NICER follow-up observations of transient activity from Terzan 1

ATel #15957; J. Homan (Eureka Scientific), Z. Arzoumanian, K. C. Gendreau, T. E. Strohmayer (NASA GSFC), S. Guillot (IRAP/CNRS), A. Sanna (Univ. of Cagliari), M. Ng, D. Chakrabarty (MIT)
on 21 Mar 2023; 21:24 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Tweet

Following the detection with MAXI/GSC of transient activity from the direction of the globular cluster Terzan 1 (ATel #15955), we observed the cluster with NICER on March 21, between 16:14 UTC and 19:01 UTC for a total exposure time of ~1.1 ks.

The average 0.5-10 keV count rate is ~220 cts/s (for 52 detectors). No Type-I X-ray bursts are detected. The current count rates are higher than in archival NICER observations of Terzan 1 performed in April/May 2020, when count rates peaked around 75 cts/s. The spectral hardness (4-10 keV / 0.5-2 keV) is lower than in the 2020 data.

A power spectrum in the 0.5-10 keV band does not reveal any significant power in excess of white noise (3 sigma upper limit of ~9% rms in the 1/16-100 Hz band). For comparison, in the 2020 data strong band-limited noise was observed in the power spectrum (~22% rms). We also conducted an acceleration search over the 0.3-2 keV and 2-10 keV bands and did not detect any significant periodicity candidates.

We performed spectral fits in the 0.7-10 keV band. An absorbed power-law fits reasonably well (reduced chi^2 = 1.11 for 730 degrees of freedom) and yields Nh=(2.18+/-0.02)x10^22 cm^-2, a power-law index of 1.98+/-0.01, and an unabsorbed flux (0.7-10 keV) of ~2.2x10^-9 erg/s/cm^2. Adding a black body component at low-energies does not significantly improve the fit. We also tried an absorbed continuum consisting of a black body plus a disk black body. This fits better (reduced chi^2 = 0.96 for 728 degrees of freedom) and yields Nh=(1.94+/-0.02)x10^22 cm^-2, a black body temperature of 1.66+/-0.02 keV, a disk black body temperature of 0.71+/-0.01 keV, and an unabsorbed flux of ~1.9x10^-9 erg/s/cm^2. The latter fit, implying a soft spectral state, would be more consistent with the observed lack of rapid X-ray variability.

Activity from Terzan 1 was most recently reported in April and August 2000 (ATel #13630, #13983, #13991) and was attributed to the transient neutron-star low-mass X-ray binary XB 1732-304. Inspection of long-term MAXI light curves suggests that the source never fully went back to quiescence after its activity in 2000.

More observations by NICER are planned for tonight and into tomorrow.

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.