NICER Observation of Strong Wind Absorption in the Soft Outburst of 4U 1630-47

ATel #11771; J. Neilsen (Villanova University), A. L. Stevens (Michigan State U.), R. Remillard (MIT) J. F. Steiner (MIT), J. Homan (Eureka Scientific & SRON), D. Altamirano (U. of Southampton), Z. Arzoumanian (GSFC), K. Gendreau (GSFC)
on 23 Jun 2018; 21:07 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Joey Neilsen (jneilsen@villanova.edu)

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In response to the MAXI transient alert regarding a new outburst of 4U 1630-47, NICER observed the black hole candidate for 1.96 ks in three exposures, with the first starting at 2018-06-11T13:37:26 (UTC). The source is clearly detected at approximately 230 cts/s (0.2-12 keV); no features (broadband noise or quasi-periodic oscillations) are present in the average power spectrum for this NICER observation. An analysis of MAXI/Swift data (ATel #11722) has indicated that this outburst is quite soft; no evidence of an initial hard state is immediately apparent (consistent with the discussion of occasionally missing hard states by Capitanio et al. 2015). 4U 1630-47 had reached a 0.5-10 keV flux of ~1e-8 erg/s/cm^2 (roughly 0.22 Crab) as of 15 June 2018.

The 0.25-12 keV NICER continuum, background-subtracted using the method of Neilsen et al. (2018), is well described by an absorbed disk blackbody (tbnew*ezdiskbb; Wilms et al. 2000; Zimmerman et al. 2005). The interstellar absorption has a column density NH=(11.72+/-0.07)e22 with an enhanced Si abundance (A_Si=2.3+/-0.1), though calibration uncertainties near the Si edge make it difficult to interpret this result robustly. The disk has a maximum temperature of (1.084+/-0.006) keV, which is similar to the result reported from the Swift XRT several days later. Together with the variability properties, this suggests that the source is in the soft state.

In addition to the absorbed disk, we find evidence of several narrow emission and absorption lines. The absorption features are consistent with Fe XXV and Fe XXVI absorption in an outflowing wind, as has been seen on many occasions in 4U 1630-47 and other, similar sources. These lines are detected at 6.74 [-0.03,+0.05] keV and 7.00 [-0.02,+0.01] keV with equivalent widths of 18 and 39 eV, respectively, and have 1-sigma widths below 0.023 keV (roughly 900 km/s). The apparent blueshift of the gas is approximately 1600-1700 km/s and is statistically non-zero at the ~3 sigma level. The precise numbers should be treated with caution in light of possible residual gain calibration uncertainty near 6-7 keV that can reach 20-30 eV, but previous observations of 4U 1630-47 with Suzaku and Chandra have revealed similarly strong absorption lines with blueshifts ranging from 200-1500 km/s (Kubota et al. 2007; Neilsen et al. 2014; Miller et al. 2015).

The emission lines are observed at (2.36+/-0.03) keV and (6.36-0.03+0.04) keV, with ~4 sigma and ~3 sigma significance; we identify them as S K and Fe K, respectively, though we cannot rule out an association between the 2.36 keV line and the gold edge from the NICER optics at this time. The lines are nominally broader than the narrow absorption though still unresolved at (0.05+/-0.03) keV. Fe K lines are commonly found in X-ray binaries, but fluorescent lines from other elements are more rare. The S K detection here is facilitated by NICER's large soft X-ray area.

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.