The gamma emission we observe in the sky through our telescopes involves a wide variety of astrophysical objects. That is why the understanding of the physical processes involved in the production of such emission requires the detailed study of objects like pulsar-wind nebulae, supernova remnants, active galactic nuclei, massive young stellar objects, X-ray binaries, and classic- and symbiotic-novae.
In particular, most of the gamma-ray emission in the Fermi-LAT 14-yr source catalogue is associated with pulsars or with blazars. However, many high energy sources remain unassociated—and unveiling their nature is quite a challenge when it comes to understanding these phenomena in depth. After ruling out possible associations with other sources, we propose for the first time that a nascent planetary nebula could be generating the observed gamma-ray emission of the Fermi-LAT source 4FGL J1846.9-0227.
A previous work towards the Fermi-LAT source 4FGL J1846.9-0227 suggested that the gamma-ray emission could be associated with a blazar; and another study, based on the presence of a massive protostar within the Fermi confidence ellipse, also proposed a possible association. However, neither of the two studies was conclusive regarding the nature of the source associated with the gamma emission. Therefore, with the primary objective of unveiling the nature of this source, we conducted a multispectral study of the region using different catalogs and observations from several telescopes like the Jansky Very Large Array (JVLA), the Atacama Large Millimeter Array (ALMA), the XMM-Newton, the Spitzer, etc.
The Fermi-LAT source 4FGL J1846.9-0227 is located in a very complex region of the Galactic plane that includes several candidate sources lying within its confidence ellipse. After discarding the blazar nature of 4FGL J1846.9-0227, a multiwavelength and comprehensive study of all the candidate sources was carried out resulting in the final selection of two of them as the most likely to be generating the gamma-ray emission: a newly discovered symbiotic binary system and a likely planetary nebula with associated radio synchrotron emission. It is worth mentioning that both objects, the closest to the centre of the 95% confidence ellipse of the Fermi-LAT source 4FGL J1846.9-0227, are associated with the two brightest XMM-Newton sources in the region.
Regarding the first candidate source (the symbiotic binary system), we found a clear association between the XMM-Newton source 4XMM J184650.6-022907, whose soft X-ray spectra were well fitted by an optically thin thermal plasma model, and the spectroscopic binary star BD-02 4739 cataloged in Gaia. Our multi-wavelength analysis, which included infrared and optical data, indicated that the spectroscopic binary star BD-02 4739 is a non shell-burning β-type white dwarf symbiotic system with a sub-giant donor star. These objects are good candidates to produce gamma-ray emission under certain conditions. However, the lack of evidence of an optical nova event associated with the discovered symbiotic system prevents us from relating gamma emission to this object.
Having ruled out the symbiotic binary star as the possible source of the gamma-ray emission, we focused on the source IRAS 18443-0231, which was previously cataloged as a planetary nebula candidate. It is important to mention that we found strong evidence that this object was actually a proto-planetary nebula. The dominant radio continuum emission in proto-planetary nebulae, which is a short-lived transition (about 1000 years) from the asymptotic giant branch to the planetary nebula phase, is expected to be thermal—although some processes such as jets and magnetic fields could provide an environment for non-thermal emission. However, there are not many cases where non-thermal radio continuum emission was observed towards this kind of object.
Interestingly, IRAS 18443-0231 exhibits a bipolar morphology in centimetre radio continuum emission with a negative spectral index, compatible with synchrotron emission, suggesting the presence of particle acceleration taking place in the jets. The non-thermal radio emission in proto-planetary nebulae suggests that the emitting electrons arise at collisions between the fast and slow asymptotic giant branch winds that are observed predominantly on the front sides of the circumstellar shells. Moreover, we found a red-shifted molecular outflow arising from the central object in positional coincidence with one of the lobes, which supports the presence of jet activity, making this source the only one with strong evidence of particle acceleration in the analysed region. Additionally, IRAS 18443-0231 is associated with a water maser, which suggests that the source could be the kind of young planetary nebula known as “water fountains”.
In conclusion, based on the presence of radio synchrotron emission, the jet-like morphology at centimetre wavelengths, the presence of molecular outflows, the associated hard X-ray emission, and its location closer to the centre of the Fermi confidence ellipse, it is suggested that IRAS 18443−0231 is the most likely counterpart of the gamma ray source 4FGL J1846.9-0227. The presence of jets and molecular material in its surroundings could explain the gamma-ray emission through mechanisms such as proton-proton collisions and relativistic Bremsstrahlung. If this is the case, it would be the first reported proto-planetary nebula related to very high energy emission, and hence, multiwavelength dedicated observations and modelling would be necessary to understand the mechanisms of gamma-rays production in this kind of source. This discovery broadens the space for new kinds of gamma-ray sources to be recognized as such.
Featured image by Alexander Andrews by Unsplash.
Very interesting!