NICER Observations of the new outburst from XTE J1908+094

ATel #12652; R. M. Ludlam (U. Michigan), R. Remillard (MIT), J. Homan (Eureka Scientific,), T. E. Strohmayer, K. Gendreau, and Z. Arzoumanian (NASA/GSFC)
on 11 Apr 2019; 20:47 UT
Credential Certification: Ron Remillard (rr@space.mit.edu)

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Renewed activity in the black hole X-ray transient, XTE J1908+094, was discovered with INTEGRAL observations on 2019-04-01 (ATel #12628; Rodriguez et al.). Accretion activity in this recurrent transient (2002, 2013) was confirmed with a precise X-ray position from the Neil Gehrels Swift Observatory on 2019-04-04 (ATel #12632; Miller et al. 2019). The Swift observation further indicated that the X-ray spectrum was soft, and it could be fit by either a power law with photon index of 4.7 or a disk-blackbody with temperature near 0.7 keV. This result was somewhat surprising, since the INTEGRAL detection was apparent at 30-100 keV.

NICER observed the source on 2019-04-06 (576 s) and 2019-04-09 (2555 s). Count rates (0.2-12 keV) were ~200 cts/s (for 52 FPM). A soft spectrum, in agreement with ATel #12632, is confirmed on the basis of the NICER hard color (HC; the ratio of count rates at 4-12 keV, relative to 2-4 keV). For XTE J1908 on 2019-04-06, HC = 0.078(4), which is significantly softer than the Crab spectrum (HC = 0.250(1)).

The energy spectra of XTE J1908 were fit, after background subtraction, using two different methods. The 2019-04-06 spectrum was fit in the range 0.3-6.0 keV, which is the most reliable range in the current response calibration. A satisfactory fit is obtained with a simple disk-blackbody model (reduced chi^2 = 0.99 for 444 degrees of freedom), indicating a column density, Nh=2.88(0.02) x 10^22 cm^-2, an inner disk temperature 0.62(1) keV, and disk-blackbody normalization 1417(54). The fit is marginally improved (chi^2 = 0.93) by adding a Comptonization term using the simpl function and assuming that the disk-blackbody provides the seed photons. In that case, neither the scattering fraction (best fit is 2%) nor the photon index is meaningfully constrained. On the other hand a power law model produces an unsatisfactory fit (chi^2 = 2.2).

The energy spectrum from 2019-04-09 was background subtracted and normalized to the Crab Nebula (Ludlam et al. 2018), which allows spectral fits out to 10 keV. The spectrum can again be described by an absorbed disk-blackbody and power-law component (chi^2 = 1.16 for 854 degrees of freedom). The neutral absorption column along the line of sight was 4.21(4) x 10^22 cm^-2 with a disk temperature of 0.60(1) keV, disk normalization of 1279(50), photon index of 2.2(2), and normalization of 0.11(7); errors are quoted at the 90% confidence level. The unabsorbed flux in the 0.4-10 keV energy band is 3.6E-9 erg/cm^2/s. There is an emission feature near 6.8 keV that is consistent with an Fe K line.

The power spectra of both observations indicate little to no measurable variability. An upper limit on the integrated fractional rms variability (0.1 - 10 Hz) in the 1 - 4 keV band is about 5%. Both the thermal energy spectrum and dearth of variability are indicative of a black hole in the soft state.

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.

References: Ludlam et al. 2018, ApJL, 858, L5