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Identification of AX J1620.1- 5002 with MAXI J1621-501

ATel #11317; Nicholas Gorgone, Chryssa Kouveliotou, George Younes (George Washington University), Jamie Kennea (Penn State University)
on 18 Feb 2018; 02:09 UT
Credential Certification: Chryssa Kouveliotou (ckouveliotou@email.gwu.edu)

Subjects: X-ray, Neutron Star

We report on behalf of the Swift/XRT Deep Galactic Plane Survey Team: We observed the INTEGRAL error region of the X-ray transient source AX J1620.1-5002 (ATEL# 11252; Lepingwell et al.) for 1 ks with Swift/XRT to constrain its location. At the reported location of AX J1620.1-5002 we determine a 3-sigma upper limit of 4.2E-02 counts/s. We detected only one source, MAXI J1621-501, in the 24' XRT field of view with over 3 sigma significance above the background (0.3-10 keV). The source count rate was high, ~2.2 counts/s, so the fluxes reported below have been corrected for pileup. The source spectrum (0.5-10 keV) is well fit both with an absorbed powerlaw or an absorbed disk blackbody continuum. For the disk blackbody we get an unabsorbed flux of 7.4 +/- 0.2 E-10 erg/s/cm^2 corresponding to a luminosity L = 6.2 +/- 1.5 E36 erg/s at the reported distance of 8.4 +/- 2 kpc (Chenevez et al., ATel 11272). The blackbody temperature is T_in=3.6 +/- 0.4 keV and the N_H is frozen at 3.7 E22 cm^-2, as derived by our combined analysis (Gorgone et al., in preparation) of NuSTAR and XRT data (Cstat = 168.75 for 206 degrees of freedom). For the powerlaw we get Cstat = 180.26 with 206 dof, with N_H frozen at 3.7E22 cm^-2, and gamma=1.12 +/- 0.08; the unabsorbed flux is 8.2E-10 +/- 0.2 erg/s/cm^2 and the luminosity is L = 8.2 +/- 1.7 E36 erg/s. Our analysis supports the conclusion by Chenevez et al. (ATEL# 11272), that the source is most likely MAXI J1621-501 and not AX J1620.1-5002 or some other source in the field.