Strong changes in the X-ray behavior of XTE J1701-462

NICER has monitored the current outburst of XTE J1701-462 (ATel #15592, #15594, #15616, #15621) on a daily basis since September 8. Following our report on the initial NICER observations (ATel #15605), the source kept gradually increasing in brightness, with the 0.5-12 keV count rate increasing from ~590 cts/s on September 8 to ~1650 ct/s early on September 20. After this peak in count rate, the source started to show strong dips in the light curve, down to count rates as low as ~920 cts/s. A 0.5-12 keV light curve can be seen in the figure linked below. Strong dipping was also observed around the peak of the 2006/2007 outburst of XTE J1701-462, when it showed so-called Cyg-like Z source behavior (Lin et al. 2009, ApJ, 696, 1257; Homan et al. 2010, ApJ, 719, 201). Although we were not able to construct a full Z track from the NICER observations made between September 20-24, the morphology of the incomplete track is broadly consistent with a Cyg-like Z source track in a hardness-intensity diagram.

The above suggests that the source must also have passed through a Sco-like Z source phase during the rise, since Sco-like Z behavior occurs at lower luminosities (see references above). Clear Sco-like Z source tracks cannot be identified in the NICER hardness-intensity diagram, likely due to a lack of coverage above 12 keV. However, we did observe flaring in the 8-12 keV band between September 10 and 20, with count rates briefly (tens of minutes) increasing by factors of up to ~2-3 (see figure). This flaring at high energies was also observed with RXTE during the Sco-like Z source phase of XTE J1701-462 during its 2006/2007 outburst. The Z source behavior observed with NICER is in line with reports on the recent radio properties of the source (ATel #15617, #15621).

A spectrum from the segment with the highest 0.5-12 keV count rate (Sep. 20.4; 1.3 ks exposure) was fit with a double thermal model (absorbed disk blackbody plus blackbody). This model did not perform as well as for the atoll soft state spectrum reported in ATel #15605, especially at the low-energy end, with a reduced chi-squared of ~1.6 for 939 degrees of freedom. Replacing the blackbody with a Comptonized blackbody slightly improved the fit (reduced chi-squared of ~1.4 for 939 d.o.f.). For this model the unabsorbed 0.5-10 keV flux was 1.8e-8 erg/cm^2/s, which, for a distance of 8.8 kpc, corresponds to a luminosity of 1.7e38 erg/s.

Rapid X-ray variability remained weak and no clear QPOs could be identified in the power spectra of the recently obtained NICER data.

NICER will continue to observe XTE J1701-462. We encourage further observations of the source with other observatories.

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.