Unveiling new H.E.S.S. sources
October 2021
The H.E.S.S. Galactic Plane Survey — or HGPS for short H.E.S.S.-SOM 2016-01, [1] — has been a very successful project. It has resulted in a very large number of γ-ray sources, many of which are so extended that they can be spatially resolved with the H.E.S.S. telescopes. It also demonstrated that the density of sources in the Galactic plane is so high that they overlap in projection, and dedicated analysis is required to properly identify and characterise them (see, e.g., H.E.S.S.-SOM 2020-02).
In a recent study [2], the H.E.S.S. collaboration has unveiled four new sources by using a method to determine the background in fields of view that contain many overlapping sources. These four sources all stand out from other H.E.S.S. Galactic sources: They are rather diffuse, and display spectra that extend to very high energies. In fact, they are most likely counterparts of sources that had been discovered by the HAWC instrument [3], which is particularly sensitive to diffuse sources of very high energy radiation. In all cases, the H.E.S.S. detections allow for a more detailed analysis of the source morphology. As a result, it was found that the exact positions of the sources (being defined by the position of peak intensity) is offset from the HAWC sources. The higher resolution of H.E.S.S. will aid the still incomplete effort of identifying the true nature and potential counterparts of these objects in other wavebands.
HESS J1854+013
HESS J1854+013 is a source of significant γ-ray emission in the H.E.S.S. map near HAWC J1852+013. This region is very susceptible to source confusion, which affects the accuracy when deriving a clear peak position. This source is ∼1 degree away from HAWC J1852+013 and the SNR G34.7-0.4 (W44), which is detected in the radio, X-ray, and GeV γ-ray bands. W44 hosts the pulsar PSR B1853+01, a 20 kyr pulsar with dE/dt= 4.3 × 1035 erg s−1 located ∼3 kpc away. W44 has not been detected by H.E.S.S. during the HGPS, but H.E.S.S. reported significant emission with only one of their analysis chains at the location of W44, without claiming a detection [4].
HESS J1907+089
HESS J1907+089 is the H.E.S.S. detection of HAWC J1907+085, and was formally reported in the HGPS as the hotspot HOTS J1907+091. Two potential counterparts are spatially coincident: the magnetar SGR 1900+14 [5] and the SNR G42.8+0.6 [6]. HAWC J1907+085 had not been found in searches by other γ-ray instruments such as MAGIC and Fermi-LAT [7] which emphasizes the difficulties of identifying extended sources of low surface brightness in the Galactic plane. A detailed study of SGR 1900+14 and its environment is presented in [8], including a model of the observed high energy (E > 100 MeV; 4FGL J1908.6+0915e) and VHE γ-ray emission (E > 100 GeV; HOTS J1907+091 and 3HWC J1907+085).
HESS J1915+115
HESS J1915+115 is the H.E.S.S. counterpart of HAWC J1914+118, an unidentified source without any obvious counterpart. Two pulsars detected at radio wavelengths [9] are found in the vicinity: PSR J1915+1149 and PSR J1915+1144 located 0.09° and 0.16° away from the HAWC source. No spin-down power has yet been reported for either pulsar. They are located at distances of 14 kpc and 7.2 kpc, respectively. Moreover, the Fermi-LAT source 4FGL J1915.3+1149 overlaps with PSR J1915+1149 and the HAWC source within the location errors. 4FGL J1915.3+1149 is associated with TXS 1913+115 and classified as an active galaxy but the association probability is not much higher than the threshold.
HESS J1928+182
HESS J1928+181 is the H.E.S.S. detection of HAWC J1928+178, a source of unknown astronomical origin. It may be associated with the middle-aged pulsar (82 kyr) PSR J1928+1746 [10],[11], located 4.3 kpc away. It has been detected in radio wavelength [12] and has a spin-down power of dE/dt = 1.6 × 1036 erg s−1. In that case, the γ-ray emission would come from inverse Compton (IC) scattering of e± from the associated pulsar wind nebula (PWN) with ambient photons. However, the lack of an X-ray counterpart — despite several sensitive observations — may indicate that the PWN is in an evolved state. Alternatively, accelerated protons from the PWN may be interacting with a nearby molecular cloud to produce γ-ray emission via π0 decay. Yet another scenario was proposed by Mori et al. [13] involving a binary system in which the variable X-ray source CXO J192812.0+174712 would be a massive star with a pulsar companion. In that case, e± from the pulsar wind would be accelerated at the shock between the pulsar and the stellar wind of the massive star and produce γ rays via synchrotron emission and IC scattering.
The four new sources detected in this investigation were identified in a particular part of the HESS Galactic Plane Survey, and a forthcoming extension of the new analysis approach is expected to result in more additions to the family of HESS sources.
References:
[1] H.E.S.S. Collaboration, Abdalla, H., Abramowski, A., et al. 2018, A&A, 612, A1, arXiv: 1804.02432
[2] Abdalla, H., Aharonian, F., Ait Benkhali, F., 2021, ApJ, 917, 6, arXiv: 2107.01425
[3] Albert, A., Alfaro, R., Alvarez, C., et al. 2020, ApJ, 905, 76, arXiv: 2007.08582
[4] H.E.S.S. Collaboration, Abdalla, H., Abramowski, A., et al. 2018, A&A, 612, A3, arXiv: 1802.05172
[5] Mazets, E. P., Golenetskij, S. V., & Guryan, Y. A. 1979, SvAL, 5, 343
[6] Fuerst, E., Reich, W., Reich, P., Handa, T., & Sofue, Y. 1987, A&AS, 69, 403
[7] Ahnen, M. L., Ansoldi, S., Antonelli, L. A., et al. 2019, MNRAS, 485, 356, arXiv:1901.03982
[8] Hnatyk, B., Hnatyk, R., Zhdanov, V., & Voitsekhovskyi, V. 2020, arXiv:2009.06081
[9] Lazarus, P., Brazier, A., Hessels, J. W. T., et al. 2015, ApJ, 812, 81, arXiv: 1504.02294
[10] Lopez-Coto, R., Marandon, V., & Brun, F. 2017, Proc. ICRC (Busan), 35, 732, arXiv: 1708.03137
[11] Jardin-Blicq, A. 2019, PhD thesis, Ruprecht-Karls-Universität Heidelberg
[12] Cordes, J. M., Freire, P. C. C., Lorimer, D. R., et al. 2006, AJ, 637, 446, arXiv: astro-ph/0509732
[13] Mori, K., An, H., Feng, Q., et al. 2020, ApJ, 897, 129, arXiv: 2005.12343