NICER observations of MAXI J1834-021: possibly a distant black hole low-mass X-ray binary
ATel #15951; J. Homan (Eureka Scientific), K. Gendreau, Z. Arzoumanian, T. E. Strohmayer (NASA/GSFC), D. Altamirano (University of Southampton). A. Sanna (Univ. of Cagliari), M. Ng (MIT)
on 17 Mar 2023; 20:59 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)
Subjects: X-ray, Transient
We report on NICER observations of the recently discovered X-ray transient MAXI J1834-021 (ATels #15929, #15932, #15939, #15940, #15946). NICER observed the source on a daily basis from March 7 to March 16 for a total exposure (after filtering) of ~42 ks. Initial count rates (0.5-10 keV, scaled to 52 detectors) were ~40 cts/s. They increased to a maximum of ~45 cts/s on March 9 and gradually dropped to ~22 cts/s on March 14 (confirming the decay reported in ATel #15946). A re-brightening occurred between March 14 and 16 to a maximum count rate of ~30 cts/s. Throughout this evolution the X-ray hardness (5-10 keV/0.5-3 keV) showed a mild anti-correlation with count rate. A plot of the light and hardness curves can be found at the link below.
A spectrum (0.5-10 keV) of a high count-rate interval (March 9) was fitted well with an absorbed (Nh=9.5+/-0.2 x 1e21 cm^-2) continuum of a power law (index 1.89+/-0.05) plus a disk blackbody (kT=0.37+/-0.03 keV). The unabsorbed 0.5-10 keV flux was ~2.4e-10 ergs/cm^2/s, with the power-law component contributing ~83% of the flux. Using the same spectral model for a low count-rate interval (March 14/15), we obtain a power-law index of 1.62+/-0.03, a disk temperature of 0.31+/-0.1 keV, and an unabsorbed flux of ~1.54e-10 ergs/cm^2/s (with a similar power-law fraction as in the high count-rate spectrum).
Power spectra in the 0.5-10 keV band revealed the presence of band-limited noise and a low-frequency QPO in all observations. The QPO frequency appeared to correlate with count rate. On March 7 the QPO was detected at ~1.8 Hz (Q-value~5.5, rms ~8.5%), reached a maximum frequency of ~2.2 Hz (Q-value~3.7, rms ~8.7%) on March 9, and gradually decreased in frequency to ~1.1 Hz (Q-value~2.7, rms ~14.3%) on March 14/15. The total fractional rms in the 1/16-100 Hz band was around 22%, with little change during the observations.
The above spectral and variability properties suggest that MAXI J1834-021 is a black hole low-mass X-ray binary that was in a hard-intermediate state during our observations. We compared the above results to observations of other sources and found, for example, that when the black hole X-ray transient MAXI J1820+070 showed low-frequency QPOs in the 1-2 Hz range, its spectral decomposition was very similar to what we find for MAXI J1834-021. Although there are exceptions, the lowest Eddington fractions at which black hole X-ray binaries are typically observed in a hard-intermediate state during an outburst is ~1% (see, e.g., Maccarone 2003, A&A, 409, 697). For an 8 solar-mass black hole that luminosity corresponds to ~1e37 erg/s. For the observed fluxes the inferred distance would then be ~20 kpc (or larger for higher Eddington fractions), but we note that this is a very rough distance estimate. A large distance could perhaps explain the reported non-detection of the source in the radio band (ATel #15939).
NICER will continue to monitor the outburst of MAXI J1834-021. Observations at other wavelengths are encouraged.
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.
NICER Light and Hardness Curve
