MAXI J1621-501 distance inferred from type-I X-ray bursts detected by JEM-X

During INTEGRAL observation of the Norma Region (revolution 1915) on February 3rd, 2018, a thermonuclear X-ray burst was detected by the JEM-X instruments at 21:51:08 UTC with a rise time of about 2s and a decay time of almost 40s as measured between 3-25 keV. The position of the burst (RA=245.09, Dec=-50.03; 2 arcmin error radius) on the JEM-X image is consistent with MAXI J1621-501 (ATel #10869, #10874), which has already been recorded as an X-ray burster (ATel #11067).

Considering that INTEGRAL cannot distinguish MAXI J1621-501 from AX J1620.1-5002 only separated by about 2 arcminutes, this suggests that the source which is currently seen active by INTEGRAL (ATel #11252) is MAXI J1621-501. The source flux is measured with the following higher values compared to the previous INTEGRAL revolution (ATel #11252): 3-10 keV: 24 ±1 mCrab, 10-25 keV: 23 ±5 mCrab, 22-60 keV: 22.5 ±1.3 mCrab The JEM-X light curves show a structure consistent with a photospheric radius expansion burst reaching the Eddington luminosity.

The burst peak flux is measured at 1.5 ±0.3 Crab between 3-25 keV corresponding to an estimated unabsorbed bolometric flux of (4.1 ±1)×10-8 erg/cm2/s. We thus derive a source distance of 8.4 ±2 kpc using the standard candle burst luminosity (LEdd = 3.8×1038 erg/s) from Kuulkers et al. 2003; A&A 399, 663.

A similar X-ray burst is again detected from the same position during INTEGRAL revolution 1916 at 2018-02-06T03:42:05. At that time the average source fluxes are measured at 30 ±2 mCrab between 3-25 keV and 40.3 ±1.5 mCrab between 22-60 keV.