NICER detects a type I X-ray burst from 4U 1730-22

On 2021 June 7 the new transient MAXI J1733−222 was detected by MAXI (ATel #14683). Subsequent localization associated this outburst with the historic X-ray transient 4U 1730−22 and its suspected quiescent counterpart CXOU J173357.5−220156 (ATel #14686, #14688, #14693, #14694). On July 6 MAXI reported a rapid brightening of the source (ATel #14757).

We have regularly monitored the ongoing outburst of 4U 1730−22 with NICER, and, on July 9 13:14 UTC, detected a type I X-ray burst from this source. The X-ray burst showed a slow and markedly concave rise, taking about 5 seconds to reach its peak luminosity. The subsequent decay lasted roughly 50 seconds and could be well described by a cooling blackbody emission component, thereby confirming the thermonuclear nature of the X-ray burst.

The peak bolometric flux of the X-ray burst was measured at 3.5 × 10-8 erg/s/cm2. The X-ray burst showed no signs of photospheric radius expansion. Assuming the empirical Eddington luminosity of 3.8 × 1038 erg/s (Kuulkers et al. 2003, A&A 399, 663), this places an upper limit on the source distance of about 10 kpc.

No burst oscillations were detected. The pre-burst emission could be well described using an absorbed disk plus power law model, yielding a disk temperature of 1.3 keV and a power law photon index of 1.75, with an absorption column density of 0.6 × 1022 cm-2. The 1-10 keV unabsorbed X-ray flux is measured at 2.3 × 10-9 erg/s/cm2.

This intensity is consistent with the latest report from MAXI (ATel #14757). The source was previously suspected to host a neutron star based on the association with CXOU J173357.5−220156, which shows a quiescent X-ray spectrum that is consistent with a neutron star atmosphere (Tomsick et al. 2007, ApJ 663, 461).

This detection marks the first X-ray burst observed from 4U 1730−22, and definitively confirms the neutron star nature of this X-ray transient. 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.