  After a long low-activity period, a gamma-ray flare from the narrow-line
Seyfert 1 PKS 1502+036 (z=0.4089) was detected by the Large Area Telescope
(LAT) on board Fermi in 2015. On 2015 December 20 the source reached a daily
peak flux, in the 0.1-300 GeV band, of (93 $\pm$ 19) $\times$10$^{-8}$ ph
cm$^{-2}$ s$^{-1}$, attaining a flux of (237 $\pm$ 71) $\times$10$^{-8}$ ph
cm$^{-2}$ s$^{-1}$ on 3-hr time-scales, which corresponds to an isotropic
luminosity of (7.3 $\pm$ 2.1) $\times$10$^{47}$ erg/s. The gamma-ray flare was
not accompanied by significant spectral changes. We report on multi-wavelength
radio-to-gamma-ray observations of PKS 1502+036 during 2008 August-2016 March
by Fermi-LAT, Swift, XMM-Newton, Catalina Real-Time Transient Survey, and the
Owens Valley Radio Observatory (OVRO). An increase in activity was observed on
2015 December 22 by Swift in optical, UV, and X-rays. The OVRO 15 GHz light
curve reached the highest flux density observed from this source on 2016
January 12, indicating a delay of about three weeks between the gamma-ray and
15 GHz emission peaks. This suggests that the gamma-ray emitting region is
located beyond the broad line region. We compared the spectral energy
distribution (SED) of an average activity state with that of the flaring state.
The two SED, with the high-energy bump modelled as an external Compton
component with seed photons from a dust torus, could be fitted by changing the
electron distribution parameters as well as the magnetic field. The fit of the
disc emission during the average state constrains the black hole mass to values
lower than 10$^8$ solar masses. The SED, high-energy emission mechanisms, and
gamma-ray properties of the source resemble those of a flat spectrum radio
quasar.
