Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT

Abstract

Context. The flux of gamma rays with energies greater than 100 MeV is dominated by diffuse emission coming from cosmic-rays (CRs) illuminating the interstellar medium (ISM) of our Galaxy through the processes of Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The study of this diffuse emission provides insight into the origin and transport of cosmic rays. Aims. We searched for gamma-ray emission from the Small Magellanic Cloud (SMC) in order to derive constraints on the cosmic-ray population and transport in an external system with properties different from the Milky Way. Methods. We analysed the first 17 months of continuous all-sky observations by the Large Area Telescope (LAT) of the Fermi mission to determine the spatial distribution, flux and spectrum of the gamma-ray emission from the SMC. We also used past radio synchrotron observations of the SMC to study the population of CR electrons specifically. Results. We obtained the first detection of the SMC in high-energy gamma rays, with an integrated >100 MeV flux of (3.7 ±0.7) × 10 −8 ph cm −2 s −1 , with additional systematic uncertainty of ≤ 16%. The emission is steady and from an extended source ∼ 3 • in size. It is not clearly correlated with the distribution of massive stars or neutral gas, nor with known pulsars or supernova remnants, but a certain correlation with supergiant shells is observed. Conclusions. The observed flux implies an upper limit on the average CR nuclei density in the SMC of ∼15% of the value measured locally in the Milky Way. The population of high-energy pulsars of the SMC may account for a substantial fraction of the gamma-ray flux, which would make the inferred CR nuclei density even lower. The average density of CR electrons derived from radio synchrotron observations is consistent with the same reduction factor but the uncertainties are large. From our current knowledge of the SMC, such a low CR density does not seem to be due to a lower rate of CR injection and rather indicates a smaller CR confinement volume characteristic size.

Components of the Extragalactic Gamma-Ray Background

“…Thus, it is expected that other star-forming galaxies emit γ -rays through the same interactions and that unresolved star-forming galaxies could provide a substantial contribution to the EGB. Thus far, the Fermi-LAT Collaboration has reported detections of two nearby irregular galaxies (the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC); Abdo et al 2010c, two starburst galaxies (M82 and NGC253; Abdo et al 2010b), and M31, a galaxy similar to our own (Abdo et al 2010g). As such, whatever the contribution to the EGB from unresolved star-forming galaxies, it will not have changed substantially in the Fermi data with respect to EGRET data as Fermi has resolved only a handful of starforming galaxies.…”

SEARCH FOR SPATIALLY EXTENDEDFERMILARGE AREA TELESCOPE SOURCES USING TWO YEARS OF DATA

“…We note that gtlike has been used in the past in several studies of source extension in the LAT Collaboration (Abdo et al 2009a(Abdo et al , 2010c. In these studies, a set of gtlike maximum likelihood fits at fixed extensions was used to build a profile of the likelihood as a function of extension.…”

“…Two nearby satellite galaxies of the Milky Way the LMC and the SMC were included in the 2FGL catalog as spatially extended sources (Abdo et al 2010c. Their extensions were significantly detected using photons with energies between 1 GeV and 100 GeV.…”

“…In addition, three extended PWNe have been detected by the LAT: MSH 15−52, Vela X, and HESS J1825−137 (Abdo et al 2010bGrondin et al 2011). Two nearby galaxies, the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), and the lobes of one radio galaxy, Centaurus A, were spatially resolved at GeV energies (Abdo et al 2010c(Abdo et al , 2010d. A number of additional sources detected at GeV energies are positionally coincident with sources that exhibit large enough extension at other wavelengths to be spatially resolvable by the LAT at GeV energies.…”

Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?

“…Besides the MW, seven external star-forming galaxies have been firmly detected in gamma rays with the Fermi Large Area Telescope (LAT), including the Large Magellanic Cloud (LMC; Abdo et al 2010a;Ackermann et al 2016), the Small Magellanic Cloud (Abdo et al 2010b), the Andromeda galaxy M31 (Abdo et al 2010c, hereafter Paper I), starburst galaxies M82 and NGC253 (Abdo et al 2010d), NGC2146 (Tang et al 2014), and Arp 220 (Peng et al 2016;Griffin et al 2016). In Paper I, based on a subset of these detections, a correlation was suggested between gamma-ray luminosity and SFR; it was later strengthened by a large systematic study of more than 60 galaxies (Ackermann et al 2012) and now appears as a possible constraint on the origin and transport of CRs (Martin 2014).…”