Classical novae are cataclysmic binary star systems in which the matter of a companion star is accreted on a white dwarf1,2. Accumulation of hydrogen in a layer eventually causes a thermonuclear explosion on the surface of the white dwarf3, brightening the white dwarf to ~105 solar luminosities and triggering ejection of the accumulated matter. Novae provide the extreme conditions required to accelerate particles, electrons or protons, to high energies. Here we present the detection of gamma rays by the MAGIC telescopes from the 2021 outburst of RS Ophiuchi, a recurrent nova with a red giant companion, which allowed us to accurately characterize the emission from a nova in the 60 GeV to 250 GeV energy range. The theoretical interpretation of the combined Fermi LAT and MAGIC data suggests that protons are accelerated to hundreds of gigaelectronvolts in the nova shock. Such protons should create bubbles of enhanced cosmic ray density, of the order of 10 pc, from the recurrent novae.
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Analysis products of MAGIC data are available at http://vobs.magic.pic.es/fits/. Low-level data are available on request.
The code for fitting the electron and proton models is available at https://opendata.magic.pic.es/download?pid=2.
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We thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF, MPG and HGF; the Italian INFN and INAF; the Swiss National Fund SNF; the ERDF under the Spanish Ministerio de Ciencia e Innovación (MICINN) (PID2019-104114RB-C31, PID2019-104114RB-C32, PID2019-104114RB-C33, PID2019-105510GB-C31,PID2019-107847RB-C41, PID2019-107847RB-C42, PID2019-107847RB-C44, PID2019-107988GB-C22); the Indian Department of Atomic Energy; the Japanese ICRR, the University of Tokyo, JSPS and MEXT; the Bulgarian Ministry of Education and Science, National RI Roadmap Project DO1-400/18.12.2020, and the Academy of Finland grant number 320045 is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia Severo Ochoa (SEV-2016-0588, SEV-2017-0709, CEX2019-000920-S), the Unidad de Excelencia María de Maeztu (CEX2019-000918-M, MDM-2015-0509-18-2) and the CERCA programme of the Generalitat de Catalunya; by the Croatian Science Foundation (HrZZ) project IP-2016-06-9782 and the University of Rijeka Project uniri-prirod-18-48; by the DFG Collaborative Research Centres SFB823/C4 and SFB876/C3; by the Polish National Research Centre grant UMO-2016/22/M/ST9/00382 and by the Brazilian MCTIC, CNPq and FAPERJ. The TJO of the Montsec Observatory (OdM) is owned by the Catalan Government and operated by the Institute for Space Studies of Catalonia (IEEC). We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We gratefully acknowledge the prompt response to the alert and the data provided by the CAOS Team. We acknowledge with thanks the ARAS database27 (https://aras-database.github.io/database/index.html). The observers who contributed worldwide data used in this research are O. Garde, V. Lecoq, L. Franco, F. Teyssier, O. Thizy, C. Boussin, P. A. Dubovsky and D. Boyd. We thank G. Principe for the advice in extending the Fermi LAT analysis below 100 MeV and F. D’Ammando for his comments on the manuscript. R.L.-C.’s work was financially supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement 754496—FELLINI.
The authors declare no competing interests.
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Extended Data Fig. 1 Optical B-band observed magnitude and the color index of RS Oph 2021 outburst from ANS and TJO compared to that of 2006 eruption.
Optical B-band observed magnitude and the color index of RS Oph 2021 outburst from ANS (red circles) and TJO (green triangle) compared to that of 2006 eruption (black crosses, computed with respect to MJD of 53775.86, ). Top three panels show the color indices, while the bottom panel shows B magnitude evolution.
Extended Data Fig. 2 Distribution of the squared angular distance between the nominal source position and the reconstructed arrival direction of events and the estimated background.
Distribution of the squared angular distance between the nominal source position and the reconstructed arrival direction of events (black crosses) and the estimated background (gray shaded area). Vertical dashed line represents the angular cut below which the number of background and excess events as well as the statistical significance of the detection are given (inset panel). Error bars represent 1-sigma statistical uncertainties in the data points.
Extended Data Fig. 3 Modeling of daily emission in proton model and electron model for first, second, third and fourth day after the nova eruption.
Modeling of daily emission in proton model (left panels) and electron model (right panels) for first, second, third and fourth day after the nova eruption (from top to bottom). The dashed line shows the gamma rays from the π0 decay and the dotted line shows the inverse Compton contribution of the secondary e ± pairs produced in hadronic interactions. dN/dE p and dN/dEe report the shape of the proton and electron energy distributions obtained from the fit. The bottom panel shows the fit residuals. Error bars represent 1-sigma statistical uncertainties in the data points.
Extended Data Fig. 4 Comparison of the photon flux measured by Fermi LAT above 100 MeV with the one measured by MAGIC above 100 GeV and with that of the V -band obtained by ANS.
Comparison of the photon flux measured by Fermi LAT above 100 MeV with the one measured by MAGIC above 100 GeV (left panel) and with that of the V band obtained by ANS (right panel). Arrows show the sequence of the flux temporal evolution and the blue line shows the linear proportionality fit in both panels. Error bars represent 1-sigma statistical uncertainties in the data points.
Cooling and acceleration time scale for protons (left panels) and electrons (right panels) for two values of ξB parameter: 10−7 G (top panels) and 3 × 10−6 G (bottom panels). Assumed parameters (see text for details): vsh = 4500 km s−1, t = 3 d, Rph = 200 R, Tph = 8460 K, np = 6 × 108 cm−3.
Optical photometry performed by ANS 1, 3, and 4 days (see the panel titles) after the outburst (blue empty markers) corrected for the Galactic absorption. Filled markers show the flux after subtraction of Hα and Hß line contributions. The thick black lines show a black-body emission used in the modeling, while the dashed line shows for comparison the average 2006 spectral fit from  (with the photosphere radius corrected to the distance of 2.45 kpc). Horizontal error bars represent the bandwidth of the filters used.
Extended Data Fig. 7 Example of H α, H β and He I λ5876 P Cygni profiles used to determine the behavior of the expansion velocity of the expanding envelope.
Example of H α, H β and He I 5876λ P Cygni profiles used to determine the behavior of the expansion velocity of the expanding envelope time after the outburst is given in the top right part of each panel). The bottom right panel shows the evolution of the velocity in time. Error bars represent 1-sigma statistical uncertainties in the data points.
Extended Data Fig. 8 Absorption of the gamma-ray emission on the radiation field of the photosphere and collision with it.
Assumed parameters: vsh = 4500 km s−1, Rph = 200 R. Temperature of the photosphere is Tph = 10780 K, 9490 K, 8460 K and 7680 K for the time after the nova onset: 1 d (black solid), 2 d (red dotted), 3 d (green dashed), 4 d (blue dot-dashed) respectively.
Extended Data Fig. 9 The maximum energy of protons obtained from the theoretical model fits to the daily gamma-ray emission.
The maximum energy of protons obtained from the theoretical model fits to the daily gamma-ray emission (points) shown in Fig. 3. Red and green line show, respectively, the scenario of proportional increase and constant value of maximum energy. Error bars represent 1-sigma statistical uncertainties in the determination of the maximum energy of protons.
Spectra of other Fermi-LAT-detected novae are shown in the top panel. Gamma-ray spectra of V407 Cyg (middle panel) and V339 Del (bottom panel) compared to the measured (red) and scaled (gray) RS Oph gamma-ray spectra. Blue triangles and arrows correspond to Fermi-LAT measurements and upper limits of V407 Cyg (top) and V339 Del (bottom). Red squares are the Fermi-LAT spectrum of RS Oph and red circles the MAGIC one. Gray squares are the Fermi-LAT scaled spectrum of RS Oph and gray circles the MAGIC one. Cyan arrows correspond to the VERITAS (V407 Cyg) and MAGIC (V339 Del) upper limits. The dashed blue lines correspond to the best-fit using a single power-law for the Fermi-LAT data. The dotted blue lines correspond to the best-fit using a power-law with an exponential cut-off for the Fermi-LAT data. Data taken from13,14,15,16,68. Error bars represent 1-sigma statistical uncertainties in the data points.
MAGIC, Fermi LAT and optical data.
MAGIC and Fermi LAT data.
Total energy versus duration of different novae.
Light curve and colour indices for 2021 and 2006 outbursts.
Number of entries versus distance from the nominal position of the source.
Fermi LAT and MAGIC daily spectra.
Photometry data with and without line contribution removed.
Line profiles at different times.
Spectra of different novae.
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Acciari, V.A., Ansoldi, S., Antonelli, L.A. et al. Proton acceleration in thermonuclear nova explosions revealed by gamma rays. Nat Astron 6, 689–697 (2022). https://doi.org/10.1038/s41550-022-01640-z