NuSTAR+Swift observations and possible dimming of the new X-ray transient MAXI J1834-021

ATel #15946; Alessio Marino (ICE-CSIC, IEEC Barcelona), Alice Borghese (IAC Tenerife), Francesco Coti Zelati, Nanda Rea (ICE-CSIC, IEEC Barcelona), Andrea Sanna (Univ. Cagliari)
on 15 Mar 2023; 10:32 UT
Credential Certification: Alessio Marino (marino@ice.csic.es)

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MAXI J1834-021 is a newly discovered X-ray transient (Negoro et al., Atel #15929; Kennea et al., #15932) the nature of which has yet to be identified. On March 7th 2023, Swift/XRT caught the source at a flux of about ~2e-10 erg/s/cm^2 in the 0.3-10 keV band. Following its discovery, the source has been the object of a number of multi-wavelength follow-up observations (Bright et al., Atel #15939, Saikia et al., #15940).

We observed the source simultaneously with NuSTAR (28 ks) and Swift/XRT (WT mode, 3.2 ks) on March 10th. Visual inspection of the light curves did not reveal any peculiar feature, such as type-I X-ray bursts or flares. The NuSTAR power density spectrum, extracted using a time bin size of 0.2 ms, is remarkably flat over the frequency range 0.01-1000 Hz and compatible with Poisson noise. We conducted preliminary acceleration searches for periodic signals over the frequency range 1-1000 Hz using the PRESTO (Ransom et al. 2002, AJ, 124, 1788) and Stingray (Huppenkothen et al. 2019, ApJ, 881, 39) software packages, without finding any plausible candidate.

The broadband Swift/XRT (0.7-10 keV) + NuSTAR (3-60 keV) spectrum can be well described (chi^2/d.o.f.=1337/1126) using a simple absorbed power-law model (tbabs*powerlaw in Xspec). As best-fit parameters, we found an absorption column density of N_H=(0.7+/-0.3)e22 cm^-2 and a power-law photon index of Gamma = 1.86+/-0.01. As a more physically motivated model, we also tried an absorbed Comptonization model (tbabs*nthcomp in Xspec), finding a good fit (chi^2/d.o.f.=1356/1125) and similar best-fit values for N_H and Gamma. Due to the lack of a clear spectral cut-off, however, we could not constrain the value of the electron temperature kTe (we set a 3-sigma lower limit of 300 keV). This preliminary analysis did not show any significant trace of a soft X-ray thermal component (i.e., from an accretion disc and/or a boundary layer) or reflection features such as the Fe K line at 6.4-6.7 keV. The absorbed X-ray flux is (3.1+/-0.3)e-10 erg/s/cm^2 in the 0.7-60 keV band. For a tentative distance of 8 kpc, we estimate an unabsorbed bolometric X-ray luminosity of the order of 3e36 erg/s.

Due to the lack of distinctive X-ray features, such as pulsations or type-I X-ray bursts, an unambiguous identification of the nature of the source is still not possible. We notice that the observed broadband spectral characteristics could be consistent with MAXI J1834-021 being a low-mass X-ray binary (either harbouring a black hole or a neutron star) in a hard state. However, the radio non-detection at 15.5 GHz (Atel #15939) and the lack of X-ray variability seem hard to reconcile with this scenario. Otherwise, the system could be a high mass X-ray binary, a scenario hinted in Atel #15940 if the bright Gaia source (G~15.8) within the Swift/XRT position circle is actually the optical counterpart of the system.

A second Swift/XRT observation (WT mode, 2.9 ks) was performed on March 13th. We noticed a drop in count-rate of a factor ~2 with respect to the previous observation. It is unclear whether such a trend could hint at the beginning of the outburst decay. An X-ray monitoring campaign of the source is ongoing. Follow-up multi–wavelength observations are encouraged.

We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for carrying out the observation. We also thank Brad Cenko and the Swift duty scientists and science planners for making the Swift Target of Opportunity observations possible.