# Summary of K2 Program GO14057

Title: K2 short-cadence observations of the prototypical transitional millisecond pulsar PSR J1023+0038

PI: Rea, Nanda (Instituto de Ciencias del Espacio (CSIC-IEEC))
CoIs: Maccarone, Thomas Joseph; Scaringi, Simone; Knigge, Christian; Coti Zelati, Francesco; Torres, Diego F.; Israel, GianLuca; Campana, Sergio; Serenelli, Aldo; Papitto, Alessandro; D'Avanzo, Paolo

The short spin periods of binary millisecond pulsars are the outcome of the accretion onto the neutron star (NS) of the mass transferred by a low mass late type companion star, through an accretion disc (Alpar et al. 1982, Nature, 300, 728). After a Gyr-long mass accretion phase during which the binary system shines as a bright low mass X-ray binary (NS-LMXB), the mass transfer rate declines and allows the activation of a, now rapidly spinning, radio/gamma-ray millisecond pulsar (MSP) powered by the rotation of its magnetic field. The tight link existing between radio MSP and NS-LMXB has been recently demonstrated by the discovery of three transitional millisecond pulsars (PSR J1023+0038, Archibald et al. 2009 Science, 324, 1411; IGR J18245-2452 in the globular cluster M28, Papitto et al. 2013 Nature 501, 517; XSS J12270-4859, de Martino et al. 2010 A&A 515, A25, Bassa et al. 2014MNRAS, 441, 1825). These sources have been observed to switch between accretion and rotation-powered emission on timescales possibly shorter than a couple of weeks, showing that such state transitions may take place on time scales compatible with those of the variations of the mass accretion rate onto the NS.  At high mass inflow rates, the radio pulsar is shut-off and the system is bright in the X-rays (LX > 10^36 erg/s). At low mass inflow rates, the magnetosphere expands up to the light cylinder activating the radio pulsar, the disc disappears and the system is faint in the X-rays (LX~10^32 erg/s). Surprisingly, in addition to the X-ray outbursts accreting state and the radio pulsar state, the three known transitional MSPs have been observed into another extremely peculiar subluminous disc state with LX~10^33 erg/s. During these states both PSR J1023+0038 (Archibald et al. 2014) and XSS J1227-4859 (Papitto et al. 2015) showed that part of the disc material is accreted onto the NS surface. These subluminous states are accompained by a sizable gamma-ray flux and a flat-radio spectrum that are typical jets signatures in accreting compact objects, suggesting that large mass outflows could be launched by the fast rotating propellering magnetosphere of these pulsars (Papitto et al. 2014 MNRAS 438, 2105; Papitto & Torres 2015, ApJ 807, 33). Observing these sources is of crucial importance to understand how accretion and ejection are coupled and the role of the magnetic fields in driving outflows and jets. Currently the timescales of the transitions are only loosly constrained to about a few months, and strong fast variability is also observed on seconds/minutes timescales in several bands. Studying the multi-band fast variability of these transitional objects is extremely important because they reflect the timescales of disc formation, or the transition into a jet-dominated outflow or of the disc evaporation, or on a faster timescales the disk/magnetospheric interaction. A transition between the radio pulsar and the subluminous disc state (or even full surface accretion state) is accompanied by a marked variation of the optical emission. In the radio pulsar state the irradiated donor star dominates the optical output. The flux typically attains a magnitude ranging between V~17-19 mag with large amplitude variations along the orbital period (e.g. Bogdanov et al. 2014; de Martino et al. 2014 MNRAS 444, 3004). During the subluminous or accretion disc state the disc dominates the optical emission producing a brightening by 1-2 magnitudes, often with an irregular fast variability from X-rays to optical/nIR (de Martino et al. 2013, A&A 550 A89; de Martino et al. 2014). The prototype of the transitional ms-pulsars, PSR J1023+0038 will be visible by Kepler 2 in Cycle 5 during Campaign 14. K2 observations in short cadence of this source will represent a unique set of data to study the fast optical variability of this object. Our group has already asked for multi-band observations XMM/Swift/Nustar/VLT to partially overlap with the K2 observations.

# Targets requested by this program that have been observed (1)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
248427628, 155.948711, 0.644619, 17.815, GO14093_LC|GO14066_LC|GO14019_LC|GO14057_LC|GO14093_SC|GO14019_SC|GO14057_SC