NICER detection of two X-ray bursts in the follow-up observations of Terzan 5 X-3

ATel #15922; Andrea Sanna (UniCA), Sebastien Guillot (IRAP-CNRS), Keith Gendreau, Zaven Arzoumanian, Tod Strohmayer (NASA/GSFC), Wataru Buz Iwakiri (Chiba Univ.), Tolga Güver (Istanbul Univ.), Deepto Chakrabarty (MIT), Diego Altamirano (Univ. of Southampton), Gaurava K. Jaisawal (DTU Space), Peter Bult (RIVM), Elizabeth C. Ferrara (UMCP, NASA/GSFC, CRESST)
on 1 Mar 2023; 22:52 UT
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Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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MAXI/GSC reported a new outburst from a source in Terzan 5 on 2023 February 27 at 17:26 UT (ATel #15917). NICER conducted observations scanning a grid of pointings centered on the MAXI coordinates, finding a source position R.A. = 267.0137 deg. and Dec. = -24.7658 deg. (J2000) consistent with being just 1 arcmin from the nominal center of the cluster. The position was later refined with Swift-XRT, suggesting the association with Terzan 5 X-3 (also known as Swift J174805.3-244637), a neutron star low-mass X-ray binary located in the globular cluster Terzan 5 (ATel #15919). Given the large number of X-ray sources in the field, Chandra observations could confirm the association of the current burster with Terzan 5 X-3.

NICER collected ~7.3 ks of pointed observations starting on 2023 February 28 18:00:40 UT. The source lightcurve shows a gradually increasing count-rate, rising from ~160 cts/s up to ~230 cts/s in the 0.5 - 10 keV energy range; we note that the low starting count rate may partially result from the initially offset NICER pointing before the source coordinates were firmly established. The power spectrum in this band does not present significant periodic signals in the frequency range 0.005-2500 Hz. However, we detected a broad-band (0.1-20 Hz) noise component with fractional rms of ~20%.

Two Type-I X-ray bursts from this source were detected on 2023 February 28 18:04:04 and 2023 March 1 06:46:26 UT. The tail of a likely third Type-I X-ray burst is partially detected starting from 2023 March 1 08:07:56 UT, suggesting a burst rate of one every ~40 minutes. Both Type-I bursts displayed a fast rise over a few seconds and exponential decay over ~100 seconds. The burst decay is significantly longer than the only previously observed burst from this source (Bahramian et al. 2014), likely implying different accretion rates or different elemental abundances in the accreted fuel. We searched for burst oscillations using a moving window of duration 2, 4, and 8 seconds and steps of 0.5 seconds in the frequency range 10-1000 Hz. We found no significant signals.

We performed spectral analysis of the persistent emission with ~2.4 ks exposure (corresponding to the last four orbits of the dataset where no Type-I bursts are present) using an absorbed disk blackbody plus blackbody model in the 1-10 keV range. The inferred hydrogen column density is (2.2\pm0.1) \times 10^{22} cm^{-2} using the tbabs model and assuming ISM abundances (Wilms et al. 2001), a value similar to that reported by Bahramian et al. (2014). The best-fit temperatures of the disk blackbody and blackbody components are 0.95\pm0.05 and 1.94\pm0.04 keV, respectively. The derived unabsorbed flux in the 0.5-10 keV range is 2.61\times10^{-9} erg/s/cm^{-2}.

Time-resolved spectroscopy of two Type-I bursts did not show any evidence of photospheric radius expansion. The bolometric peak fluxes of these bursts are estimated to be 1.27\pm0.09 and 2.00\pm0.37 \times 10^{-8} erg/s/cm^{-2}, corresponding to ~30% and ~50% Eddington luminosity (for a NS mass of 1.4 Msun and assuming a distance of 5.9 kpc for Terzan 5), respectively. The maximum blackbody temperatures are 2.05\pm0.08 and 2.17\pm0.22 keV, respectively, while the apparent emitting radius of the blackbody settles around 6 and 7 km for about 30 seconds in the cooling tail of both bursts.

Further NICER observations are planned, and we encourage additional observations of this source with other facilities. 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.