Further NICER observations of V4641 Sgr: spectral turn-over and iron emission lines

ATel #14992; Douglas J. K. Buisson (Southampton), Joey Neilsen (Villanova), Sara Motta (INAF-OAB), Federico Vincentelli (Villanova), Tomaso Belloni (INAF-OAB), Erik Kuulkers (ESA/ESTEC), Andrea Sanna (University of Cagliari), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC)
on 21 Oct 2021; 16:57 UT
Credential Certification: Douglas Buisson (d.j.k.buisson@soton.ac.uk)

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NICER has been monitoring the new outburst of V4641 Sgr following its detection by MAXI (ATel #14968) and confirmation with INTEGRAL, Swift, and NICER (ATel #14971). At the time of writing, 3 snapshots are available, from Oct 13,14,16, with effective exposures totaling 1500 s. The three spectra have similar shapes, though the overall brightness appears to be slightly lower on the 14th and slightly higher on the 16th.

In each spectrum, there is a strong roll-over at high energy relative to a simple power law. Fitting with a phenomenological cut-off power law gives a very hard index (~0.75) and low cut-off (~2.3 keV). A diskbb gives a similar quality of fit (chi^2_red=1.58 rather than 1.41 for 572/571 d.o.f. over 0.7-10 keV; both models give good continuum fits above 2keV), with T_in~1.3 keV, although the normalisation (~10) corresponds to a much lower luminosity than expected for a disc around a 6M_Sun black hole. For each of these models, the fitted nH~2*10^21 cm^-2. The observed 2-10 keV flux changes from 3.8*10^-10 erg s^-1 cm^-2 on Oct 13 to 3.5*10^-10 erg s^-1 cm^-2 on Oct 14 to 4.7*10^-10 erg s^-1 cm^-2 on Oct 16. During the more recent observations diskbb temperatures are slightly higher; fitting the diskbb model to the spectrum at energies >2.5 keV gives 1.27-0.04+0.03, 1.28-0.06+0.07, 1.37-0.03+0.03 keV, respectively.

There are also emission features in the X-ray spectrum, with the most prominent lines located in the iron K band. Stacking the data shows a double line shape, with peaks around 6.7 and 6.97 keV, corresponding to the alpha lines of Fe XXV and Fe XXVI. Modelling these with Gaussian emission lines gives XSPEC Norm*10^4=3.7+-0.8 (5.2+-1.0) and Sigma<0.09 (0.07) for Fe XXV (Fe XXVI). There is also a slight soft excess around 1 keV in the spectrum; similar features were observed in neutron star systems during states with very hard power-law illumination (Her X-1: Jimenez-Garate et al. 2002, ApJ, 578, 391; LMC X-4: Neilsen et al. 2009, ApJ, 696, 182; Ballantyne et al. 2012, ApJ, 747, L35).

Interestingly, despite variations in the underlying continuum, we do not find evidence of statistically significant changes in the emission lines. This may reflect the S/N in our brief snapshots or it may indicate evolution of the geometry or ionization of the emission line region.

Further NICER observations are planned; we encourage multi-wavelength follow-up.

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.

NICER spectra of V4641 Sgr