# Summary of K2 Program GO11102

Title: Probing the Circumstellar Environment of the Supergiant Fast X-ray Transient IGR J17544-2619 using K2

PI: Boyd, Patricia T. (UMBC and NASA's Goddard Space Flight Center)
CoIs: Corbet, Robin; Torpin, Trevor J; Coley, Joel Barry; Krimm, Hans A; Pottschmidt, Katja

Scientific Goals and Objectives
We request Campaign 11 K2 monitoring to investigate accretion in the short period Supergiant
Fast X-ray Transient (SFXT) IGR J17544-2619, the prototypical SFXT consisting of a neutron star (NS) in a ~4.9 d orbit around an O9 Ib donor star (Drave et al. 2014). The source shows large scale variability ~6 orders of magnitude 10^3110^38 erg s1 on timescales of hours
(Romano et al. 2015). This large dynamic range is suggestive of an accretion inhibition mechanism.
The maximum luminosity found by Romano et al. (2015) using Swift is near Eddington 
for a 1.4Msun NS, in variance with direct Bondi-Hoyle accretion where the
typical luminosity is ~10^3510^36 erg s1 (Chaty 2011). Different mechanisms have
been proposed to explain the mysterious accretion inhibition: the presence of large clumps in the
stellar wind (int Zand 2005; Walter & Zurita Heras 2007), magnetic gating mechanisms involving
NSs with rotation periods on the order of ~1000 s and magnetic fields on the order of
10^14 G (Bozzo et al. 2008), and the presence of a short-lived transient accretion disc (Romano et al.
2015). Of the mechanisms considered, the formation and dissipation of a transient accretion disc
resulting from a low-velocity high-density stellar wind might explain the giant outburst Romano
et al. (2015). While the presence of a cyclotron resonance scattering feature at ~17 keV suggesting
a magnetic field of ~10^12 G rules out a magnetar, the ingestion of a massive clump could also be
consistent with the variability seen in IGR J17544-2619 (Bhalerao et al. 2015).
For a complete understanding of accretion in Supergiant Fast X-ray Transients, comparing
data in different energy regimes is essential. A high precision optical campaign using K2 is
complimentary to long-term X-ray campaigns using INTEGRAL and Swift BAT and pointed X-ray
observations using XMM-Newton and NuSTAR (Drave et al. 2014; Romano et al. 2015; Bhalerao et
al. 2015). While the X-ray data offers useful insights to probe the environment close to the NS, K2 optical data will reveal the mechanisms at work around the donor.

Methodology & Relevance
We request short cadence mode to investigate variability on orbital and sub-orbital timescales using
traditional Fourier analysis techniques and Dynamic Power Spectra. To search for the presence of a
transient accretion disc, we will cross-correlate the K2 optical data with X-ray data from Swift BAT.
We expect the optical data to lag the X-ray data, which could be attributed to reprocessing from the
donor or a possible transient accretion disc (Scaringi et al. 2015). We will monitor the behavior
of outbursts in the optical and X-ray using the discrete cross correlation function. Together,
these techniques could reveal insights on the physical processes that inhibits matter accretion in
prototypical SFXTs. The mechanism leading to the extreme dynamic range in luminosity found in
SFXTs remains mysterious. While different inhibition mechanisms were proposed to explain the
~10^3110^32 erg s1 low luminosity state, the ~10^38 erg s1 outburst found in IGR J17544-2619 is
puzzling.  Our use of Kepler's short cadence mode will investigate the behavior of matter accretion in extreme envirnoments, a key problem addressed in the Physics of the Cosmos.

References
Bhalerao, V., Romano, P., Tomsick, J., et al. 2015, MNRAS, 447, 2274
Bozzo, E., Falanga, M., & Stella, L. 2008, ApJ, 683, 1031
Chaty, S. 2011, Evolution of Compact Binaries, eds. L. Schmidtobreick, M. R. Schreiber, & C. Tappert, ASP Conf. Ser., 447, 29
Drave, S. P., Bird, A. J., Sidoli, L., et al. 2014, MNRAS, 439, 2175
int Zand, J. J. M. 2005, A&A, 441, L1
Romano, P., Bozzo, E., Mangano, V., et al. 2015, A&A, 576, L4
Scaringi, S., Maccarone, T. J., Hynes, R. I., et al. 2015, MNRAS, 451, 3857
Walter, R., & Zurita Heras, J. 2007, A&A, 476, 335

# 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