Detection of Type-I X-ray bursts from MAXI J1621-501

ATel #11067; Peter Bult (NASA/GSFC), Nicholas Gorgone, George Younes, Chryssa Kouveliotou (George Washington University), Fiona Harrison (Caltech), on behalf of the Swift Deep Galactic Plane Survey team
on 16 Dec 2017; 17:20 UT
Credential Certification: Chryssa Kouveliotou (ckouveliotou@email.gwu.edu)

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We observed the recently discovered X-ray transient MAXI J1621-501 (ATel #10969; ATel #10876; ATel #10874; ATel #10869) on 2017-12-02 (MJD 58089.96) and 2017-12-03 (MJD 58090.64) with NuSTAR, as part of the Swift/XRT Deep Galactic Plane Survey (PI: C. Kouveliotou) and the NuSTAR Legacy Survey. The light curve of the first observation (ObID 90301328002) shows two Type I X-ray bursts, establishing the nature of this source as an accreting neutron star. The two X-ray bursts occur 3.5 hours apart, and show similar phenomenology. The first event peaks at ~850 ct/s and lasts for 30 seconds, whereas the second peaks at ~400 ct/s and lasts for 35 seconds. No oscillations were detected in either burst. Preliminary analysis of the spectrum of the brightest burst indicates that it is well described by a blackbody with temperature kT=1.86 +-0.04 keV. The burst peak flux integrated over 2 seconds is ~4x10e-8 erg/cm^2. MAXI/GSC detected a third burst from the vicinity of MAXI J1621-501 on 2017 October 19 with a similar duration and spectrum (H. Negoro, private communication). The large MAXI/GSC uncertainties of the burst and source determination, however, prevented the MAXI team from conclusively associating the two. Given our NuSTAR results, it is highly likely that all three bursts originate from MAXI J1621-501. Further analysis of the MAXI and NuSTAR data is underway.