The 26th anniversary outburst of jet-driving symbiotic binary MWC 560: results from Chandra, Swift, and optical spectroscopy
Adrian B. Lucy, J. L. Sokoloski (Columbia), U. Munari (INAF Padova), N. P. M. Kuin (UCL MSSL, Swift), M. J. Darnley (LJMU), G. J. M. Luna (IAFE), C. Knigge (Southampton), P. Valisa, A. Milani (ANS)
The symbiotic star MWC 560 = V694 Mon, which is believed to usually drive a jet along the line of sight (e.g., Schmid et al. 2001), is undergoing a sustained outburst (ATel #8653) rivaling its previous brightest outburst of 1990 (Tomov et al. 1990, Leibowitz and Formiggini 2015). We present initial results from a multi-wavelength observational campaign, including evidence for dramatic enhancement in the soft (<2 keV) X-rays.

Chandra observed the system for 24.76 ks on Mar. 8.285 and 24.76 ks on UT Mar. 9.079, detecting 5.1E-3 +/- 0.3E-3 counts/second. Preliminary models require at least two distinct spectral components, which we fit with absorbed, optically thin thermal plasmas (wabs*apec XSPEC models): a strong soft component with kT= 0.15-0.3 keV (1-sigma + systematic uncertainty confidence intervals) absorbed by a hydrogen column nH=0.35-0.8 E22cm^-2 (unabsorbed 0.1-15 keV flux = 2.3-8.9 E-13erg/cm^2/s, unabsorbed 0.1-15 keV luminosity = 1.7-6.7 E32erg/s [d/2.5 kpc]^2), and a hard component (see below for details; unabsorbed flux = 0.6-3.4 E-13erg/cm^2/s, unabsorbed luminosity = 0.45-2.5 E32erg/s [d/2.5 kpc]^2).

A 2007 Sept. 27 XMM spectrum obtained by Stute & Sahai (2009) showed a strong hard X-ray component, likely from the accretion disk boundary layer, and a weak soft X-ray component that they contend was from a jet shock. Our soft component shows a temperature and absorbing column consistent with 2007, but its unabsorbed flux has been enhanced by a factor between 6 and 25.

Meanwhile, the hard X-ray component (fixed at the kT=11.26 keV obtained by Stute & Sahai) has declined to 0.1-0.5 of its 2007 unabsorbed flux. 2.5-3.5 keV photon counts require either a decline in the column absorbing the hard component from nH=22.-35. to 1.4-5.2 E22cm^-2 between 2007 and 2016, or the addition of a kT>2.2 keV intermediate component. Both options are feasible; absorption of boundary layer X-rays is known to vary by large factors in at least two other symbiotic stars (e.g., Nunez et al. 2015), and a shock with velocity > 1300 km/s could yield the intermediate component.

We obtained optical spectra at Asiago (Padova, INAF) with the 1.22m + B&C and 1.82m + Echelle, and at the Liverpool Telescope. On 2016 Mar. 8 and 10, Balmer troughs only reached up to -2100 km/s and were contiguous with their extended emission lines. Velocities exceeding -3000 km/s have not been seen in 2016. In contrast, during the 1990 outburst, detached Balmer absorption lines achieved record velocities up to -6000 km/s (Tomov et al. 1990). An optical spectrum taken at Asiago on 2007 Oct. 30, a month after the Stute & Sahai XMM spectrum, shows Balmer troughs detached from their emission lines and extending from about -850 to -2300 km/s.

Swift UVM2 magnitudes on 2016 Mar. 2-5 varied between 10.65-11.4 on timescales of minutes to days, comparable to V and B band flickering. Swift UV grism data on Mar. 2, 5, and 9 show a 1900-2900A spectrum similar to one obtained by IUE on 1991 Jun. 10, more than a year after the 1990 outburst peak. Preliminary reductions suggest that the Mg II 2800A emission line might be unusually broad and blue relative to both outburst and non-outburst IUE spectra in the 1990s, and >20% variability between our three spectra is seen in both the Mg II line and Fe II (>2600A) emission complexes.

UBVRI photometric and spectroscopic monitoring is continuing at Asiago and by the ANS Collaboration, with Swift taking UV spectra and UVM2 photometry every few days through at least the end of March. We expect the evolution of MWC 560 to remain interesting for at least the next few months.

We thank Belinda Wilkes, Neil Gehrels, and the Chandra/Swift teams for executing these DDT/ToO observations. We acknowledge with thanks the use of optical data from the AAVSO.
optical,ultra-violet,x-ray,binary,cataclysmic variable,star,variables