Evidence for GeV emission of the superluminous supernova SN 2017egm

TL;DR

SN 2017egm, a nearby hydrogen-poor SLSN, is tested for GeV gamma-ray emission powered by a nascent millisecond magnetar. Using 15 years of Fermi-LAT data, the authors detect a transient GeV signal in a few months after explosion, with localization consistent with the SN and a significance that withstands rigorous validation. The observed rise, peak luminosity, and spectral shape agree with magnetar-nebula model predictions, providing evidence for a magnetar central engine in this SLSN. The result demonstrates that GeV emission from SLSNe is detectable and informs future gamma-ray missions and survey strategies, while placing constraints on nebula magnetization $ε_B$.

Abstract

Superluminous supernovae (SLSNe) are a new class of transients with luminosities $\sim10 -100$ times larger than the usual core-collapse supernovae (SNe). Their origin is still unclear and one widely discussed scenario involves a millisecond magnetar central engine. The GeV-TeV emission of SLSNe has been predicted in the literature but has not been convincingly detected yet. Here we report the results of the search for $γ$-ray emission in the direction of SN 2017egm, one of the closest SLSNe detected so far, using 15 years of {\it Fermi}-LAT Pass 8 data. There is a transient $γ$-ray source appearing about 2 months after this event and lasting a few months. Monte Carlo simulations show that the $γ$-ray signal has a global significance of {\it at least} 4$σ$. Both the peak time and the luminosity of the GeV emission are consistent with the magnetar model prediction, suggesting that such a GeV transient is the high-energy counterpart of SN 2017egm and the central engine of this SLSNe is a young magnetar.

Figures (7)

• Figure 1: Light curve of the $\gamma$-ray emission from the direction of SN 2017egm between 500 MeV and 500 GeV in 2-month bins. The zero point is set to the onset of the supernova, 2017 May 23. Shaded regions show the TS values (right axis). For TS values less than 4, 95% confidence level upper limits are presented.
• Figure 2: TS maps for different epochs: (a) from 2017 July 23 to 2017 November 23, (b) from 2008 August 4 to 2017 July 23, and (c) from 2017 November 23 to 2023 August 4. The TS maps display $3^{\circ}\times 3^{\circ}$ region centered at SN 2017egm for the data between 500 MeV and 500 GeV. The optical position of SN 2017egm is represented by the blue cross symbol. The circles in panel (a) are the 68% (inner) and 95% (outer) error uncertainties of the $\gamma$-ray source localization.
• Figure 3: $\rm \sqrt{TS}$ distribution of 20000 random sky simulations. The blue curve is the TS distribution expected from Chernoff's theorem Chernoff1954.
• Figure 4: A comparison between the theoretical prediction (blue dashed line) and the observed $\gamma$-ray light curve (red points, in 2-month bin) of SN 2017egm. The theoretical curve considers a magnetar-powered supernova model Vurm_2021. The nebula magnetization is assumed to be $\varepsilon_{B} = 10^{-4}$. The dashed line shows the spin-down luminosity of the central magnetar.
• Figure S1: Spectral energy distribution of the new $\gamma$-ray source.