Recent Swift Monitoring of Black Hole Candidate GX 339-4

ATel #12690; J. Chen, S. Ali, M. Balakrishnan, N. Kebede, J. M. Miller, M. T. Reynolds, B. E. Tetarenko, D. Vozza (Univ. of Michigan)
on 25 Apr 2019; 14:35 UT
Credential Certification: Mark Reynolds (markrey@umich.edu)

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GX 339-4 is one of the most frequently recurring black-hole low-mass X-ray binaries in our Galaxy, having undergone more than 20 outbursts since its discovery (Tetarenko+ 2016). Following on previous Atels (#12420, #12413, #12322), we have looked at the 6 most recent observations starting from the beginning of March (OBSID: 00032898195) in XRT and UVOT.

GX 339-4 has been in the hard-state since mid-2015 (ATel #7962), and continual detections are made since then. Recently, on April 9th (OBSID: 00032898207), GX 339-4 was observed with XRT (1.8ks in pc mode). We have mitigated pile-up by using an annulus around the center of the source (10-60 arcseconds) for the source region. The extracted spectrum was re-binned with grppha and the spectrum had a net count rate of 0.89 counts per second.

Using an absorbed power law (tbabs*po), the 0.5-10 keV spectrum had a photon index of 1.35 +/- 0.17, which matches with previous observations made back in January 2019 (ATels #12413, #12322). This value continues to suggest that GX 339-4 is still in the "low-hard" state. The model returned a column density of (4.1 +/- 1.7)E21 cm^-2 and flux of (2.1 +/- 0.2)E-10 erg/cm^2/s with (chi^2/dof = 63/71). (All values are quoted with 90% confidence errors.) Comparing the flux values with recent observations made in the start (OBSID: 00032898195) and end (OBSID: 00032898205) of March, GX 339-4 shows a decline in brightness in 0.5-10 keV by a factor of ~1.5 from 5.21 to 3.46 (E-10 erg/cm^2/s).

We have also examined observations made with SWIFT/UVOT and the source has been consistently detected. An observation on March 16 (OBSID: 00032898202) with an exposure time of 820s shows a UVW2-band flux of (1.61 +/- 0.26)E-2 mJy, while a 1.3ks UVW2-band observation on March 27 (OBSID: 00032898205) reveals a flux of (1.28 +\- 0.21)E-2 mJy. A recent detection on April 9 (OBSID: 00032898207) with an exposure time of 1.4ks shows a UVM2-band flux of (1.02 +/- 0.22)E-2 mJy. Both extractions have a source radius of 5". The flux is observed to continue its decay in both the X-ray and UV bands over the past month.

We will continue our multi-wavelength monitoring of GX 339-4 with SWIFT and we thank the Neil Gehrels Swift Observatory for executing this observation.

References:
Tetarenko et al., 2016, ApJS, 222, 15