Detection of highly ionized iron during a giant flare of GX 301-2

ATel #8870; Felix Fuerst (Caltech), Ingo Kreykenbohm (Remeis/FAU/ECAP), Peter Kretschmar (ESAC), Ralf Ballhausen (Remeis/FAU/ECAP), Katja Pottschmidt (CRESST/GSFC/UMBC)
on 25 Mar 2016; 21:01 UT
Credential Certification: Felix Fuerst (ff@caltech.edu)

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GX 301-2 is a well known accreting X-ray pulsar on an eccentric orbit (P=41.5d, e=0.47; Koh et al. 1997, ApJ 479, 933) around its extremely massive companion star (M=43 Msun), showing regular outbursts centered about 1.4d before periastron passage. This pre-periastron flare is thought to be due to the neutron star intercepting a gas stream flowing from the optical companion to the neutron star (Leahy & Kostka 2008, MNRAS 384, 747).

On March 24th, Swift/BAT triggered on an extremely bright outburst of GX 301-2 as reported in ATel #8860 and GCN #19221. While the outburst coincides with the typical pre-periastron flare, it is about a factor of 4 brighter than average flares in the 15-50keV band of BAT.

We analyzed the XRT data resulting from the BAT trigger and find very large variability in count-rate and hardness over the 1200s duration of the observation. The observation covers about 2 cycles of the 685s pulse period, however, the pulse profile is not obvious in the XRT light curve.

The X-ray flux varies drastically during the observation. Between ~400s-900s after the start of the observation the source was found in a lower flux state, with (6.3 +/- 0.2)e-9 erg/s/cm^2. During the rest of the observation the observed flux was a factor of ~2.5 higher. During the lower flux state, the hardness (between the soft 2-5keV and hard 5-10keV band) was larger by about a factor of 5 compared to the high flux state.

We extracted spectra separately for the high and low state and find remarkable variation, in particular in the iron line band. While in the lower, harder state the iron line complex can be fitted with two narrow lines centered at 6.387+/-0.018 (neutral K-alpha) and 6.97+-0.06 (likely a blend of H-like and K-beta), this does not describe the line profile in the softer, brighter state. Instead a third line is needed, centered at 6.76+/-0.04, i.e., close to He-like iron.

GX 301-2 is known for its very prominent iron line, however, during typical pre-periastron flares no higher ionization states of iron then XIV have been found (Fuerst et al., 2011, A&A 535, A9).

We kept the photon fluxes of the neutral and H-like iron line fixed between the two spectra, resulting in an equivalent width of ~550eV in the lower and ~225eV in higher state for the neutral line. These equivalent widths are somewhat lower but close to typically found values (Fuerst et al., 2011).

The source is also known for its very large absorption column on the order of 1e24/cm^2 (e.g., Kreykenbohm et al. 2004, A&A 427, 975). In the XRT observation we find N_H=(28.2+/-1.6)e22/cm^2 in the lower state and N_H=(19.4+/-0.8)e22/cm^2 in the higher state, assuming the continuum (a power law with photon-index Gamma=0.19+/-0.06) did not change. These columns are significantly lower than typical values during the pre-periastron flare.

The decreased absorption and higher ionization species indicate that large parts of the material surrounding the X-ray source have been ionized by the very intense X-ray radiation. The immediate increase of the column and reduction of the ionization state with a drop in flux during the middle part of the observation indicate that the iron line is produced very close to the X-ray source.

As this behavior of GX 301-2 is completely new and has never been studied before, we encourage multi-wavelength follow-up observations. Swift/XRT will continue to monitor the source for the next 5 days (PI Kreykenbohm). Optical studies of the wind of the companion, Wray 977, are in particular encouraged.