Discovery of a new extragalactic population of energetic particles

We report the discovery of a statistically significant hardening in the Fermi-LAT $\gamma$-ray spectrum of Centaurus A’s (Cen A) core, with the spectral index hardening from ${\mathrm{\Gamma }}_1=2.73\pm 0.02$ to ${\mathrm{\Gamma }}_1=2.29\pm 0.07$ at a break energy of $(2.6\pm 0.3)\mathrm{GeV}$. Using a likelihood analysis, we find no evidence for flux variability in Cen A’s core light curve above or below the spectral break when considering the entire 8 year period. Interestingly, however, the first $\sim 3.5\mathrm{years}$ of the low energy light curve shows evidence of flux variability at the $\sim 3.5\sigma$ confidence level. To understand the origin of this spectral break, we assume that the low energy component below the break feature originates from leptons in Centaurus A’s radio jet, and we investigate the possibility that the high energy component above the spectral break is due to an additional source of very high energy particles near the core of Cen A. We show for the first time that the observed $\gamma$-ray spectrum of an active galactic nucleus is compatible with either a very large localized enhancement (referred to as a spike) in the dark matter halo profile or a population of millisecond pulsars. Our work constitutes the first robust indication that new $\gamma$-ray production mechanisms can explain the emission from active galaxies and could provide tantalizing first evidence for the clustering of heavy dark matter particles around black holes.

Figure 1

A $17°\times 17°$ TS map of all 0.1–300 GeV photons that passed the selection criteria, for the entire 8 year data set. The color scale is in units of TS. Three new point sources were found, we note however that two of these apparent point sources are located on the boundary of Cen A’s lobes, shown by the red contour lines.

Figure 2

The 0.1 to 300 GeV spectrum of Cen A’s core, as seen by the Fermi-LAT detector. The broken power-law model, shown in blue, is preferred over the power law model, shown in red, with a significance $>5\sigma$. The grey histogram shows the TS value for each spectral bin. The bins with a $\mathrm{TS}<25$, are replaced with an upper limit at 95% confidence level.

Figure 3

Above: light curve of 0.1–2.6 GeV flux Below: light curve of 2.6–300 GeV flux, binned in six month temporal bins. The red horizontal band shows the 8-year averaged flux for each energy range.

Figure 4

A combined spectrum of our LAT analysis, shown with black circles, and the H.E.S.S. spectrum above $E_{\gamma }=250\mathrm{GeV}$, taken from [10], shown in red. The broken power-law model, shown in blue, is preferred over the power law model, shown in red, with a significance $>5\sigma$. The grey histogram, with the right-hand y-axis, shows the TS value for each spectral bin for the LAT data points. The bin with a $\mathrm{TS}<25$ is replaced with an upper limit at 95% confidence level. Note that the last upper limit is not shown since it overlaps in energy with the H.E.S.S. data points. The broken power law fit allows for a smooth transition with the $\gamma$-ray spectrum reported here and the H.E.S.S. spectrum.

Figure 5

Best fit to the $\gamma$-ray spectrum of Cen A, obtained by assuming a single power law plus the prompt emission from 3 TeV DM particles annihilating into $t\overline{t}$ with a cross section of $1.6\times {10}^{-32}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, and a spike in the DM density profile, with slope ${\gamma }_{\text{sp}}=7/3$.

Figure 6

Best fit to the $\gamma$-ray spectrum of Cen A, obtained by assuming a single power law plus the prompt emission from 3 TeV DM particles annihilating into $b\overline{b}$ with a cross section of $1.4\times {10}^{-32}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, and a spike in the DM density profile, with slope ${\gamma }_{\text{sp}}=7/3$.

Figure 7

Best fit to the $\gamma$-ray spectrum of Cen A, obtained by assuming a single power law plus the prompt emission from 400 GeV DM particles annihilating into ${\tau }^{+}{\tau }^{-}$ with a cross section of $4\times {10}^{-33}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, and a spike in the DM density profile, with slope ${\gamma }_{\text{sp}}=7/3$.

Figure 8

Contribution to the $\gamma$-ray spectrum of Cen A from a single power law plus the prompt emission from 3 TeV DM particles annihilating into $b\overline{b}$ with the canonical cross section of $3\times {10}^{-26}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, for a DM density following the NFW profile. The DM-induced emission is several orders of magnitude below the data.

Figure 9

Best fit for a population of MSPs plus a power law.

Figure 10

Best fit for a power law, a population of MSPs and a DM candidate of 30 TeV plus a spike.