GX 339-4 in the low-hard state from early NICER and NuSTAR observations
ATel #15585; Federico Garcia (IAR, Argentina), Mariano Mendez (U. Groningen, NL), Diego Altamirano (U. Southampton, UK), Liang Zhang (IHEP, China), Konstantinos Karpouzas (U. Groningen, NL)
on 2 Sep 2022; 21:50 UT
Credential Certification: Federico Garcia (fgarcia@iar.unlp.edu.ar)
Subjects: X-ray, Black Hole
Following the X-ray detection with MAXI (ATel #15577) and Swift/XRT (ATel #15578) of a possible outburst of the black-hole candidate GX 339-4, and a radio detection with MeerKAT (ATel #15580), we triggered observations of this source with NICER (0.3-12 keV) and NuSTAR (3-70 keV). NICER observations span from 2022-08-29 at 20:41 UTC to 2022-09-02 at 14:19 UTC for a total net exposure of 3.3ks, while NuSTAR observations are from 2022-09-01 at15:35 UTC to 2022-09-02 at 01:50 UTC, with a net exposure of 21 ks.
A preliminary spectral and timing analysis of the quick-look data shows that GX 339-4 is in the Low-Hard State (LHS), with a total unabsorbed flux (1-sigma errors given in parentheses) of 7.2(5)e-10 erg/cm^2/s in the 2-20 keV energy band. We fitted simultaneously the NICER and NuSTAR spectra with a model consisting of a multicolor disk-blackbody (diskbb), a thermal Comptonisation (nthcomp) component, and a relatively narrow Gaussian to represent a significant Fe line detected in the residuals. The best-fitting temperature of the disc is kTin = 0.5(1) keV, and the power-law index and electron temperature of the Comptonising component are, respectively, Gamma = 1.56(1) and kTe = 56(15) keV. The total flux is dominated by nthcomp (~99%).
The full-band power density spectra of both instruments can be well fitted with two zero-centered Lorentzian functions with FWHM of, respectively, 0.17(1) Hz and 2.0(1) Hz, with a total fractional rms amplitude of 35-40% in the 0.1-10 Hz frequency band. We did not detect any significant QPO.
We encourage multi-wavelength follow-up of this new outburst of GX 339-4. We will revisit the source with NICER and NuSTAR in the upcoming days/weeks.
We thank the NICER and NuSTAR teams for promptly scheduling the observations.
