NICER timing results indicate a state transition in Swift J1727.8-1613

ATel #16273; Niek Bollemeijer (University of Amsterdam), Phil Uttley (University of Amsterdam), Douglas Buisson, Jeroen Homan (Eureka Scientific), Diego Altamirano (University of Southampton), James F. Steiner (Center for Astrophysics | Harvard & Smithsonian), Keith C. Gendreau (NASA GSFC), Zaven Arzoumanian (NASA GSFC), Tod E. Strohmayer (NASA GSFC), A. Sanna (Univ. of Cagliari), on behalf of the NICER team
on 8 Oct 2023; 20:00 UT
Credential Certification: Phil Uttley (p.uttley@uva.nl)

Tweet

The black hole candidate X-ray binary Swift J1727.8-1613 was discovered in X-ray emission on August 24, 2023 (ATel #16205, #16206, #16219), and has since been monitored closely with NICER, revealing a steady evolution in timing and spectral properties from the hard state to the hard intermediate state (HIMS; ATel #16247), marked by type C QPOs increasing in frequency up to 8 Hz, broadband (0.1-10 Hz) noise with fractional rms > 11% in the 4-10 keV band, and substantial power-law emission (scattering fraction > 0.3). Here we report results on the recent evolution of timing properties, which suggest that Swift J1727.8-1613 underwent a HIMS to soft/soft-intermediate (SIMS) state transition between October 4 and October 5 2023.

Swift J1727.8-1613 was observed by NICER throughout October 4 and 5, with observations taking place every ISS orbit, approximately every 90 minutes. After removal of data from detectors with shredded GTIs due to instrumental light leak and the high source count rate (https://heasarc.gsfc.nasa.gov/docs/nicer/analysis_threads/heasoft632/#shredding), timing analysis shows that the 0.1-10 Hz 4-10 keV fractional rms decreased from 10% on October 4 to 4% on October 5, while the prominent 7 Hz QPO is no longer detected. A dynamic power spectrum made from successive observations indicates that these changes occurred during 03:00-11:00 UT October 5, suggesting that a state transition from the HIMS to the SIMS or soft state occurred during this time. Since sources in the SIMS can show weak band-limited noise instead of a QPO, it is not yet possible to determine whether Swift J1727.8-1613 has entered the SIMS or the full soft state. In either case, a transition on October 5 would be consistent with the observed radio quenching and subsequent radio flaring observed on October 5 and 6 (ATel #16271).

Based on a visual comparison, NICER spectra remain disk-dominated below 3 keV but show evidence for significant softening or suppression of the power-law flux at higher energies between October 4 and 5, consistent with the state transition interpretation and with the MAXI light curves, which show a large drop in 10-20 keV flux from October 4 to October 5 (http://maxi.riken.jp/star_data/J1727-162/J1727-162.html).

NICER observations are continuing until October 9 2023 when solar avoidance constraints will prevent further observations until February 2024. A schedule of upcoming observations can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. We would like to thank F. Capitanio and the IXPE team for allowing us to report on the timing properties of the October 4 NICER data, which was obtained in support of the IXPE observing campaign on Swift J1727.8-1613.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.