NICER observations indicate that Swift J1727.8-1603 is deep in the hard intermediate state and evolving towards the soft intermediate state

ATel #16247; Niek Bollemeijer (University of Amsterdam), Phil Uttley (University of Amsterdam), Douglas Buisson, Jeroen Homan (Eureka Scientific), Diego Altamirano (University of Southampton), Keith C. Gendreau (NASA GSFC), Zaven Arzoumanian (NASA GSFC), Tod E. Strohmayer (NASA GSFC), Andrea Sanna (Univ. of Cagliari), on behalf of the NICER team
on 19 Sep 2023; 21:33 UT
Credential Certification: Phil Uttley (p.uttley@uva.nl)

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Swift J1727.8-1603 was first detected in X-rays on August 24, 2023 (ATel #16205, #16206, #16219), and has since been monitored closely with NICER. Here, we report on preliminary spectral and timing analyses from NICER observations made on 2023 September 18 and 19. The spectral and timing properties of Swift J1727.8-1603 show that the source is currently in the Hard Intermediate State (HIMS) and may be close to entering the Soft Intermediate State (SIMS).

Following removal of data from detectors with "shredded" GTIs due to light leak and the high source count rates (https://heasarc.gsfc.nasa.gov/docs/nicer/analysis_threads/heasoft632/#shredding), timing analysis shows a clear QPO with a central frequency varying primarily between 4 and 6 Hz. At the end of the observation on September 18, the frequency increased to ~8 Hz and the amplitude of the broadband noise decreased, but was still clearly present. The 0.1-10 Hz fractional rms in the 4-10 keV band decreased from 17% to 11% during the observation, while the 4-10 keV QPO fractional rms decreased from 5% to 3%. The QPO frequencies and fractional rms suggest that the source did not yet reach the SIMS, but is very close to it. The QPO frequency increases appear to closely track flaring in the soft X-ray flux observed by both NICER and MAXI (http://maxi.riken.jp/star_data/J1727-162/J1727-162.html), while the hard X-ray flux measured by MAXI and Swift/BAT continues to decline.

The NICER spectrum can be described by a disk black body with a scattered power law tail, subject to absorption by neutral material (with N_H=0.2-0.25x10^21cm^-2), using the XSPEC model "tbabs*simpl*diskbb". Throughout most of September 18, when the QPO frequency was between 3.5 and 5 Hz, disk emission formed the spectral peak (kT=0.6 keV) but a majority of photons were still in the scattered tail (Gamma=2.4, scattered fraction f_Scat=0.7). In the observation segment with the highest QPO frequency, around September 19 06:00 UT, the disk was hotter (kT=0.9 keV) and the power law softer and proportionally weaker, Gamma=2.5; f_Scat=0.3. In all cases, a broad iron K alpha line is also present, with little change in its amplitude.

NICER observations are continuing. A schedule of upcoming observations can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html .

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.