Discovery of a point-like very-high-energy gamma-ray source in Monoceros

TL;DR

This paper reports the discovery of a new very-high-energy gamma-ray source near the Monoceros SNR, using H.E.S.S. observations, with potential associations to X-ray and lower-energy gamma-ray sources, but no clear molecular cloud counterpart.

Contribution

The study presents the first detection of a point-like VHE gamma-ray source in the Monoceros region with no prior identification at other wavelengths.

Findings

Discovery of gamma-ray source HESS J0632+058 near Monoceros SNR

No associated molecular cloud detected in CO data

Possible links to X-ray and lower-energy gamma-ray sources

Abstract

The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) gamma-ray emission and two as yet unidentified high-energy EGRET sources. Sensitive VHE observations are required to probe acceleration processes in this region. The H.E.S.S. telescope array has been used to search for very high-energy gamma-ray sources in this region. CO data from the NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for gamma-ray production via hadronic interactions. We announce the discovery of a new gamma-ray source, HESS J0632+058, located close to the rim of the Monoceros SNR. This source is unresolved by H.E.S.S. and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray source 1RXS J063258.3+054857, the Be-star MWC 148 and/or the lower energy gamma-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data.

Figures

Full text

*Methods. * The H.E.S.S. telescope array has been used to search for very high-energy $\gamma$-ray sources in this region. CO data from the NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for $\gamma$-ray production via hadronic interactions.

*Results. * We announce the discovery of a new $\gamma$-ray source, HESS J0632+058, located close to the rim of the Monoceros SNR. This source is unresolved by H.E.S.S. and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray source 1RXS J063258.3+054857, the Be-star MWC 148 and/or the lower energy $\gamma$-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data.

††offprints: J.A.Hinton@leeds.ac.uk, Armand.Fiasson@lpta.in2p3.fr

1 Introduction

Shell-type supernova remnants (SNRs) have been identified as particle accelerators via their very-high-energy (VHE; $E>100$ GeV) $\gamma$-ray and non-thermal X-ray emission (see e.g. Aharonian et al. (2006a) and Koyama et al. (1997)). It has been suggested that interactions of particles accelerated in SNR with nearby molecular clouds should produce detectable $\gamma$-ray emission Aharonian et al. (1994). For this reason the well-known Monoceros Loop SNR (G 205.5+0.5, distance $\sim$1.6 kpc Graham et al. (1982); Leahy et al. (1986)), with its apparent interaction with the Rosette Nebula (a young stellar cluster/molecular cloud complex, distance $1.4\pm 0.1$ kpc Hensberge et al. (2000)) is a prime target for observations with VHE $\gamma$-ray instruments.

For the case of hadronic cosmic rays (CRs) interacting in the interstellar medium to produce pions and hence $\gamma$-rays via $\pi^{0}$ decay, a spatial correlation between $\gamma$-ray emission and tracers of interstellar gas is expected. Such a correlation was used to infer the presence of a population of recently accelerated CR hadrons in the Galactic Centre region Aharonian et al. (2006b). This discovery highlights the importance of accurate mapping of available target material for the interpretation of TeV $\gamma$-ray emission. The NANTEN 4 m diameter sub-mm telescope at Las Campanas observatory, Chile, has been conducting a 12CO ($J$=1$\rightarrow$0) survey of the Galactic plane since 1996 Mizuno & Fukui (2004). The Monoceros region is covered by this survey and the NANTEN data are used here to trace the target material for interactions of accelerated hadrons.

2 H.E.S.S. Observations and Results

The observations described here took place between March 2004 and March 2006 and comprise 13.5 hours of data after data quality selection and dead-time correction. The data were taken over a wide range of zenith angles from 29 to 59 degrees, leading to a mean energy threshold of 400 GeV with so-called standard cuts used here for spectral analysis and 750 GeV with the hard cuts used here for the source search and position fitting. These cuts are described in detail in Aharonian et al. (2006c).

A search in this region for point-like emission was made using a 0.11*∘* On source region and a ring of mean radius 0.5*∘* for Off source background estimation (see Berge et al. (2006) for details). Fig. 1 shows the resulting significance map, together with CO data from NANTEN, radio contours and the positions of all Be-stars in this region. The peak significance in the field is $7.1\sigma$. The number of statistical trials associated with a search of the entire field of view, in $0.01^{\circ}$ steps along both axes, is $\approx 10^{5}$. The measured peak significance corresponds to $5.3\sigma$ after accounting for these trials. A completely independent analysis based on a fit of camera images to a shower model (Model Analysis described in de Naurois (2006)), yields a significance of $7.3\sigma$ ($5.6\sigma$ post-trials).

The best fit position of the new source is $6^{\mathrm{h}}32^{\mathrm{m}}58.3^{\mathrm{s}}$, $+5^{\circ}48^{\prime}20^{\prime\prime}$ (RA/Dec. J2000) with 28*′′* statistical errors on each axis, and is hence identified as HESS J0632+057. Systematic errors are estimated at 20*′′* on each axis. There is no evidence for intrinsic extension of the source and we derive a limit on the rms size of the emission region of $2^{\prime}$ (at 95% confidence), under the assumption that the source follows a Gaussian profile. This source size upper limit is shown as a dashed circle in the bottom panel of Fig. 1. Fig. 2 demonstrates the point-like nature of the source. The angular distribution of excess $\gamma$-ray-like events with respect to the best fit position is shown together with the expected distribution for a point-like source.

The reconstructed energy spectrum of the source is consistent with a power-law: $dN/dE=k(E/1\mathrm{TeV})^{-\Gamma}$ with photon index $\Gamma=2.53\pm 0.26_{stat}\pm 0.20_{sys}$ and a flux normalisation $k=9.1\pm 1.7_{stat}\pm 3.0_{sys}\times 10^{-13}$ cm*-2s-1TeV-1*. Fig. 3 shows the H.E.S.S. spectrum together with that for the unidentified EGRET source 3EG J0634+0521 (discussed below) and an upper limit derived for TeV emission from 3EG J0634+0521 using the HEGRA telescope array Aharonian et al. (2004), converted from an integral to a differential flux using the spectral shape measured by H.E.S.S. We find no evidence for flux variability of HESS J0632+057 within our dataset. However, we note that due to the weakness of the source and sparse sampling of the light-curve, intrinsic variability of the source is not strongly constrained. The bulk of the available data was taken in two short periods in December 2004 (P1, 4.7 hours) and November/December 2005 (P2, 6.2 hours). The integral fluxes (above 1 TeV) in these two periods were: $6.3\pm 1.8\times 10^{-13}$ cm*-2* s*-1* (P1) and $6.4\pm 1.5\times 10^{-13}$ cm*-2* s*-1* (P2).

Amongst the candidate VHE sources in this field is the 34 ms binary pulsar SAX J0635.2+0533. There is no significant $\gamma$-ray emission at the position of this object and we derive a 99% confidence upper limit on the integral flux, $F(>1\mathrm{TeV})$, of $2.6\times 10^{-13}$ cm*-2* s*-1*, assuming an $E^{-2}$ type spectrum.

3 Possible Associations of HESS J0632+057

The new VHE source HESS J0632+057 lies in a complex region and several associations with objects known at other wavelengths seem plausible. We therefore consider each of these potential counterparts in turn.

The Monoceros Loop SNR is rather old in comparison to the known VHE $\gamma$-ray shell-type SNRs RX J1713.7$-$3946 Aharonian et al. (2006a), RX J0852.0$-$4622 Aharonian et al. (2005b) and Cas-A Aharonian et al. (2001). All these objects have estimated ages less than $\sim 2000$ years, in contrast the Monoceros Loop SNR has an age of $\sim 3\times 10^{4}$ years Leahy et al. (1986). This supernova remnant therefore appears to be in a different evolutionary phase (late Sedov or Radiative) compared to these known VHE sources. However, CR acceleration may occur even at this later evolutionary stage (see for example Yamazaki et al. (2006)). The principal challenge for a scenario involving the Monoceros Loop is to explain the very localised VHE emission at only one point on the SNR limb. The interaction of the SNR with a compact molecular cloud is one possible solution. In this scenario (and indeed any $\pi^{0}$ decay scenario) for the observed $\gamma$-ray emission, a correlation is expected between the TeV emission and the distribution of target material. An unresolved molecular cloud listed in a CO survey at 115 GHz (Oliver et al. 1996) lies rather close to HESS J0623+057, at $l=205.75$ $b=-1.31$. The distance estimate for this cloud (1.6 kpc) is consistent with that for the Monoceros SNR, making it a potential target for hadrons accelerated in the SNR. However, as can be seen clearly in the NANTEN data in Fig. 1, the intensity peak of this cloud is significantly shifted to the East of the H.E.S.S. source. We find no evidence in the NANTEN data for any clouds along the line of sight to the H.E.S.S. source.

3EG J0634+0521 is an unidentified EGRET source Hartman et al. (1999) with positional uncertainties such that HESS J0632+057 lies close to the 99% confidence contour. Given that this source is flagged as possibly extended or confused, a positional coincidence of these two objects seems plausible. Furthermore, the reported third EGRET catalogue flux above 100 MeV ($(25.5\pm 5.1)\times 10^{-8}$ photons cm*-2* s*-1* with a photon index of $2.03\pm 0.26$, see Fig. 3), is consistent with an extrapolation of the H.E.S.S. spectrum. A global fit of the two spectra gives a photon index of 2.41$\pm$0.06.

1RXS J063258.3+054857 is a faint ROSAT source Voges et al. (2000) which lies 36*′′* from the H.E.S.S. source with a positional uncertainty of 21*′′* (see Fig. 1 bottom). Given the uncertainties on the positions of both objects this X-ray source can certainly be considered a potential counterpart of HESS J0632+057. The chance probability of the coincidence of a ROSAT Faint Source Catalogue source within the H.E.S.S. error circle is estimated as 0.1% by scaling the total number of sources in the field of view. The ROSAT source is rather weak, with only 4 counts detected above 0.9 keV, spectral comparison is therefore rather difficult. In the scenario where the $\gamma$-ray emission is interpreted as inverse Compton emission from a population of energetic electrons, the ROSAT source could be naturally ascribed to the synchrotron emission of the same electron population. However, the low level of the X-ray emission ($\sim 10^{-13}$ erg cm*-2* s*-1*) in comparison with the TeV flux ($\sim 10^{-12}$ erg cm*-2* s*-1*) implies a very low magnetic field ($\ll 3\mu$G) unless a strong radiation source exists in the neighbourhood of the emission region and/or the X-ray emission suffers from substantial absorption. Observations at $>4$ keV are required to resolve this absorption issue. In a $\pi^{0}$ decay scenario for the $\gamma$-ray source, secondary electron production via muon decay is expected along with $\gamma$-ray emission. The synchrotron emission of these secondary electrons would in general produce a weaker X-ray source than the IC scenario, probably compatible with the measured ROSAT flux.

MWC 148 (HD 259440) is a massive emission-line star of spectral type B0pe which lies within the H.E.S.S. error circle. The chance probability of this coincidence is hard to assess, as there was no a-priori selection of stellar objects as potential $\gamma$-ray sources. However, given the presence of only 3 Be-type stars in the field of view of the H.E.S.S. observation (see Fig. 1) and the solid angle of the H.E.S.S. error circle, the naive chance probability of the association is $10^{-4}$. Stars of this spectral type have winds with typical velocities and mass loss rates of 1000 km s*-1* and $10^{-7}M_{\odot}/$year, respectively. Plausible acceleration sites are in strong internal or external shocks of the stellar wind. We estimate that an efficiency of 1-10% in the conversion of the kinetic energy of the wind into $\gamma$-ray emission would be required to explain the H.E.S.S. flux (assuming this star lies at the distance of the Rosette Nebula). However, as no associations of similar stars with point-like $\gamma$-ray sources were found in the H.E.S.S. survey of the inner Galaxy, this scenario seems rather unlikely.

A related possibility is that MWC 148 is part of a binary system with an, as yet undetected, compact companion. Such a system might then resemble the known VHE $\gamma$-ray source PSR B1259-63/SS 2883 Aharonian et al. (2005a). Further multi-wavelength observations are required to confirm or refute this scenario.

Acknowledgements.

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 French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Particle Physics and Astronomy Research Council (PPARC), the IPNP of the Charles University, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. 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. The NANTEN project is financially supported from JSPS (Japan Society for the Promotion of Science) Core-to-Core Program, MEXT Grant-in-Aid for Scientific Research on Priority Areas, and SORST-JST (Solution Oriented Research for Science and Technology: Japan Science and Technology Agency). We would also like to thank Stan Owocki and James Urquhart for very useful discussions.