A Neil Gehrels Swift Observatory Snapshot of the Black Hole Candidate XTE J1908+094

ATel #12632; J. M. Miller, M. Reynolds, B. Tetarenko, S. Ali, M. Balakrishnan, J. Chen, D. Vozza (University of Michigan)
on 5 Apr 2019; 21:53 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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XTE J1908+094 is a black hole candidate and recurrent transient. During its 2013 outburst, complex, relativistic radio jets were resolved with the VLBA and EVN (Rushton et al. 2017). A preliminary black hole spin measurement of a = 0.75 +/- 0.09 was obtained based on modeling of BeppoSAX spectra (Miller et al. 2009; also see in't Zand et al. 2002); subsequent studies during the 2013 outburst did not achieve the sensitivity required to update this measurement (Tao et al. 2015, Zhang et al. 2015).

Following the detection of a new outburst in ATEL #12628 (Rodriguez et al. 2019), we requested a snapshot with the Neil Gehrels Swift Observatory to confirm the outburst in soft X-rays. The field was observed starting on 2019-04-04 at 06:34:41 UT, with a net exposure of 986s. A very strong source is detected at the known position of XTE J1908+094.

In this "photon counting" mode observation, there is significant pile-up and the core of the event distribution is black. We extracted a spectrum from an annulus (18-90 arcseconds), and grouped the data according to the optimal binning scheme of Kaastra & Bleeker (2016) via the HEASOFT ftool "ftgrouppha". The 1-10 keV spectrum can be fit acceptably (chi^2/dof = 50.7/50) with an absorbed power-law; however, this requires an implausible, steep index (Gamma = 4.7 +/- 0.1) and an equivalent neutral hydrogen column density that is several times higher than expected (N_H = 4.6 +/- 0.2 E+22 cm^-2; whereas N_H ~ 1.8 E+22 is expected via Dickey & Lockman 1990). If this is astrophysical, it may indicate dust along the line of sight. When the expected value is used, a power-law fit becomes unacceptable (chi^2/dof = 535/51).

Fits to the spectrum with an absorbed disk blackbody (Mitsuda et al. 1984) model are not as good (chi^2/dof = 98/50), but yield more plausible parameter values (kT = 0.69 +/- 0.03 keV, N_H = 2.8 +/- 0.1 E+22 cm^-2). Mixed models, with either a disk blackbody and a power-law, or a Comptonized disk blackbody, still require a very steep power-law index and a column density significantly above predictions. Finally adopting a mixed model, tbabs*(diskbb+powerlaw), and enforcing an upper-limit of Gamma = 3 on the power-law index, a good fit (chi^2/dof = 51/49) is achieved with broadly plausible parameters: N_H = 3.5 +/- 0.1 E+22 cm^-2, kT = 0.49 +/- 0.03, K_dbb = 4421 (+2700, -1600), Gamma = 3, K_pow = 0.75 +/- 0.09.

This model gives an absorbed flux of 6.9 E-10 erg/cm^2/s in the 1-10 keV band, and an unabsorbed flux of 3.4 E-9 erg/cm^2/s in the same band. The distance to XTE J1908+094 is unconstrained though it might be measured via proper motion measurements in radio in the future. Assuming a distance of 8 kpc, the unabsorbed flux implies a luminosity of 2.6 E+37 erg/s, or an Eddington fraction of 0.02 for a 10 Msun black hole. The spectral parameters that we have measured are broadly consistent with these estimates of luminosity and Eddington fraction.

We thank the Neil Gehrels Swift Observatory for executing this observation.

References

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