NuSTAR observations of a type-I X-ray burst and preliminary pulse detection identify SAX J1748.9-2021 in outburst

ATel #15048; Sean Pike (Caltech), Nobuyuki Kawai (Tokyo Tech), Tatehiro Mihara (RIKEN), Hitoshi Negoro (Nihon U.)
on 19 Nov 2021; 05:45 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sean Pike (spike@caltech.edu)

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We report on NuSTAR observations of the recent outbursting source MAXI J1749-204 in the globular cluster NGC 6440. The MAXI error circle was consistent with two previously known accreting millisecond pulsars (AMP), SAX J1748.9-2021 and NGC 6440 X-2 (Negoro et al. 2021, Atel #15033). NuSTAR observed the outbursting source on 2021 November 18 (UTC) for a total exposure time of 39 ks. With a position of RA=17:48:52, Dec=-20:21:53 (+/- 45"), the source is coincident with both of the AMPs. The source spectrum is described well by a blackbody with temperature kT_bb=0.46 keV, scattered into a power law with index Gamma=1.8 by a Comptonizing medium with electron temperature kT_e=26 keV. We also included an absorbing column, for which we found a density of nH=6e22 cm^-2. We found a source flux (absorbed, 2-10 keV) of 3.6e-10 erg/cm^2/s. We performed a Z^2_1 search (Buccheri et al. 1983) for pulsations in the frequency ranges 435-455 Hz and 195-215 Hz, corresponding to the known pulse frequencies of SAX J1748.9-2021 (442.36 Hz, Altamirano et al. 2008) and NGC 6440 X-2 (205.89 Hz, Altamirano et al. 2010), respectively. While the most significant signal in the 195-215 Hz range had a 92% false alarm probability (0.1-sigma confidence), we found preliminary evidence for pulsations with frequency 423 Hz at the 2-sigma confidence level. More detailed timing analysis, including orbital corrections and NuSTAR clock corrections are necessary to more accurately determine the significance of this detection as well as the precise frequency of pulsations. We also observed a type-I X-ray burst with a duration of ~100 s. This, in addition to the preliminary pulse detection at 423 Hz, leads us to conclude that the outbursting source MAXI J1749-204 is SAX J1748.9-2021, which has been shown to exhibit type-I bursts. We encourage further observations of the source.