NICER observations of the X-ray transient MAXI J1807+132

ATel #13139; Stefano Rapisarda (Shanghai Astronomical Observatory), Peter Bult (USRA/NASA/GSFC), Renee Ludlam (Cahill Center for Astrophysics, CalTech), Wenda Zhang (Astronomical Institute of the Czech Academy of Sciences), Wenfei Yu (Shanghai Astronomical Observatory)
on 27 Sep 2019; 18:33 UT
Credential Certification: Renee Ludlam (rmludlam@umich.edu)

Tweet

Following the detection of the transient MAXI J1807+132 by MAXI/GSC on 2019 September 10 (ATel #13097) and subsequent X-ray and radio observations (ATel #13107), we have triggered NICER ToO observations on 2019 September 16. We report preliminary results from ~15 ks of NICER observations collected between 2019 September 16 and 26 (MJD 58742 and 58752, respectively). Up to MJD 58743 NICER collected ~9.6 ks of data. In this period, the source had a NICER mean count rate of ~200 c/s in the 0.5-10 keV energy band. After these observations, NICER lost visibility of the target until it started observing again on MJD 58749 (23 September 2019), when the source count rate had dropped to about ~50 c/s. From this date to MJD 58752 the count rate decreased to ~14 c/s, this may be interpreted as a possible transition to quiescence.

We performed preliminary spectral and timing analysis of the source before the visibility gap. The broadband X-ray spectrum (0.5-10 keV) clearly shows a hard power-law and soft X-ray component. Fitting the data with an absorbed power-law and a multi-temperature blackbody, we obtained a spectral index of Γ ~2.0, black body temperature of ~0.12 keV, and neutral gas column density of ~1.6 10²¹ cm^-2. Residuals between 6 and 7 keV suggest the presence of a broad iron line.

The power spectrum of the source in the 0.5-10 keV energy band is characterized by band limited noise with significant excess power at ~18 Hz. The shape of the Poisson noise-subtracted power spectrum can be well approximated as the sum of a broad and a narrower Lorentzian. The former has a characteristic frequency of ~2 Hz and coherence Q ~0.2, the latter has a characteristic frequency of ~18 Hz and coherence Q ~1.5. The narrow component has a fractional rms of ~6% in the 0.5-10 keV energy band and may be precursor of a true QPO.

The integrated 0.01-80 Hz fractional rms in the 0.5-10 keV energy band is ~23%. This value is almost the same below 2 keV, but it doubles above 5 keV. The main reason for this difference is power excess around ~7 Hz and ~18 Hz.

Based on results from our preliminary analysis, the nature of the transient (accreting black hole or neutron star) remains uncertain. However, we note that our fits are part of an exploratory analysis, therefore the reported values of best-fit parameters represent only an indicative description of the spectral and timing properties of the source.

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.