Identifying the Gamma-ray Emission of the Nearby Galaxy M83

Yi Xing Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China; [email protected]; [email protected] Zhongxiang Wang Department of Astronomy, School of Physics and Astronomy, Yunnan University, Kunming 650091, China Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China; [email protected]; [email protected]

Abstract

We report on the detection of a $\gamma$ -ray source at the position of the nearby star-forming galaxy (SFG) M83, which is found from our analysis of 14 years of the data obtained with the Large Area Telescope (LAT) on-board Fermi Gamma-ray Space Telescope (Fermi). The source is weakly detected, with a significance of $\sim 5\sigma$ , and its emission can be described with an exponentially cutoff power law. At a distance of 4.61 Mpc, the source’s $\gamma$ -ray luminosity is $\sim 1.4\times 10^{39}$ erg s^-1, roughly along the correlation line between the $\gamma$ -ray and IR luminosities determined for nearby SFGs. Because of the weak detection, the source spectrum can not be used for checking its similarity with those of other SFGs. Given the positional matches and the empirical expectation for $\gamma$ -ray emission from M83 due to the galaxy’s star-forming activity, we conclude that the $\gamma$ -ray source is the likely counterpart to m83. The detection thus adds another member to the group of approximately a dozen SFGs, whose $\gamma$ -ray emissions mostly have a cosmic-ray origin.

Gamma-ray sources (633); Starburst galaxies (1570)

1 Introduction

Among more than 6000 $\gamma$ -ray sources detected with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) in all sky, the dominant class is active galactic nuclei (AGN; Abdollahi et al. 2022), whose high-energy emission is mostly radiated from their jets. Non-active galaxies thus do not show such emission. However there are approximately a dozen of the galaxies, either within the local group or nearby, have been detected at $\gamma$ -rays (Ajello et al., 2020; Xi et al., 2020). While there are complications in the production of the $\gamma$ -ray emissions observed from these galaxies, for example AGN possibly hiding in some of them (Peng et al., 2019) and the $\gamma$ -ray emission of the local-group galaxy M31 being considered consisting of different components (Li et al., 2016; Pshirkov et al., 2016; Ackermann et al., 2017; Karwin et al., 2019; Zimmer et al., 2022; Xing et al., 2023), $\gamma$ -ray luminosities of most of them well correlate with infrared (IR) or radio 1.4 GHz luminosities (Abdo et al., 2010b; Ackermann et al., 2012; Ajello et al., 2020; Xi et al., 2020). This correlation is considered as an indicator for the cosmic ray (CR) origin of the gamma-ray emissions. Supernova remnants (SNRs) produce CRs as their shock fronts serve as particle accelerators (e.g., Bykov et al. 2018), and the accelerated particles emit at radio frequencies through the synchrotron process and at high energies through the proton-proton collisions and/or leptonic processes (i.e., bremsstrahlung or inverse Compton scattering; e.g., Dermer 1986). On the other hand, SNRs are the results of massive stars ( $M\gtrsim 8\,M_{\sun}$ ) evolving to the end of their lives on relatively short, $\sim 10^{7}$ yr timescales, and their densities are thus closely related to star-formation of a galaxy. Given that the IR luminosities are an indicator of star-formation rates of a galaxy, a correlation between them and corresponding $\gamma$ -ray luminosities is naturally expected for star-forming galaxies (see, e.g., Domingo-Santamaría & Torres 2005; Lacki et al. 2010; Ackermann et al. 2012, and references therein).

Along this expected correlation, efforts have been made to detect nearby star-forming galaxies at $\gamma$ -rays. Thus far, approximately a dozen of them have been reported with the detection (see, e.g., Ajello et al. 2020; Xi et al. 2020, and references therein). Here we report on the likely detection of another one, the nearby galaxy M83 (also known as NGC 5236 or the Southern Pinwheel).

Refer to caption — Figure 1: TS maps of a $3\arcdeg\times 3\arcdeg$ region centered at M83 in the energy ranges of 0.1–500 GeV (left panel) and 0.5–500 GeV (middle panel). The only 4FGL-DR3 catalog source in the region, a blazar candidate (marked by green pluses), is removed in the maps. The green crosses mark the position of M83, which is within the 2 $\sigma$ error circle (marked by the green circles) determined for the residual excess emission at the location. There is also weak excess emission (whose 2 $\sigma$ error circle is marked by green dashed circles) NE to the center. Right panel: the same as the middle panel, but the NE source is removed as a point source. For each TS map, the image scale is 0 $\fdg$ 05 pixel^-1, and the color bar indicates the TS value range.

M83 is often referred to as a grand-design spiral galaxy. It is nearly face-on to us ( $i\sim 25\arcdeg$ , Sofue et al. 1999), at a distance of 4.61 Mpc (Saha et al., 2006). There have been extensively studies of the galaxy over the whole wavelength range. Its star formation is relatively active, having a total star-formation rate of 5 $M_{\sun}$ yr^-1 (Kennicutt, 1998). Thus it was listed in the star-forming galaxy sample selected by Ackermann et al. (2012), to be searched for CR-induced $\gamma$ -ray emission. A flux upper limit was reported in Ackermann et al. (2012), while only 3 years of the Fermi-LAT data were used.

Now with 14 years of the data having been collected with Fermi-LAT, we conducted a search for M83’s $\gamma$ -ray emission. We found a likely counterpart and report the results. Below we describe the details of our analysis and provide the corresponding results in Section 2. The results are discussed in Section 3.

2 Analysis and Results

2.1 Fermi-LAT Data and Source Model

We selected 0.1–500 GeV LAT events from the updated Fermi Pass 8 database in a region of interest (RoI) that has a size of 20^∘ $\times$ 20^∘ and the center at the central position of M83. Since the galaxy appears to have a size $\sim 12\arcmin$ in the sky, which is not resolvable in the LAT data due to its large point spread function (PSF), we treat M83 as a point source through this paper. The time period of the LAT data was from 2008-08-04 15:43:39 (UTC) to 2022-09-26 23:16:35 (UTC), slightly more than 14 yrs. The CLEAN event class was used. We included the events with zenith angles less than 90 deg and excluded the events with quality flags of ‘bad’. Both these are recommended by the LAT team¹¹1http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/.

We constructed the source model by including all sources within 20 deg from M83. The positions and the spectral parameters of these sources are provided in the Fermi LAT 12-year source catalog (4FGL-DR3; Abdollahi et al. 2022). We set the spectral parameters of the sources within 5 deg from M83 free, and froze the other parameters at their catalog values. The spectral model gll_iem_v07.fit was used for the Galactic diffuse emission, and the spectral file iso_P8R3_CLEAN_V3_v1.txt for the extragalactic diffuse emission. The normalizations of these two diffuse components were set free in the following analyses.

Table 1: Likelihood analysis results with the PL, PLEC, and LP models

Model	Best-fit parameters	$\log(L)$	TS
PL	$\Gamma=2.0\pm 0.2$	265522.5	14
PLEC	$\Gamma=0.00\pm 0.03$	265526.9	22
	$E_{c}=2.8\pm 0.8$
LP	$\alpha=0.00\pm 0.06$	265526.1	20
	$\beta=0.67\pm 0.14$

2.2 Likelihood Analysis

We performed the standard binned likelihood analysis to the whole data in 0.1–500 GeV and updated the parameter values for the sources within 5 deg from M83. With the obtained results, we calculated a 0.1–500 GeV residual Test Statistic (TS) map of a $\mathrm{3^{o}\times 3^{o}}$ region centered at M83. The field is rather clean, with only one catalog source (4FGL J1335.3 $-$ 2949; see Section 2.3) in the region. This source was included in the source model and removed in the TS map, which is shown in the left panel of Figure 1. As can be seen, there is weak excess emission at the position of M83. The maximum TS value is approximately 14, corresponding to a $\sim$ 3.7 $\sigma$ detection significance. Because in the analysis below (Section 2.4 and Table 2) we have found that the low energy range for significant detection is $\gtrsim$ 0.5 GeV, we also calculated the 0.5–500 GeV residual TS map (shown in the middle panel of Figure 1). The maximum TS value of the excess emission at M83 is improved to be $\sim$ 25, now at a $\sim$ 5 $\sigma$ detection significance. We ran gtfindsrc in the Fermitools to the 0.5–500 GeV data to determine the position, and obtained R.A.=204 $\fdg$ 23, Decl.= $-$ 29 $\fdg$ 83 (equinox J2000.0), with a 1 $\sigma$ nominal uncertainty of 0 $\fdg$ 04. M83 is 0 $\fdg$ 04 away from this position and thus within the 1 $\sigma$ error circle.

We then added this new source, the possible counterpart to M83, in the source model as a point source and repeated the likelihood analysis in 0.1–500 GeV. Given the low TS value of the source, we considered three models to fit its emission. One is a simple power law (PL), $dN/dE=N_{0}E^{-\Gamma}$ , where $\Gamma$ is the photon index, and the other two are a PL with an exponential cutoff (PLEC), $dN/dE=N_{0}E^{-\Gamma}\exp(-E/E_{c})$ , where $E_{c}$ is the cutoff energy, and a Log-Parabola (LP) function, $dN/dE=N_{0}(E/E_{b})^{-[\alpha+\beta\log(E/E_{b})]}$ , where $E_{b}$ is a scale parameter and was fixed at 1 GeV in our analysis. The results obtained with the three models are given in Table 1. The PLEC model provided the largest TS value, $\simeq$ 22. By comparing the log-likelihood values, that is $\sqrt{-2log(L_{i}/L_{j})}$ , where $L_{i/j}$ are the maximum likelihood values from model $i$ and $j$ , the PLEC and LP models are found to be more favored than the PL at $\sim$ 3.0 $\sigma$ and $\sim$ 2.7 $\sigma$ significances respectively. Although $\gamma$ -ray emissions of most star-forming galaxies can be well described with a PL model (or a LP model; e.g., Ajello et al. 2020), below we adopted the PLEC model because of the largest TS value resulting from it. The corresponding 0.1–500 GeV photon flux for the source was $F_{0.1-500}\simeq 1.2\pm 0.4\times 10^{-10}$ photon cm^-2 s^-1.

2.3 Analysis for Nearby Sources

Since the emission at the position of M83 was weak, we conducted extra-checks to ensure the detection. First as shown in the TS maps (Figure 1), another excess emission is present, which is north-east (NE) to M83. Although it does not appear like a point source, we ran gtfindsrc to the 0.5–500 GeV data, and obtained a position of R.A.=205 $\fdg$ 3, Decl.= $-$ 28 $\fdg$ 9 (equinox J2000.0), with a 1 $\sigma$ nominal uncertainty of 0 $\fdg$ 2. This position is 1.3 deg away from M83, nearly outside of the 68% containment angle of the LAT PSF in $>$ 0.5 GeV band²²2https://www.slac.stanford.edu/exp/glast/groups/canda/lat_Performance.htm. We added this source in the source model and performed the likelihood analysis. The source could be totally removed in the TS map (shown in the right panel of Figure 1), and the results for the emission at M83 were nearly the same as above. These suggest that the NE excess emission did not affect our analysis results for the source at M83.

Second, the catalog source, 4FGL J1335.3 $-$ 2949, is located very close to M83 (see Figure 1). It has an angular separation of 0 $\fdg$ 35 from M83 and its 1 $\sigma$ positional uncertainty is 0 $\fdg$ 02, given in 4FGL-DR3 (Abdollahi et al., 2022). Thus this source and M83’s source are outside of the error circle of each other. Because it is relatively bright, it could contaminate the detection of the source at M83. We calculated the 0.5–500 GeV TS map with the sources (including the NE one) in the $\mathrm{3^{o}\times 3^{o}}$ region kept. 4FGL J1335.3 $-$ 2949 is clearly seen as the brightest one (the TS value was $\sim$ 206) in the field (left panel of Figure 2).

This source is a blazar candidate but did not show significant $\gamma$ -ray variations in 12 years of the Fermi-LAT data (Abdollahi et al., 2022). We extracted its 0.1–500 GeV light curve by setting 60-day time bins and performing the likelihood analysis to each time-bin data. In the extraction, only the normalization parameters of the sources within 5 deg from M83 were set free. As a check, we also extracted a light curve of the source at M83 with the same setup. The resulting light curves and TS curves for the two sources are shown in Figure 3, for which only flux data points with TS $\geq$ 4 were kept in the light curves. As can be seen, this blazar candidate had a high TS value in the beginning of the data at $\sim$ MJD 55000 and likely showed a flaring event after MJD 59000 (note that the latter part was not covered in the LAT 12-yr source catalog). The source at M83 did not show any obvious variations.

To reduce any contamination possibly caused by the variations of 4FGL J1335.3 $-$ 2949, we selected the LAT data during the time period of MJD 55042–59000 (marked as dotted lines in Figure 3). The 0.5–500 GeV TS map of this time period was calculated and is shown in the right panel of Figure 2. The sources in the TS-map region were kept. The TS value for the blazar candidate is reduced to $\sim$ 100, and the source at M83, appearing as an extension of the former to the east direction, is revealed. Thus the flaring activity of the blazar candidate could affect our analysis and weaken the visibility of the source at M83, but when the flares were filtered out, the detection of this source was proved to be true.

2.4 Spectral Analysis

We extracted the $\gamma$ -ray spectrum of the source at M83 by performing maximum likelihood analysis to the LAT data in 10 evenly divided energy bands in logarithm from 0.1–500 GeV. In the extraction, the spectral normalizations of the sources within 5 deg from M83 were set as free parameters, while all the other parameters of the sources were fixed at the values obtained from the above maximum likelihood analysis. The emission was set to be a PL with $\Gamma$ fixed to 2. For the result, we kept only spectral data points when TS $\geq 4$ ( $\geq$ 2 $\sigma$ significance) and derived 95% flux upper limits otherwise, where for the latter a Bayesian approch implemented in the Python tool IntegralUpperLimit (provided in the Fermi Science tools) was used. The obtained spectrum is plotted as black points in Figure 4, and the flux and TS values of the spectral data points are provided in Table 2. In the energy band of 0.2–0.5 GeV, the TS value is 5, but the flux has a large uncertainty, 0.42 $\pm$ 0.62 $\times$ 10^-12 erg cm^-2 s^-1, likely caused by the contamination of the nearby blazar candidate (because of large containment angles of the LAT PSF at the low energies; see also Figure 4). Thus for this data point, we report an upper limit instead. In addition, the spectrum of the nearby blazar candidate was obtained with the same setup, and is also shown in Figure 4 for comparison.

2.5 Variability Analysis

We checked the source at M83 for any long-term variability in 0.1–500 GeV by calculating the variability index TS_var (Nolan et al., 2012). We set 87 time bins with each bin consisting of 60-day data and derived the fluxes or flux upper limits for the source (i.e., shown in Figure 3). If the emission is constant, TS_var would be distributed as a $\chi^{2}$ distribution with 86 degrees of freedom. A variable source would be identified if TS_var is larger than 119.4 (at a 99% confidence level). The computed TS_var for the source is 74.8, lower than the threshold value. Since this source is faint, we also constructed its 2-yr binned light curve (see Figure 3) and checked for variability. For 7 time bins (i.e., 6 degrees of freedom), TS ${}_{var}>16.8$ is required for a variable source. We obtained TS ${}_{var}\simeq 11.3$ . Thus there were no significant long-term variations found for this source.

Table 2: Flux Measurements

Band	$G_{M83}/10^{-12}$	TS
(GeV)	(erg cm^-2 s^-1)
0.15 (0.1–0.2)	0.61	0
0.36 (0.2–0.5)	0.74	5
0.84 (0.5–1.3)	0.34 $\pm$ 0.19	7
1.97 (1.3–3.0)	0.20 $\pm$ 0.10	6
4.62 (3.0–7.1)	0.17 $\pm$ 0.09	5
10.83 (7.1–16.6)	0.31 $\pm$ 0.14	13
25.37 (16.6–38.8)	0.31	0
59.46 (38.8–91.0)	0.89	0
139.36 (91.0–213.3)	1.58	0
326.60 (213.3–500.0)	3.69	0

Note: Fluxes without uncertainties are the 95 $\%$ upper limits.

3 Discussion

By analyzing the Fermi-LAT data for the M83 region, we have found a faint $\gamma$ -ray source at the position of M83. The source’s emission is preferably described with a curved-function model (a PLEC or a LP). Although the detection significance for the source is low, only $\sim 4.7\sigma$ , and a nearby blazar candidate complicates the detection (particularly at the $\lesssim$ 0.5 GeV low energies), our detailed analysis has ensured the existence of the source. Given the high positional coincidence between the $\gamma$ -ray source and M83 and the expectation for $\gamma$ -ray emission from this star-forming galaxy, we conclude that this $\gamma$ -ray source is the likely counterpart to M83.

At a distance of 4.61 Mpc (Saha et al., 2006), the $\gamma$ -ray luminosity (in 0.1–500 GeV) of the source is $\sim 1.4\pm 0.5\times 10^{39}$ erg s^-1, higher than that of the Milky way (Ackermann et al., 2012) and lower than that of NGC 253 (a spiral galaxy with a central starburst region; Abdo et al. 2010a; Ajello et al. 2020). The 2–1000 $\mu$ m IR luminosity of M83 is approximately $8.7\times 10^{43}$ erg s^-1 (Ackermann et al. 2012; note that a source distance of 3.7 Mpc was used in Ackermann et al. 2012). If we use the parameters obtained in Ajello et al. (2020) for the $\gamma$ -ray–IR luminosity correlation, the predicted $\gamma$ -ray luminosity for M83 would be $\sim 7.9\times 10^{39}$ erg s^-1. Further considering a dispersion of 0.3 ( $\gamma$ -ray–luminosity residuals with respect to the correlation line in log space; Ackermann et al. 2012; Ajello et al. 2020), the luminosity range would be $\sim 4.0$ – $16\times 10^{39}$ erg s^-1. Thus the observed luminosity is slightly below the correlation line but can be considered to be consistent with it, given significant uncertainties such as on the distance and IR properties (Ajello et al. 2020 and references therein).

Because of the weak detection of M83, its $\gamma$ -ray spectrum only contains 4 data points (Figure 4). The model fits we obtained in Section 2.2, either the PLEC or the LP, appear to be highly curved (for example, $\beta\approx 0.67$ in the LP model), which are different from those of the other star-forming galaxies. The latter are mostly described with a PL without significant curvature required (e.g., Ajello et al. 2020). The difference could be due to the weak detection causing the emission property not well determined, and remains to be resolved when more LAT data for M83 are collected. Ajello et al. (2020) have normalized the $\gamma$ -ray spectra of 9 star-forming galaxies by simply scaling the spectra to a common value at 1 GeV, and obtained a best-fit model that is in the form of a smoothly broken power law. In Figure 4, we show this best fit by aligning it with the first flux measurement of M83’s spectrum, since the energy range of the data point is 0.5–1.3 GeV, approximately at 1 GeV. As can be seen, the spectrum of M83 is approximately consistent with the best fit. Given this and the $\gamma$ -ray luminosity indicated above being approximately in the right range, the $\gamma$ -ray emission of M83 likely has the same origin as the other nearby galaxies, arising from CRs and related to the star-formation activity (Ackermann et al., 2012). Thus M83 is likely another member of this gourp of the $\gamma$ -ray–emitting, star-forming galaxies. Hopefully with more LAT data collected in the future, both the detection significance and the quality of the $\gamma$ -ray spectrum will be improved, helping provide more information for the high-energy properties of M83.

This research is supported by the Original Innovation Program of the Chinese Academy of Sciences (E085021002), the Basic Research Program of Yunnan Province No. 202201AS070005, and the National Natural Science Foundation of China No. 12273033.

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