Mid-IR detection of GX 339-4 with VLT/VISIR, and upcoming JWST observation

ATel #15596; David M. Russell (NYU Abu Dhabi), Poshak Gandhi (Univ. of Southampton), Kevin Alabarta, M. Cristina Baglio (NYU Abu Dhabi), Tomaso Belloni (INAF-OA Brera), Piergiorgio Casella (INAF-OA Roma), Chiara Ceccobello (Chalmers Univ. of Technology, Sweden), Maria Diaz Trigo (ESO Garching), Elena Gallo (Univ. of Michigan), Jeroen Homan (Eureka Scientific), Karri Koljonen (Norwegian Univ. of Science and Technology, Trondheim), Sera Markoff (Univ. Amsterdam), James C. A. Miller-Jones (ICRAR, Curtin Univ.), Kieran S. O’Brien (Durham Univ.), Jérôme Rodriguez (Universite de Paris), Thomas D. Russell (INAF/IASF Palermo), Payaswini Saikia (NYU Abu Dhabi), Tariq Shahbaz (IAC & ULL), Gregory Silvakoff (Univ. of Alberta), Roberto Soria (NAOC, ICAS, Beijing & Univ. of Sydney), Alexandra Tetarenko (Texas Tech)
on 7 Sep 2022; 19:00 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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A new outburst from the recurrent transient X-ray binary, GX 339-4, was detected in 2022 August. The X-ray flux increased from 23 to 28 August in MAXI (2-20 keV) and Swift/BAT (15-50 keV; ATel #15577), and a brightening in more sensitive Swift/XRT observations was reported to have started around 6 August (ATel #15578). A radio detection at 1.284 GHz with MeerKAT on 27 August (ATel #15580), and X-ray spectral and timing analysis from NICER (0.3-12 keV) and NuSTAR (3-70 keV) observations on 29 August to 2 September (ATel #15585), confirmed that the source is in a hard X-ray state in the rising stage of a new outburst.

We observed GX 339-4 at mid-infrared wavelengths with VISIR on the Very Large Telescope (UT2) in Chile from 23:53 UT on 5 September to 00:50 UT on 6 September (MJD 59828.0), under ESO Programme ID 109.23CD (PI: D. Russell). The observations were taken in the B10.7 filter (central wavelength 10.64 microns, FWHM 1.34 microns) using a chopping/nodding routine. The standard star HD169916 was observed at 23:40 UT on 5 September, at a similar airmass and time as GX 339-4. The data were reduced and combined in the gasgano environment and photometry was performed using IRAF. GX 339-4 was detected with a S/N of ~25. Using the single standard star, we estimate a preliminary flux density of 23.8 +/- 2.6 mJy at 10.64 microns. The error is dominated by a conservative, systematic flux error of 10% since only a single standard star was observed.

Adopting an extinction towards GX 339-4 of A_V = 3.25 +/- 0.50 mag (Gandhi et al. 2011), and a mid-IR extinction of A_Lambda/A_V of 0.03 at ~10 microns (Weingartner & Draine 2001), the de-reddened flux density at 28 THz is 26.2 +/- 2.6 mJy. Using the 1.284 GHz MeerKAT radio detection of 2.24 +/- 0.05 mJy taken 10 days before the VISIR observation (ATel #15580), we estimate a radio to mid-IR spectral index of alpha ~ 0.25 (where F_nu ~ nu^alpha), assuming there is no spectral curvature or break. This is similar to previous measurements of the radio to mid-IR spectral index (alpha ~ 0.29; Gandhi et al. 2011) and the radio to far-IR spectral index (alpha ~ 0.32 to 0.38; Corbel et al. 2013, MNRAS, 431, L107). However, the source will likely have brightened slightly during those 10 days (the MAXI light curve shows a very slight brightening in that period) so the spectrum may be slightly flatter (alpha <= 0.25) since the radio may have brightened by the time of the VISIR observation.

JWST is scheduled to observe GX 339-4 as part of Cycle 1 ToO programme 1586 (PI: P. Gandhi) in the mid-IR (~5-12 microns), on 10 September starting UT 00:42 UT and ending 03:03 UT. Multiwavelength coordination with this time window is encouraged, especially with high time resolution facilities. Strong multiwavelength sub-second variability is characteristic to the hard state of this source (e.g. Gandhi et al. 2010 MNRAS 407 2166 , Casella et al. 2010 MNRAS 404 L21). Additional multiwavelength coordination throughout the outburst is also strongly encouraged (e.g., through SmartNet, http://isdc.unige.ch/smartnet/ ).