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Register a new userHESS J1741-302: a hidden accelerator in the Galactic plane
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Added by
Ekrem O\\u{g}uzhan Ang\\\"uner
Publication date
2017
fields
Physics
and research's language is
English
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The H.E.S.S. collaboration has discovered a new very high energy (VHE, E $>$ 0.1 TeV) $gamma$-ray source, HESS J1741-302, located in the Galactic plane. Despite several attempts to constrain its nature, no plausible counterpart has been found so far at X-ray and MeV/GeV $gamma$-ray energies, and the source remains unidentified. An analysis of 145-hour of observations of HESS J1741-302 at VHEs has revealed a steady and relatively weak TeV source ($sim$1$%$ of the Crab Nebula flux), with a spectral index of $Gamma$ = 2.3 $pm$ 0.2$_{text{stat}}$ $pm$ 0.2$_{text{sys}}$, extending to energies up to 10 TeV without any clear signature of a cut-off. In a hadronic scenario, such a spectrum implies an object with particle acceleration up to energies of several hundred TeV. Contrary to most H.E.S.S. unidentified sources, the angular size of HESS J1741-302 is compatible with the H.E.S.S. point spread function at VHEs, with an extension constrained to be below 0.068$^{circ}$ at a 99$%$ confidence level. The $gamma$-ray emission detected by H.E.S.S. can be explained both within a hadronic scenario, due to collisions of protons with energies of hundreds of TeV with dense molecular clouds, and in a leptonic scenario, as a relic pulsar wind nebula, possibly powered by the middle-aged (20 kyr) pulsar PSR B1737-30. A binary scenario, related to the compact radio source 1LC 358.266+0.038 found to be spatially coincident with the best fit position of HESS J1741-302, is also envisaged.
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The unidentified very-high-energy (VHE; E $>$ 0.1 TeV) $gamma$-ray source, HESS J1826$-$130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady $gamma$-ray flux from HESS J1826$-$130, which appears extended with a half-width of 0.21$^{circ}$ $pm$ 0.02$^{circ}_{text{stat}}$ $pm$ 0.05$^{circ}_{text{sys}}$. The source spectrum is best fit with either a power-law function with a spectral index $Gamma$ = 1.78 $pm$ 0.10$_{text{stat}}$ $pm$ 0.20$_{text{sys}}$ and an exponential cut-off at 15.2$^{+5.5}_{-3.2}$ TeV, or a broken power-law with $Gamma_{1}$ = 1.96 $pm$ 0.06$_{text{stat}}$ $pm$ 0.20$_{text{sys}}$, $Gamma_{2}$ = 3.59 $pm$ 0.69$_{text{stat}}$ $pm$ 0.20$_{text{sys}}$ for energies below and above $E_{rm{br}}$ = 11.2 $pm$ 2.7 TeV, respectively. The VHE flux from HESS J1826$-$130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula (PWN), HESS J1825$-$137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826$-$130 VHE emission related to PSR J1826$-$1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826$-$130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to $gtrsim$200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants (SNRs), molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.
With the Suzaku satellite, we observed an unidentified TeV gamma-ray source HESS J1741$-$302 and its surroundings. No diffuse or point-like X-ray sources are detected from the bright southern emission peak of HESS J1741$-$302. From its neighborhood, we found a new intermediate polar candidate at the position of $(alpha, delta)_{rm J2000.0} = (timeform{17h40m35.6s}, timeform{-30D14m16s})$, which is designated as Suzaku J174035.6$-$301416. The spectrum of Suzaku J174035.6$-$301416 exhibits emission lines at the energy of 6.4, 6.7 and 7.0 keV, which can be assigned as the K$alpha$ lines from neutral, He-like and H-like iron, respectively. A coherent pulsation is found at a period of 432.1 $pm$ 0.1 s. The pulse profile is quasi-sinusoidal in the hard X-ray band (4$-$8 keV), but is more complicated in the soft X-ray band (1$-$3 keV). The moderate period of pulsation, the energy flux, and the presence of the iron K$alpha$ lines indicate that Suzaku J174035.6$-$301416 is likely an intermediate polar, a subclass of magnetized white dwarf binaries (cataclysmic variables). Based on these discoveries, we give some implications on the origin of GCDX and brief comments on HESS J1741$-$302 and PSR B1737$-$30.
HESS J1943+213 is a very-high-energy (VHE; $>$100 GeV) $gamma$-ray source in the direction of the Galactic Plane. Studies exploring the classification of the source are converging towards its identification as an extreme synchrotron BL Lac object. Here we present 38 hours of VERITAS observations of HESS J1943+213 taken over two years. The source is detected with $sim$20 standard deviations significance, showing a remarkably stable flux and spectrum in VHE $gamma$-rays. Multi-frequency very-long-baseline array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with European VLBI Network and detect this component in the 4.6 GHz and the 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, $Fermi$-LAT, $Swift$-XRT, FLWO 48$$ telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron-self-Compton model. The well-measured $gamma$-ray peak of the SED with VERITAS and $Fermi$-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary $gamma$-rays from ultra-high-energy cosmic ray-initiated electromagnetic cascades to the $gamma$-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and $gamma$-ray bands. No statistically significant flux or spectral variability is detected.
We present a detailed analysis of the gamma-ray emission from HESS J1745-303 with the data obtained by the Fermi Gamma-ray Space Telescope in the first ~29 months observation.The source can be clearly detected at the level of ~18-sigma and ~6-sigma in 1-20 GeV and 10-20 GeV respectively. Different from the results obtained by the Compton Gamma-ray Observatory, we do not find any evidence of variability. Most of emission in 10-20 GeV is found to coincide with the region C of HESS J1745-303. A simple power-law is sufficient to describe the GeV spectrum with a photon index of ~2.6. The power-law spectrum inferred in the GeV regime can be connected to that of a particular spatial component of HESS J1745-303 in 1-10 TeV without any spectral break. These properties impose independent constraints for understanding the nature of this dark particle accelerator.
Recent HESS observations of the ~200 pc scale diffuse gamma-ray emission from the central molecular zone (CMZ) suggest the presence of a PeV cosmic-ray accelerator (PeVatron) located in the inner 10 pc region of the Galactic Center. Interestingly, the gamma-ray spectrum of the point-like source (HESS J1745-290) in the Galactic Center shows a cutoff at ~10 TeV, implying a cutoff around 100 TeV in the cosmic-ray proton spectrum. Here we propose that the gamma-ray emission from the inner and the outer regions may be explained self-consistently by run-away protons from a single, yet fading accelerator. In this model, gamma rays from the CMZ region are produced by protons injected in the past, while gamma rays from the inner region are produced by protons injected more recently. We suggest that the blast wave formed in a tidal disruption event (TDE) caused by the supermassive black hole (Sgr A*) could serve as such a fading accelerator. With typical parameters of the TDE blast wave, gamma-ray spectra of both the CMZ region and HESS J1745-290 can be reproduced simultaneously. Meanwhile, we find that the cosmic-ray energy density profile in the CMZ region may also be reproduced in the fading accelerator model when appropriate combinations of the particle injection history and the diffusion coefficient of cosmic rays are adopted.
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