Swift and Chandra Observations of GRS 1915+105 in an unusual, very low-flux state
ATel #12743; J. M. Miller, M. Balakrishnan, M. T. Reynolds, N. Trueba, A. Zoghbi (Univ. of Michigan), J. Kaastra (SRON), T. Kallman (NASA/GSFC), D. Proga (UNLV)
on 10 May 2019; 16:48 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Black Hole, Transient
Referred to by ATel #: 12755, 12761, 12765, 12769, 12770, 12772, 12773, 12788, 12806, 12839, 12848, 12855, 13652
Through years of monitoring with RXTE, the Swift/BAT, and MAXI, GRS 1915+105 has displayed a range of extreme flux variations. However, when it has registered a low flux in hard X-rays, it has always been bright in soft X-rays, and vice-versa. However, recent
weeks and months have seen a continual dimming of the source in all X-ray monitoring bands (also see #12742).
As of 2019-05-08, the BAT count rate is 0.01 cnts/cm^2/s, or about 50 mCrab. Count rates in the soft MAXI bands range between 25-50 mCrab. This is confirmed by several pointed Swift/XRT observations; in ObsID 00034292129 (start time: 2019-05-08 at 20:39:22), we measure a 1-10 keV flux of 4.4 E-10 erg/cm^2/s, or an unabsorbed flux of 5.9 E-10 erg/cm^2/s. For a distance of 8.6 kpc (Reid et al. 2014), this translates to a luminosity of just 5.3 E+36 erg/s. For a 12.4 Msun black hole (Reid et al. 2014), this implies an Eddington fraction of L/L_Edd ~ 0.003.
We were granted a Chandra DDT observation using the HETGS (start time: 2019-04-30). The source flux was approximately 50 mCrab. Like recent Swift/XRT spectra, the continuum measured by Chandra is nominally consistent with an unusually hot, 2-3 keV disk blackbody. Numerous extremely deep absorption lines are detected; the strongest are found in the Fe K band. A range of charge states, not just He-like Fe XXV and H-like Fe XXVI, must contribute to the absorption. There is no evidence of a blue-shift indicative of a disk wind in the time-averaged spectrum (see, e.g., Miller et al. 2016). However, extremely strong, nearly sinusoidal flux variations with a quasi-period of 1250s are seen in the zeroth order and dispersed events. Caution is warranted as this is close to the dither period of Chandra, but the variations are seen in different extraction regions, and phase-selected spectra appear to show significant continuum variations, as well as absorption line flux variations and phase-dependent blue-shifts of a few*100 km/s. This is qualitatively similar to variations seen in prior "heartbeat" oscillations (eg., Zoghbi et al. 2016).
We have also been granted a NuSTAR DDT observation to study the broadband continuum and absorption during this state. The observation was made on 2019-05-05, and we will report results when the data become available.
Last, we have initiated a series of daily Swift/XRT exposures, and we will report on developments from this monitoring program.
Observations in other wavelengths, especially radio and IR, are strongly encouraged.
We thank: Brad Cenko, Jamie Kennea, and the Neil Gehrels Swift Observatory team; Belinda Wilkes, the Chandra Director's Office
including Harvey Tananbaum, and Norbert Schulz; and Fiona Harrison, Karl Forster, and the NuSTAR team.
References:
Miller, J. M., et al., 2016, ApJ, 821, L9
Reid, M., et al., 2014, ApJ, 796, 2
Zoghbi, A., et al., 2016, ApJ, 833, 165