode data are piled-up due to the high count rate of the source (~2.5 cts/s). After correcting for pile-up effects, we performed a spectral analysis adopting the Nthcomp model. We found that the Swift/XRT spectrum is generally consistent with the NICER spectrum.

Further NICER observations of this source are underway. Additional multi-wavelength follow-up is encouraged. We thank the Swift and NICER teams for the prompt scheduling of the reported observations.