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Appendix A: YY Her and ASASSN-13ax

Two additional objects, the symbiotic nova YY Her and dwarf nova ASASSN-13ax, were observed as part of the broader campaign, but neither was detected to have GeV or TeV emission. In this appendix we report the values of the differential upper limits from those two sources. The Fermi-LAT analysis was configured as described for V339 Del with the ROIs centred on the optical positions for YY Her and ASASSN-13ax. Data were selected within the time windows reported, preceding the optical peak and containing approximately the following two weeks of observations. The region model was derived from the 2FGL catalogue and contains all sources within 20° of the ROI centre. The spectra for YY Her and ASASSN-13ax were described using a power law with the photon index fixed to 2.2. The flux normalization for the source of interest and the normalization for the isotropic template were left free to vary, while the other source parameters were fixed either to catalogue values or refitted values if the source flux was detected in the analysis window and deviated by more than 1σ from the catalogue value. The upper limits were calculated at 95% C.L. using the Bayesian method provided with the Fermi Science Tools⁵.

The upper limits from MAGIC observations were computed in 5 bins per decade in energy. The photon spectral index assumed for both sources was 2.6.

YY Her is a symbiotic nova system that undergoes a recurrent pattern of outbursts. MAGIC observations of YY Her occurred on the night of April 22, seven days after the optical maximum. The observations were motivated by the symbiotic nature of the source. In the absence of detectable emission, flux upper limits at 95% C.L. were calculated to be 5.0 × 10^-12 ph cm^-2 s^-1 above 300 GeV. Emission was not detected in the LAT over the interval 2013 April 10 to April 30 (MJD 56 392.5 to 56 412.5). Upper limits at 95% C.L. were calculated to be 2.8 × 10^-8 ph cm^-2 s^-1 above 100 MeV. Differential upper limits obtained from the Fermi-LAT and MAGIC observations of YY Her are summarized in Table A.1.

ASASSN-13ax is a member of a different class of cataclysmic variables, the dwarf novae, which are known for significantly weaker optical outbursts (typically 2–8 mag) than classical novae. The source was observed by MAGIC because of its very strong optical outburst of 7.7 mag (Stanek et al. 2013). Instead of undergoing a thermonuclear explosion on the surface of the WD, these outbursts are caused by the gravitational energy release from an instability in the accretion disk surrounding the WD. The MAGIC observations were performed on two consecutive nights starting on 2013 July 4, soon after the optical outburst seen on 2013 July 1. In the absence of detectable VHE emission, upper limits at 95% C.L. were calculated to be 1.5 × 10^-12 ph cm^-2 s^-1 above 300 GeV. Emission was not detected in the LAT over the interval 2013 June 25 to July 15

(MJD 56 468.5 to 56 488.5). Upper limits at 95% C.L. were calculated to be 1.6 × 10^-8 ph cm^-2 s^-1 above 100 MeV. Differential upper limits obtained from the Fermi-LAT and MAGIC observations of ASASSN-13ax are summarized in Table A.2

© ESO, 2015