HESS J1930+188: The high energy side of the Bull's Eye Pulsar Wind Nebula
November 2021
With the first sources published by the LHAASO experiment this year, the number of observatories studying the cosmos at very high photon energies has grown further. This allows comparisons for cross-validations and the combination of information from facilities that have different strengths. One object that has been detected independently in sky surveys obtained with the three facilities HESS [1], HAWC [2], and LHAASO [3], but was first published as a discovery obtained in a dedicated search with the VERITAS array [4], is the Pulsar Wind Nebula (PWN) G54.1+0.3. The nebula surrounds the young and energetic pulsar PSR J1930+1852. The source is listed as HESS J1930+188 in the HESS source catalog. PWN are the most numerous class of Galactic gamma-ray emitters and make up most of the sources in the Galactic Plane Survey of HESS that are firmly associated with a specific object [1].
HESS J1930+188 is the second-most northern source detected in the HESS Galactic Plane Survey (HGPS, see SOM 2016-01) and coincides spatially with the source VER J1930+188 [4]. Subsequent surveys of the entire northern sky detected emission from the same location (2HWC J1930+188, LHAASO J1929+1745), all instruments revealing consistent spectra in mutually comparable energy ranges. In this part of the Galactic plane the density of gamma-ray source is rather low, allowing a firm association of HESS J1930+188 with PSR J1930+1852 and its surrounding PWN G54.1+0.3. At an estimated distance of approximately 6.5 kpc (21.000 light-years), G54.1+0.3 is placed in the Perseus arm of the Milky Way and turns out to be one of the more distant VHE emitting sources in the HESS Galactic Plane Survey.
The Pulsar J1930+1852, detected in 2003 [5], spins at 136 ms and has a characteristic age of 2900 years and a spin-down luminosity of 1.2 1037 erg s−1. Radio and X-ray timing reveal large scatter in the spin-down evolution, implying significant glitching activity and/or timing noise. The rotational modulation in the radio- and X-ray bands, as well as its flux and spectral characteristics are typical for a Crab-like pulsar.
PSR J1930+1852 powers the surrounding PWN G54.1+0.3, the 'Bull's Eye Nebula' (see figure 1), the 'eye' being shaped by a toroidal region of particle acceleration. X-ray studies reveal that the emission is less energetic at larger distances from the pulsar. This trend is likely due to 'synchrotron cooling' (particles lose energy as they radiate while gradually moving away from the center of the nebula) and so-called adiabatic expansion. The cooling efficiency is about 20 times lower/higher than in the prominent PWN of the Crab and the Vela pulsar, respectively - both of which are more nearby and well-studied VHE-emitting sources. An energy-dependent morphology, such as the radial softening of the X-ray spectra is seen in many VHE sources but is difficult to establish in the more distant and smaller objects.
The VHE energy source HESS J1930+188 is only slightly extended and about 30 arcsec off-set with respect to the pulsar [1]. Measuring such offsets and extensions requires the higher astrometric accuracy available with the stereoscopic H.E.S.S. array. The all-sky instrument HAWC on the other hand, has obtained a deeper exposure of this northern source, indicating that the diffuse emission beyond the marginally resolved source (see figure 1) may actually contain further sources. It is unclear, however, whether this diffuse emission is related to HESS J1930+188. The LHAASO experiment provides higher sensitivity at the highest energy end of the gamma-ray spectrum and reveals that the source can still be detected out to several tens of TeV. This extends beyond the high-energy end of 10 TeV in the spectrum obtained with HESS (see figure 2). The spectra obtained with the different VHE experiments are consistent despite the different angular resolution, implying that the diffuse extended emission towards the neighboring HAWC source 2HWC J1928+178 does not add significant flux.
The gamma-ray emission from the PWN can be explained as resulting from Inverse Compton scattering of electrons accelerated at the PWN termination shock on ambient photon fields. G54.1+0.3 is surrounded by a star-forming loop of at least 11 massive young stars (so-called pre-main sequence stars). Their age suggests star formation linked to the progenitor star of PSR J1930+1852, which would imply significant amounts of interstellar material surrounding the expanding PWN G54.1+0.3 [6]. This might result in a more complex explanation of the gamma-ray spectrum, including gamma-ray emission from the collisions of protons accelerated in the PWN with the dense matter left over in the formation of the young stars depicted in figure 3.
References:
[1] The H.E.S.S. Galactic plane survey, H.E.S.S. Collaboration, AA 612, A1 (2018)
[2] VERITAS and Fermi-LAT Observations of TeV Gamma-Ray Sources Discovered by HAWC in the 2HWC Catalog, Abeysekara, A. U. et al., ApJ 866, 24 (2018)
[3] Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources, Z. Cao et al., Nature 594, 33–36 (2021)
[4] Discovery of Very High Energy γ-ray Emission from the SNR G54.1+0.3, Acciari et al., ApJ 719, L69 (2010)
[5] Discovery of a 136 Millisecond Radio and X-Ray Pulsar in Supernova Remnant G54.1+0.3, Camilo, F.,et al., ApJ, 574, L71 (2002)
[6] Star-Forming Infrared Loop around G54.1+0.3, Koo, B.-C., et al., ApJ, 673, L147 (2008)
[7] TeV emission of Galactic plane sources with HAWC and H.E.S.S., H.E.S.S. Collaboration and HAWC Collaboration, ApJ, 2021ApJ...917....6A (2021), arXiv:2107.01425