# Summary of K2 Program GO11093

Title: Monitoring the archetype of supergiant fast X-ray transients

PI: Posselt, Bettina (Pennsylvania State University)
CoIs: Kargaltsev, Oleg; Pavlov, George

We propose to monitor the light curve of the archetype supergiant fast X-ray transient (SFXT) - IGR J17544-2619 - with Kepler during campaign 11. The rare SFXTs are high mass X-ray binaries (HMXBs) consisting of a neutron star (NS) and an OB supergiant companion. The X-ray activity of SFXTs covers a huge dynamical range of 5 orders of magnitude on time scales of a few hours. This is very unusual since normal supergiant HMXBs display a fairly constant average luminosity with typical variations of a factor of 10-50 on shorter time scales (minutes to about an hour). The SFXT phenomenon has been interpreted as accretion onto the NS from a clumpy wind from the OB donor. However, there are several open questions regarding this interpretation. The prototypical SFXT IGR J17544-2619 (J17544 in the following), for example, exhibited a giant X-ray outburst in 2014, spanning 6 orders of magnitude in luminosity, which is difficult to explain in the clumpy-wind accretion scenario. Romano et al. 2015 proposed instead the formation of a transient accretion disk around the NS. One key to understand the nature of SFXTs is multiwavelength monitoring of these objects.

J17544 is located at 3 kpc and consists of a NS orbiting a V=12.65 mag O9Ib star with an inferred orbital period of 4.926+/-0.001d (Pellizza et al. 2006; Rahoui et al. 2008, Clark et al. 2009). From daily X-ray monitoring observations we know that J17544 shows X-ray activity for about 50% of the time (Romano 2015a). Large outbursts of IGR J17544 are picked up automatically by the Swift Burst Alert Telescope about 3-4 times a year. 
Due to the outburst duration of just a few hours and the difficulty to schedule follow-up observations soon enough, there exist no contemporaneous optical data before, during or right after an X-ray outburst for any SFXT. Yet, simultaneous optical-X-ray observations during an outburst could provide useful insights into the relation between the activity of the donor O star and the accretion-powered X-ray luminosity, as well as on the accretion process itself. Observed optical magnitude changes during an outburst would be even more significant if the binary is well monitored and characterized at these wavelengths immediately before the outburst. Kepler is the ideal instrument to obtain the needed high-accuracy photometric monitoring over a long time. During the Kepler visit of about 80 days, there is a 90% chance that there will be a bright X-ray outburst from J17544 (which Swift would automatically detect and slew to the target for X-ray observations). A Kepler photometric light curve at optical wavelength will allow us to search for eclipses and potentially unusual activity of the O-star before, during, and after the X-ray outburst. The light curve during the quiescent phase is needed to characterize the activity of the O9Ib star and statistically pin down deviations in the wake of outbursts.

# Targets requested by this program that have been observed (1)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
223217668, 268.605289, -26.33127, 11.939, GO11093_LC|GO11113_LC|GO11101_LC|GO11102_LC|GO11127_LC|GO11102_SC