HESS J1826$-$130: A Very Hard $\gamma$-Ray Spectrum Source in the Galactic Plane

TL;DR

HESS J1826-130 is a very hard gamma-ray source in the Galactic plane with a spectrum indicating acceleration of particles up to hundreds of TeV, likely due to cosmic-ray interactions with molecular clouds.

Contribution

This paper reports the discovery and spectral analysis of HESS J1826-130, highlighting its unusually hard spectrum and potential hadronic origin involving cosmic-ray interactions.

Findings

Spectral index of 1.6 with a cutoff at 12 TeV.

No clear counterpart at other wavelengths.

Likely hadronic origin involving cosmic-ray interactions.

Abstract

HESS J1826$-$130 is an unidentified hard spectrum source discovered by H.E.S.S. along the Galactic plane, the spectral index being $\Gamma$ = 1.6 with an exponential cut-off at about 12 TeV. While the source does not have a clear counterpart at longer wavelengths, the very hard spectrum emission at TeV energies implies that electrons or protons accelerated up to several hundreds of TeV are responsible for the emission. In the hadronic case, the VHE emission can be produced by runaway cosmic-rays colliding with the dense molecular clouds spatially coincident with the H.E.S.S. source.

Full text

1 INTRODUCTION

In the past decade, observations with the High Energy Stereoscopic System (H.E.S.S.) have discovered tens of very high energy (VHE, $E>0.1$ TeV) $\gamma$-ray sources111See http://tevcat.uchicago.edu for an updated list of known VHE $\gamma$-ray sources. by scanning a large fraction of the Galactic Plane [1].

HESS J1826$-$130 is an unidentified extended source discovered by the H.E.S.S. telescope array in the multi-TeV domain. The source had been previously hidden in the extended tail of emission from the bright nearby source HESS J1825$-$137 [2]. For such sources affected by source confusion, an investigation in energy bands can provide an additional powerful tool for new discoveries [3]. HESS J1826$-$130 shows one of the hardest spectra ever observed in VHE $\gamma$-rays, extending up to at least few tens of TeV. The shell-type supernova remnants (SNRs), G018.1$-$00.1 and G018.6$-$00.2 [4], are located in the vicinity of the source along with the $\gamma$-ray pulsar PSR J1826$-$1256.

In this work, the analysis of 206 h of H.E.S.S. data from the region around HESS J1826$-$130 will be presented. Finally, possible VHE $\gamma$-ray emission scenarios related to the origin of the source will be briefly discussed.

2 H.E.S.S. OBSERVATIONS and RESULTS

2.1 The H.E.S.S. Telescopes

H.E.S.S. is an array of five imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia, 1800 m above sea level. H.E.S.S. in phase I comprised of four 13 m diameter telescopes which have been fully operational since 2004. A fifth telescope was added in the center of the array and has been operational since September 2012. The H.E.S.S. phase I array configuration is sensitive to $\gamma$-ray energies between 100 GeV and several tens of TeV. With the addition of the fifth telescope, the energy threshold was lowered to some tens of GeV. The VHE H.E.S.S. data presented in this paper were taken with the H.E.S.S. phase I array configuration, which can measure extensive air showers with an angular resolution better than ${0.1}^{\circ}$ and an average energy resolution of 15$\%$ at an energy of 1 TeV [5].

2.2 Detection and Morphological Analysis

The observations of the field of view around HESS J1826$-$130 were carried out between 2004 and 2015, corresponding to an acceptance corrected live-time of 206 h of H.E.S.S. phase I data after the application of the quality selection criteria [5]. The data have been analyzed with the H.E.S.S. analysis package for shower reconstruction and the multivariate analysis technique [6] has been applied for providing an improved discrimination between hadrons and $\gamma$ rays. In order to provide improved angular resolution and reduce contamination from the bright and nearby source HESS J1825$-$137, the source position and morphology have been obtained with a hard cut configuration which requires a minimum of 160 photo-electrons per image. The cosmic-ray background level was estimated using the ring background model [7]. Using this dataset, HESS J1826$-$130 is detected with a statistical significance of 21.0$\sigma$, determined by using Equation (17) in [8]. Figure 1 shows the acceptance corrected and smoothed with the H.E.S.S. point spread function (PSF) VHE $\gamma$-ray excess map of the region around HESS J1826$-$130 at energies greater than E = 2 TeV.

The source remained unnoticed and hidden for the standard source detection techniques due to its relatively low brightness with respect to the nearby bright source HESS J1825$-$137. During a study of a possible energy-dependent morphology of HESS J1826$-$130, a collection of images for events with energies above a set of energy thresholds (E $>$ 1, 2, 3, 4, and 5 TeV) was created (see Fig. 2). As it can be seen from the figure, HESS J1826$-$130 becomes more visible with increasing energy threshold.

The position and extension of HESS J1826$-$130 was determined by fitting a two-dimensional Gaussian model convolved with the H.E.S.S. PSF to the excess event distribution. The centroid of the 2D Gaussian corresponding to the best fit position of the source is R.A. (J2000): $18^{\textsuperscript{h}}$26${}^{\textsuperscript{m}}$0.2${}^{\textsuperscript{s}}$ $\pm$ 7.0${}^{\textsuperscript{s}}_{\rm stat}$ $\pm$ 1.3${}^{\textsuperscript{s}}_{\rm sys}$ and Dec. (J2000): $-$13*∘*02’ $\pm$ 1.8’stat $\pm$ 0.3’sys. The extension of the source is estimated as 0.17∘ $\pm$ 0.02${}^{\circ}_{\rm stat}$ $\pm$ 0.05${}^{\circ}_{\rm sys}$.

2.3 Spectral Analysis

A circular region with a radius of 0.22*∘* centered at the best fit position of HESS J1826$-$130 (e.g. Fig. 1, green circle) was used as an integration region for extracting the differential VHE $\gamma$-ray spectrum of the source. The spectrum was derived using the forward folding technique [9], and is well described by a power-law with an exponential cut-off function $dN/dE$ = $\Phi_{0}$ ($E/1\textnormal{ TeV}$)-Γ $\exp$($-E/E_{c}$) with $\Phi_{0}$ = (8.62 $\pm$ 0.73stat $\pm$ 1.72sys) $\times$ $10^{13}$ cm*-2* s*-1* TeV*-1*, $\Gamma$ = 1.61 $\pm$ 0.11stat $\pm$ 0.20sys and a cut-off energy of $E_{c}$ = 12.5${}^{+4.0}_{-2.4}$ TeV. This spectral model is preferred over a simple power-law model at a statistical significance level of 4.0$\sigma$ from log-likelihood ratio test. The time variability of the emission of the source was investigated at different timescales by fitting the integral flux to a constant. In all the timescales, from run-wise ($\sim$30 min) to year-wise light curves, the $\chi^{2}$ fit results in compatibility of the emission with a constant flux.

The integration region used for extracting the VHE spectrum of HESS J1826$-$130 is strongly contaminated by HESS J1825$-$137, especially at lower energies ($\sim$40$\%$ below 1.5 TeV and $\sim$20$\%$ above 1.5 TeV), and the spectrum presented in this section is affected by this contamination. Further spectral studies (simultaneous fitting of both source spectra) are being performed for obtaining the intrinsic spectrum of HESS J1826$-$130. Given the relatively softer spectrum of HESS J1825$-$137, the intrinsic spectrum of HESS J1826$-$130 is expected to be even harder with respect to the spectral results presented in this section.

3 MULTIWAVELENGTH COUNTERPARTS

Two giant molecular clouds (GMC) with masses of few 105 M*⨀* at 3.7 kpc and 4.7 kpc have been found along the line of sight of to HESS J1826$-$130 when studying NANTEN data [10]. The ambient gas densities of these GMCs are estimated of the order of $\sim$600 cm*-3*.

The SNR G018.1$-$00.1 is located between 4 and 6.3 kpc [4] (corresponding to the projected physical distance of $\sim$33 pc from the best fit position of HESS J1826$-$130), while the SNR G018.6$-$00.2 is located between 4 and 5.2 kpc [4] (corresponding to the projected physical distance of $\sim$18 pc from the best fit position of HESS J1826$-$130).

The $\gamma$-ray pulsar PSR J1826$-$1256 is also located in the vicinity of the source as shown in Fig. 1. This pulsar has a spin-down luminosity of $\dot{\textnormal{E}}$ = 3.6 $\times$ 1036 erg/s and a characteristic age of $\tau_{c}$ = 14.4 kyr. Note that pulsars with $\tau_{c}$ $<$ 150 kyr and with $\dot{E}$/$D_{\rm PSR}^{2}$ $\geq$ 1034 erg s*-1* kpc*-2* are known to power pulsar wind nebulae (PWNe) that are detectable at very high energies [11]. This pulsar has been detected both in X-ray energies [12] and in high energy $\gamma$-rays [13]. No distance estimation for this pulsar is known from dispersion measure [14] since it is a radio-quiet pulsar.

4 DISCUSSION

The emission from HESS J1826$-$130 is characterized by a very hard spectrum ($\Gamma$ = 1.6) with an exponential cut-off at about 12 TeV. Such a spectrum, together with a spatial coincidence of a dense molecular hydrogen region, suggests that the radiation could be produced by several hundred TeV protons colliding with the gas. To explain the very hard spectrum, one can think of a hadronic scenario in which runaway protons accelerated by the close shell-type SNRs G018.1$-$00.1 or G018.6$-$00.2 are emitting TeV photons when interacting with the dense ambient gas found along the line of sight. The proton spectrum should extend up to at least several hundreds of TeV. The $\gamma$-ray luminosity, Lγ, of the source is 8 $\times$ 1033 ergs for a distance of 4 kpc. This translates into an energy output in accelerated protons, W${}_{\textnormal{pp}}$ = Lγ $\times$ t${}_{\textnormal{pp}}$, of 6 $\times$ 1049 (n / 1 cm*-3*)-1 erg, while ambient gas densities from NANTEN data analysis are of the order of 600 cm*-3*. In the hadronic scenario, the hardness of the photon spectrum could be the effect of the highest energy protons diffusing in dense clouds, while the lower energy protons might be still confined within the accelerating source or efficiently excluded from the clouds if diffusion coefficient inside the clouds is suppressed.

A leptonic scenario, where electrons accelerated by the pulsar PSR J1826$-$1256 are up-scattering CMB or IR photons, can also explain the VHE emission. Such as hard spectrum at H.E.S.S. energies can be produced by an uncooled electron population with spectral index close to -2.0 and a cutoff at around 70 TeV. The energy output (W${}_{\textnormal{e}}$) in accelerated electrons is 2 $\times$ 1047 ergs for a distance of 7 kpc. This source has a spectrum very similar to other few PWNe, in particular, Vela X [15]. HESS J1826$-$130 could be an indication of a distinctive PWN population, with very hard spectra and relatively high cutoff energies

5 CONCLUSIONS

The unidentified TeV source, HESS J1826$-$130, stands out in the H.E.S.S. Galactic Plane Survey for its particularly hard spectrum. A proton or electron population extending up to several hundred TeV is needed to produce the TeV emission from HESS J1826$-$130. Currently, studies to better constrain its spectrum are under way.

6 ACKNOWLEDGMENTS

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Czech Science Foundation, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, the University of Namibia, the Innsbruck University, the Austrian Science Fund (FWF), and the Austrian Federal Ministry for Science, Research and Economy, and by the University of Adelaide and the Australian Research Council. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO, ChIPS, and Sherpa. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of the ATNF pulsar catalog database (http://www.atnf.csiro.au/research/pulsar/psrcat/). The NANTEN project is based on the mutual agreement between Nagoya University and the Carnegie Institution of Washington. Sabrina Casanova and Ekrem Oğuzhan Angüner acknowledge the support from the Polish National Science Center under the Opus Grant UMO-2014/13/B/ST9/00945.