HESS J1858+020 is a weak gamma-ray source that does not have any clear
cataloged counterpart at any wavelengths. Recently, the source G35.6-0.4 was
re-identified as a SNR. The HESS source lies towards the southern border of
this remnant. The purpose of this work is to investigate the interstellar
medium around the mentioned sources in order to look for possible counterparts
of the very-high energy emission. Using the 13CO J=1-0 line from the Galactic
Ring Survey and mid-IR data from GLIMPSE we analyze the environs of HESS
J1858+020 and SNR G35.6-0.4. The 13CO data show the presence of a molecular
cloud towards the southern border of SNR G35.6-0.4 and at the same distance as
the remnant. This cloud is composed by two molecular clumps, one, over the SNR
shell and the other located at the center of HESS J1858+020. We estimate a
molecular mass and a density of ~ 5 X 10^{3} Msun and ~ 500 cm^{-3},
respectively for each clump. Considering the gamma-ray flux observed towards
HESS J1858+020, we estimate that a molecular cloud with a density of at least
150 cm^{-3} could explain the very-high energy emission hadronically. Thus, we
suggest that the gamma-ray emission detected in HESS J1858+020 is due to
hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the
region is active in star formation, which could be considered as an alternative
or complementary possibility to explain the very-high energy emission.Comment: 6 pages, 3 figures, accepted for publication in A&A Lette

Giacani, E.

Paron, S.

Astronomy and Astrophysics

English

arXiv

S. Paron

E. Giacani

CiteSeerX

Identifying the counterpart of HESS J1858+020

Aims. HESS J1858+020 is a weak γ-ray source that does not have any clear cataloged counterpart
at any wavelengths. Recently, the source G35.6-0.4 was re-identified as a SNR. The HESS source lies towards the 
southern border of this remnant. The purpose of this work is to investigate the interstellar medium around 
the mentioned sources to look for possible counterparts of the very high energy emission.
        Methods. Using the 13CO 
J=1–0 line from the Galactic Ring Survey and mid-IR data from GLIMPSE we analyze the environs of 
HESS J1858+020 and SNR G35.6-0.4.
        Results. The 13CO data show the presence of a molecular cloud towards the southern 
border of SNR G35.6-0.4 and at the same distance as the remnant. This cloud is composed of two molecular clumps, 
one over the SNR shell and the other located at the center of HESS J1858+020. We estimate a molecular mass 
and a density of $\sim $$5 \times 10^{3}$ $M_\odot$ and ~500 cm-3 respectively for
each clump. Considering the gamma-ray flux observed towards HESS J1858+020,
we estimate that a molecular cloud with a density of at least 150 cm-3 could explain the
very high energy emission hadronically. Thus, we suggest that the γ-ray emission detected 
in HESS J1858+020 is due to hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the region is 
active in star formation, which could be considered as an alternative or complementary possibility to explain 
the very high energy emission

EDP Sciences OAI-PMH repository (1.2.0)

Aims: HESS J1858+020 is a weak γ-ray source that does not  have any clear cataloged counterpart at any wavelengths. Recently, the  source G35.6-0.4 was re-identified as a SNR. The HESS source lies  towards the southern border of this remnant. The purpose of this work is  to investigate the interstellar medium around the mentioned sources to  look for possible counterparts of the very high energy emission.   Methods: Using the 13CO J=1-0 line from the Galactic Ring  Survey and mid-IR data from GLIMPSE we analyze the environs of HESS  J1858+020 and SNR G35.6-0.4.  Results: The 13CO data  show the presence of a molecular cloud towards the southern border of  SNR G35.6-0.4 and at the same distance as the remnant. This cloud is  composed of two molecular clumps, one over the SNR shell and the other  located at the center of HESS J1858+020. We estimate a molecular mass  and a density of ˜ 5 × 103 M_ȯ and ~500  cm-3 respectively for each clump. Considering the gamma-ray  flux observed towards HESS J1858+020, we estimate that a molecular cloud  with a density of at least 150 cm-3 could explain the very  high energy emission hadronically. Thus, we suggest that the γ-ray  emission detected in HESS J1858+020 is due to hadronic mechanism.  Additionally, analyzing mid-IR emission, we find that the region is  active in star formation, which could be considered as an alternative or  complementary possibility to explain the very high energy emission.Fil: Paron, Sergio Ariel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Giacani, Elsa Beatriz. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin

Paron, Sergio Ariel

Giacani, Elsa Beatriz

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

A&A 509, L4 (2010)DOI: 10.1051/0004-6361/200913694c© ESO 2010Astronomy&AstrophysicsLetter to the EditorIdentifying the counterpart of HESS J1858+020S. Paron and E. GiacaniInstituto de Astronomía y Física del Espacio (CONICET-UBA), CC 67, Suc. 28, 1428 Buenos Aires, Argentinae-mail: sparon@iafe.uba.arReceived 18 November 2009 / Accepted 14 December 2009ABSTRACTAims. HESS J1858+020 is a weak γ-ray source that does not have any clear cataloged counterpart at any wavelengths. Recently, thesource G35.6-0.4 was re-identified as a SNR. The HESS source lies towards the southern border of this remnant. The purpose of thiswork is to investigate the interstellar medium around the mentioned sources to look for possible counterparts of the very high energyemission.Methods. Using the 13CO J = 1–0 line from the Galactic Ring Survey and mid-IR data from GLIMPSE we analyze the environs ofHESS J1858+020 and SNR G35.6-0.4.Results. The 13CO data show the presence of a molecular cloud towards the southern border of SNR G35.6-0.4 and at the samedistance as the remnant. This cloud is composed of two molecular clumps, one over the SNR shell and the other located at thecenter of HESS J1858+020. We estimate a molecular mass and a density of ∼5 × 103 M and ∼500 cm−3 respectively for eachclump. Considering the gamma-ray flux observed towards HESS J1858+020, we estimate that a molecular cloud with a density ofat least 150 cm−3 could explain the very high energy emission hadronically. Thus, we suggest that the γ-ray emission detected inHESS J1858+020 is due to hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the region is active in starformation, which could be considered as an alternative or complementary possibility to explain the very high energy emission.Key words. ISM: clouds – ISM: supernova remnants – gamma rays: ISM – ISM: individual objects: HESS J1858+0201. IntroductionAmong the rich population of TeV γ-ray sources, several areassociated with Galactic phenomena which include supernovaremnants (SNRs), pulsar wind nebulae (PWNe) and binary sys-tems (Hinton 2008). Extragalactic TeV sources are associatedwith active AGNs (mostly blazars). There are however a numberof γ-ray sources which lack as yet a clear identification or an es-tablished origin. In this paper we focus on the very high energysource HESS J1858+020.HESS J1858+020 is a weak γ-ray source detected withHESS (the High Energy Stereoscopic System), an array of airCherenkov telescopes. Though it is a nearly point-like source,its morphology shows a slight extension of ∼5′ along its majoraxis. The source has been detected at a significance level of 7σwith a differential spectral index of 2.2 ± 0.1. At the present,HESS J1858+020 does not have any clear cataloged counterpartat any wavelengths. The pulsar PSR J1858+0143, which is en-ergetic enough to power HESS J1858+020, is located far awayfrom the center of the gamma source, thus an association be-tween them is unlikely (Aharonian et al. 2008a).Recently Green (2009), using radio continuum data from theVery Large Array (VLA) Galactic Plane Survey (VGPS, Stilet al. 2006), re-identified the radio continuum source G35.6-0.4as a supernova remnant (SNR). This remnant is seen in projec-tion over the northern border of HESS J1858+020. Green (2009)estimated an age of 30 000 years for the SNR and a distance of∼10.5 kpc based on the proximity of the remnant with the HII re-gion G35.5-0.0. As pointed out in several works (see Aharonian& Atoyan 1996; Yamazaki et al. 2006; Gabici et al. 2007, 2009),an SNR, in particular an old remnant (i.e. age larger than a few10 000 years), interacting with a molecular cloud could explainthe origin of the γ-ray emission via pion decay from proton-proton collisions. In this context, we expect a correlation be-tween γ-ray emission and matter concentration.In this letter we study the interstellar medium around thesources G35.6-0.4 and HESS J1858+020 to investigate the pos-sible origin for the very high energy emission.2. DataTo analyze the ISM towards HESS J1858+020 we used the 13COJ = 1–0 emission obtained from the Galactic Ring Survey. Thissurvey maps the Galactic Ring with an angular and spectral res-olution of 46′′ and 0.2 km s−1, respectively (see Jackson et al.2006). We also used the mosaiced images from GLIMPSE andMIPSGAL and the GLIMPSE Point-Source Catalog (GPSC) inthe Spitzer-IRAC (3.6, 4.5, 5.8 and 8 μm). IRAC has an an-gular resolution between 1.′′5 and 1.′′9 (see Fazio et al. 2004;Werner et al. 2004). MIPSGAL is a survey of the same regionas GLIMPSE, using the MIPS instrument (24 and 70 μm) onSpitzer. The MIPSGAL resolution at 24 μm is 6′′.3. Results and discussionAnalyzing the whole 13CO J = 1–0 data cube in a region con-taining SNR G25.6-0.4 and HESS J1858+020, we find a molec-ular cloud lying in the southern border of the remnant in thevelocity range between 51 and 59 km s−1. This velocity rangecoincides with the systemic velocity of G25.6-0.4 derived fromits estimated distance (Green 2009). Figure 1 shows a three colorimage where red represents the Spitzer-IRAC 8 μm band, greenArticle published by EDP Sciences Page 1 of 3A&A 509, L4 (2010)35.700 35.600 35.500-0.300-0.400-0.500-0.600Galactic longitudeGalactic latitudeHESS J1858+020Fig. 1. Three color image of the analyzed region (red= 8 μm,green= 24 μm, and blue= 20 cm radio continuum emission). The blackcontours delineate the radio continuum emission with levels of 20 and28 K. The white contours are the 13CO J = 1–0 emission integratedbetween 51 and 59 km s−1, its levels are 3.2 and 5 K km s−1. The yellowcircle represents the source HESS J1858+020.is the MIPSGAL emission at 24 μm and blue is the radio con-tinuum emission of G35.6-0.4 at 20 cm from the VGPS. Blackcontours delineate this last emission, and white contours are the13CO J = 1–0 emission integrated between 51 and 59 km s−1. Asmentioned above, the extension of the HESS source is slightlyelliptical with ∼5′ along its major axis. As the extent and an-gle are not well constrained, we represent in Fig. 1 the posi-tion and the extension of HESS J1858+020 with a circle of5′ in diameter. Can be seen from this figure that the molec-ular cloud presents two well-defined clumps, one centered atl = 35.◦60, b = −0.◦53 over the SNR shell, and the other cen-tered at l = 35.◦58, b = −0.◦58, in coincidence with the center ofHESS J1858+020. The γ-ray emission could be related to bothmolecular clumps.By inspecting the 13CO spectra we found that the clump lo-cated over the SNR shell shows some possible kinematical ev-idence of shocked gas. As Fig. 2 displays, the spectra are notsymmetric and present a slight spectral shoulder or a less intensecomponent at “blueshifted” velocities. It could be evidence ofturbulent motion in the gas, may be produced by the SNR shock(see e.g. Falgarone et al. 1994). Taking into account the posi-tional and kinematical agreement between the molecular gas andthe SNR G35.6-0.4, we suggest that the remnant is interactingwith the adjacent molecular cloud.Using the 13CO J = 1–0 line and assuming local thermo-dynamic equilibrium (LTE) we estimate the H2 column densitytowards the molecular clumps described above. We useN(13CO) = 2.42 × 1014 Tex∫τ13dv1 − exp(−5.29/Tex)to obtain the 13CO column density. τ13 is the optical depth ofthe line, we assume that Tex is 10 K and the 13CO emission isoptically thin. We use the relation N(H2)/N(13CO)∼ 5×105 (e.g.Simon et al. 2001). Finally, we estimate a molecular mass and a50 60Velocity (km/s)24T (K)Fig. 2. 13CO J = 1–0 spectra obtained towards the maximum of themolecular clump that lies over the SNR shell. The spectra, which showbrightness temperature vs. velocity, correspond to nine positions ob-served in a region of about 50′′ × 50′′ centered at the molecular clumppeak.density of ∼5 × 103 M and ∼500 cm−3, respectively, for eachmolecular clump. The mass value was obtained fromM = μ mH∑[D2 Ω N(H2)],where Ω is the solid angle subtended by the 13CO J = 1–0 beamsize, mH is the hydrogen mass, while μ, the mean molecularweight, is assumed to be 2.8 by taking into account a relativehelium abundance of 25%, and D is the distance assumed to be10.5 kpc. Summation was performed over all the observed posi-tions within the 3.5 K km s−1s contour level (not shown in Figs. 1and 3).We can estimate the required density matter in the γ-ray pro-duction region for hadrons from the observed γ-ray flux of HESSJ1858+020. FromdFγ/dE(> Emin) = N0(E/1 TeV)−Γwe obtain Fγ ∼ 1.2 × 10−12 cm−2 s−1. Here Emin = 0.5 TeV, Γ =2.17, N0 = 0.6× 10−12 cm−2 s−1 (Aharonian et al. 2008a). UsingEq. (16) in Torres et al. (2003), we obtain a density of about150 cm−3 assuming an acceleration efficiency of hadrons on theorder of 3% and a supernova power of 1051 erg. Thus the molec-ular gas in positionally coincidence with HESS J1858+020is densely enough to generate the very high energy emissionhadronically.The presence of at least two bright sources embedded in thesouthern molecular clump can be seen in Fig. 1 in the IR emis-sion of the region studied here. The mid-IR emission also showsthe presence of hot dust and policyclic aromatic hydrocarbons(PAHs), green and red in Fig. 1, respectively, which suggestsan active star forming region. In order to confirm this, we car-ried out an IR photometric study of the sources that lie over themolecular clump that is centered at the HESS source position.In Fig. 3 we present a three color image with threeIRAC-Spitzer bands: red is 8 μm, green is 4.5 μm and blueis 3.6 μm. As in Fig. 1 the white contours are the 13COJ = 1–0 emission integrated between 51 and 59 km s−1. Tolook for tracers of star formation activity, we use the GLIMPSEPoint-Source Catalog to perform photometry. Considering onlyPage 2 of 3S. Paron and E. Giacani: Indentifying the counterpart of HESS J1858+02035.640 35.600 35.560 35.520-0.480-0.500-0.520-0.540-0.560-0.580-0.600-0.620Galactic longitudeGalactic latitudeHESS J1858+020Fig. 3. Three color image of IRAC-Spitzer bands (red= 8 μm,green= 4.5 μm, and blue= 3.6 μm). The white contours are the 13COJ = 1–0 emission integrated between 51 and 59 km s−1, its levels are 3.2and 5 K km s−1. The crosses are YSO candidates according to the pho-tometric study described in the text. Green crosses are Class I sourcesand cyan crosses represent Class II sources.sources that have been detected in the four Spitzer-IRAC bands,we found 32 sources within a circular area of 80′′ in radius,centered at the molecular clump. According to the criteriapresented by Allen et al. (2004), we found six sources as youngstellar object (YSO) candidates. Two of them are Class I objects(protostar with circumstellar envelope; green crosses in Fig. 3),and the others are Class II objects (young stars with disk-onlyemission; cyan crosses in Fig. 3). From Fig. 3 a bright sourcecan be noticed that lies almost at the center of the molecularclump, but in this case we cannot perform photometry sinceit has some bands with null fluxes in the GLIMPSE catalog.However, from Fig. 3 it can be appreciated that this sourceappears slightly extended in the 4.5 μm emission (green), whichaccording to Cyganowski et al. (2008) may suggest that itcould be a MYSO that drives outflows. For the sources thatare also detected in the 2MASS JHK-bands, we derive theirspectral energy distribution (SED) by fitting the fluxes using thetool developed by Robitaille et al. (2007), which is availableonline1. This was possible only for three Class II sources, withthe result that they are indeed young objects (∼105 years). Weconclude that this region is active in star formation, which canbe considered as an alternative or complementary possibilityto explain the very high energy emission. HESS J1858+020could be a similar case as SNR W28. Several molecularclumps and star-forming regions were found towards W28and proposed to be candidates to explain the very-high energyemission detected in the region (Aharonian et al. 2008b).Further sub-mm observations, high CO transitions and tracers of1 http://caravan.astro.wisc.edu/protostars/star formation will provide more accurate mass and density es-timates and allow us to search for perturbed gas by both SNRshock and star formation processes.4. SummaryIn this work we investigate the ISM towards the southern borderof the recently re-identified source G35.6-0.4 as a SNR, whichcoincides with the northern portion of the very high energysource HESS J1858+020. Analyzing the 13CO J = 1–0 emis-sion we find a molecular cloud in positional and kinematicalagreement with the SNR, which suggests that the moleculargas is affected by the remnant shock. The discovered molecularcloud is composed by two clumps, each with a mass and den-sity of ∼5 × 103 M and ∼500 cm−3, respectively. Consideringthe gamma-ray flux observed towards HESS J1858+020, we es-timate that a molecular cloud with a density of at least 150 cm−3could explain the very high energy emission hadronically. Thuswe suggest that the interaction between the SNR G35.6-0.4and the molecular cloud is responsible for the γ-ray emission.Additionally, analyzing mid-IR emission, we find that the re-gion is active in star formation, which could be considered asan alternative or complementary possibility to explain the veryhigh energy emission. This SNR-molecular cloud-HESS sourcecomplex could be a similar case as the high-energy sources de-tected towards SNR W28. Further sub-mm observations willprovide more accurate mass and density estimates and allow usto search for perturbed gas by both SNR shock and star forma-tion processes.Acknowledgements. S.P. and E.G. are members of the Carrera del investigadorcientífico of CONICET, Argentina. This work was partially supported by theCONICET grant PIP 112-200801-02166, UBACYT A023 and ANPCYT PICT-2007-00902.ReferencesAharonian, F. A., & Atoyan, A. M. 1996, A&A, 309, 917Aharonian, F., Akhperjanian, A. G., Barres de Almeida, U., et al. 2008a, A&A,477, 353Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2008b, A&A, 481,401Allen, L. E., Calvet, N., D’Alessio, P., et al. 2004, ApJS, 154, 363Cyganowski, C. J., Whitney, B. A., Holden, E., et al. 2008, AJ, 136, 2391Falgarone, E., Lis, D. C., Phillips, T. G., et al. 1994, ApJ, 436, 728Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154, 10Gabici, S., Aharonian, F. A., & Blasi, P. 2007, Ap&SS, 309, 365Gabici, S., Aharonian, F. A., & Casanova, S. 2009, MNRAS, 396, 1629Green, D. A. 2009, MNRAS, 399, 177Hinton, J. 2008, J. Phys. Conf. Ser., 110, 062011Jackson, J. M., Rathborne, J. M., Shah, R. Y., et al. 2006, ApJS, 163, 145Robitaille, T. P., Whitney, B. A., Indebetouw, R., & Wood, K. 2007, ApJS, 169,328Simon, R., Jackson, J. M., Clemens, D. P., Bania, T. M., & Heyer, M. H. 2001,ApJ, 551, 747Stil, J. M., Taylor, A. R., Dickey, J. M., et al. 2006, AJ, 132, 1158Torres, D. F., Romero, G. E., Dame, T. M., Combi, J. A., & Butt, Y. M. 2003,Phys. Rep., 382, 303Werner, M. W., Roellig, T. L., Low, F. J., et al. 2004, ApJS, 154, 1Yamazaki, R., Kohri, K., Bamba, A., et al. 2006, MNRAS, 371, 1975Page 3 of 3

CONICET Digital

A&A 509, L4 (2010)DOI: 10.1051/0004-6361/200913694c© ESO 2010Astronomy&AstrophysicsLetter to the EditorIdentifying the counterpart of HESS J1858+020S. Paron and E. GiacaniInstituto de Astronomía y Física del Espacio (CONICET-UBA), CC 67, Suc. 28, 1428 Buenos Aires, Argentinae-mail: sparon@iafe.uba.arReceived 18 November 2009 / Accepted 14 December 2009ABSTRACTAims. HESS J1858+020 is a weak γ-ray source that does not have any clear cataloged counterpart at any wavelengths. Recently, thesource G35.6-0.4 was re-identified as a SNR. The HESS source lies towards the southern border of this remnant. The purpose of thiswork is to investigate the interstellar medium around the mentioned sources to look for possible counterparts of the very high energyemission.Methods. Using the 13CO J = 1–0 line from the Galactic Ring Survey and mid-IR data from GLIMPSE we analyze the environs ofHESS J1858+020 and SNR G35.6-0.4.Results. The 13CO data show the presence of a molecular cloud towards the southern border of SNR G35.6-0.4 and at the samedistance as the remnant. This cloud is composed of two molecular clumps, one over the SNR shell and the other located at thecenter of HESS J1858+020. We estimate a molecular mass and a density of ∼5 × 103 M and ∼500 cm−3 respectively for eachclump. Considering the gamma-ray flux observed towards HESS J1858+020, we estimate that a molecular cloud with a density ofat least 150 cm−3 could explain the very high energy emission hadronically. Thus, we suggest that the γ-ray emission detected inHESS J1858+020 is due to hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the region is active in starformation, which could be considered as an alternative or complementary possibility to explain the very high energy emission.Key words. ISM: clouds – ISM: supernova remnants – gamma rays: ISM – ISM: individual objects: HESS J1858+0201. IntroductionAmong the rich population of TeV γ-ray sources, several areassociated with Galactic phenomena which include supernovaremnants (SNRs), pulsar wind nebulae (PWNe) and binary sys-tems (Hinton 2008). Extragalactic TeV sources are associatedwith active AGNs (mostly blazars). There are however a numberof γ-ray sources which lack as yet a clear identification or an es-tablished origin. In this paper we focus on the very high energysource HESS J1858+020.HESS J1858+020 is a weak γ-ray source detected withHESS (the High Energy Stereoscopic System), an array of airCherenkov telescopes. Though it is a nearly point-like source,its morphology shows a slight extension of ∼5′ along its majoraxis. The source has been detected at a significance level of 7σwith a diﬀerential spectral index of 2.2 ± 0.1. At the present,HESS J1858+020 does not have any clear cataloged counterpartat any wavelengths. The pulsar PSR J1858+0143, which is en-ergetic enough to power HESS J1858+020, is located far awayfrom the center of the gamma source, thus an association be-tween them is unlikely (Aharonian et al. 2008a).Recently Green (2009), using radio continuum data from theVery Large Array (VLA) Galactic Plane Survey (VGPS, Stilet al. 2006), re-identified the radio continuum source G35.6-0.4as a supernova remnant (SNR). This remnant is seen in projec-tion over the northern border of HESS J1858+020. Green (2009)estimated an age of 30 000 years for the SNR and a distance of∼10.5 kpc based on the proximity of the remnant with the HII re-gion G35.5-0.0. As pointed out in several works (see Aharonian& Atoyan 1996; Yamazaki et al. 2006; Gabici et al. 2007, 2009),an SNR, in particular an old remnant (i.e. age larger than a few10 000 years), interacting with a molecular cloud could explainthe origin of the γ-ray emission via pion decay from proton-proton collisions. In this context, we expect a correlation be-tween γ-ray emission and matter concentration.In this letter we study the interstellar medium around thesources G35.6-0.4 and HESS J1858+020 to investigate the pos-sible origin for the very high energy emission.2. DataTo analyze the ISM towards HESS J1858+020 we used the 13COJ = 1–0 emission obtained from the Galactic Ring Survey. Thissurvey maps the Galactic Ring with an angular and spectral res-olution of 46′′ and 0.2 km s−1, respectively (see Jackson et al.2006). We also used the mosaiced images from GLIMPSE andMIPSGAL and the GLIMPSE Point-Source Catalog (GPSC) inthe Spitzer-IRAC (3.6, 4.5, 5.8 and 8 μm). IRAC has an an-gular resolution between 1.′′5 and 1.′′9 (see Fazio et al. 2004;Werner et al. 2004). MIPSGAL is a survey of the same regionas GLIMPSE, using the MIPS instrument (24 and 70 μm) onSpitzer. The MIPSGAL resolution at 24 μm is 6′′.3. Results and discussionAnalyzing the whole 13CO J = 1–0 data cube in a region con-taining SNR G25.6-0.4 and HESS J1858+020, we find a molec-ular cloud lying in the southern border of the remnant in thevelocity range between 51 and 59 km s−1. This velocity rangecoincides with the systemic velocity of G25.6-0.4 derived fromits estimated distance (Green 2009). Figure 1 shows a three colorimage where red represents the Spitzer-IRAC 8 μm band, greenArticle published by EDP Sciences Page 1 of 3A&A 509, L4 (2010)35.700 35.600 35.500-0.300-0.400-0.500-0.600Galactic longitudeGalactic latitudeHESS J1858+020Fig. 1. Three color image of the analyzed region (red= 8 μm,green= 24 μm, and blue= 20 cm radio continuum emission). The blackcontours delineate the radio continuum emission with levels of 20 and28 K. The white contours are the 13CO J = 1–0 emission integratedbetween 51 and 59 km s−1, its levels are 3.2 and 5 K km s−1. The yellowcircle represents the source HESS J1858+020.is the MIPSGAL emission at 24 μm and blue is the radio con-tinuum emission of G35.6-0.4 at 20 cm from the VGPS. Blackcontours delineate this last emission, and white contours are the13CO J = 1–0 emission integrated between 51 and 59 km s−1. Asmentioned above, the extension of the HESS source is slightlyelliptical with ∼5′ along its major axis. As the extent and an-gle are not well constrained, we represent in Fig. 1 the posi-tion and the extension of HESS J1858+020 with a circle of5′ in diameter. Can be seen from this figure that the molec-ular cloud presents two well-defined clumps, one centered atl = 35.◦60, b = −0.◦53 over the SNR shell, and the other cen-tered at l = 35.◦58, b = −0.◦58, in coincidence with the center ofHESS J1858+020. The γ-ray emission could be related to bothmolecular clumps.By inspecting the 13CO spectra we found that the clump lo-cated over the SNR shell shows some possible kinematical ev-idence of shocked gas. As Fig. 2 displays, the spectra are notsymmetric and present a slight spectral shoulder or a less intensecomponent at “blueshifted” velocities. It could be evidence ofturbulent motion in the gas, may be produced by the SNR shock(see e.g. Falgarone et al. 1994). Taking into account the posi-tional and kinematical agreement between the molecular gas andthe SNR G35.6-0.4, we suggest that the remnant is interactingwith the adjacent molecular cloud.Using the 13CO J = 1–0 line and assuming local thermo-dynamic equilibrium (LTE) we estimate the H2 column densitytowards the molecular clumps described above. We useN(13CO) = 2.42 × 1014 Tex∫τ13dv1 − exp(−5.29/Tex)to obtain the 13CO column density. τ13 is the optical depth ofthe line, we assume that Tex is 10 K and the 13CO emission isoptically thin. We use the relation N(H2)/N(13CO)∼ 5×105 (e.g.Simon et al. 2001). Finally, we estimate a molecular mass and a50 60Velocity (km/s)24T (K)Fig. 2. 13CO J = 1–0 spectra obtained towards the maximum of themolecular clump that lies over the SNR shell. The spectra, which showbrightness temperature vs. velocity, correspond to nine positions ob-served in a region of about 50′′ × 50′′ centered at the molecular clumppeak.density of ∼5 × 103 M and ∼500 cm−3, respectively, for eachmolecular clump. The mass value was obtained fromM = μ mH∑[D2 Ω N(H2)],where Ω is the solid angle subtended by the 13CO J = 1–0 beamsize, mH is the hydrogen mass, while μ, the mean molecularweight, is assumed to be 2.8 by taking into account a relativehelium abundance of 25%, and D is the distance assumed to be10.5 kpc. Summation was performed over all the observed posi-tions within the 3.5 K km s−1s contour level (not shown in Figs. 1and 3).We can estimate the required density matter in the γ-ray pro-duction region for hadrons from the observed γ-ray flux of HESSJ1858+020. FromdFγ/dE(> Emin) = N0(E/1 TeV)−Γwe obtain Fγ ∼ 1.2 × 10−12 cm−2 s−1. Here Emin = 0.5 TeV, Γ =2.17, N0 = 0.6× 10−12 cm−2 s−1 (Aharonian et al. 2008a). UsingEq. (16) in Torres et al. (2003), we obtain a density of about150 cm−3 assuming an acceleration eﬃciency of hadrons on theorder of 3% and a supernova power of 1051 erg. Thus the molec-ular gas in positionally coincidence with HESS J1858+020is densely enough to generate the very high energy emissionhadronically.The presence of at least two bright sources embedded in thesouthern molecular clump can be seen in Fig. 1 in the IR emis-sion of the region studied here. The mid-IR emission also showsthe presence of hot dust and policyclic aromatic hydrocarbons(PAHs), green and red in Fig. 1, respectively, which suggestsan active star forming region. In order to confirm this, we car-ried out an IR photometric study of the sources that lie over themolecular clump that is centered at the HESS source position.In Fig. 3 we present a three color image with threeIRAC-Spitzer bands: red is 8 μm, green is 4.5 μm and blueis 3.6 μm. As in Fig. 1 the white contours are the 13COJ = 1–0 emission integrated between 51 and 59 km s−1. Tolook for tracers of star formation activity, we use the GLIMPSEPoint-Source Catalog to perform photometry. Considering onlyPage 2 of 3S. Paron and E. Giacani: Indentifying the counterpart of HESS J1858+02035.640 35.600 35.560 35.520-0.480-0.500-0.520-0.540-0.560-0.580-0.600-0.620Galactic longitudeGalactic latitudeHESS J1858+020Fig. 3. Three color image of IRAC-Spitzer bands (red= 8 μm,green= 4.5 μm, and blue= 3.6 μm). The white contours are the 13COJ = 1–0 emission integrated between 51 and 59 km s−1, its levels are 3.2and 5 K km s−1. The crosses are YSO candidates according to the pho-tometric study described in the text. Green crosses are Class I sourcesand cyan crosses represent Class II sources.sources that have been detected in the four Spitzer-IRAC bands,we found 32 sources within a circular area of 80′′ in radius,centered at the molecular clump. According to the criteriapresented by Allen et al. (2004), we found six sources as youngstellar object (YSO) candidates. Two of them are Class I objects(protostar with circumstellar envelope; green crosses in Fig. 3),and the others are Class II objects (young stars with disk-onlyemission; cyan crosses in Fig. 3). From Fig. 3 a bright sourcecan be noticed that lies almost at the center of the molecularclump, but in this case we cannot perform photometry sinceit has some bands with null fluxes in the GLIMPSE catalog.However, from Fig. 3 it can be appreciated that this sourceappears slightly extended in the 4.5 μm emission (green), whichaccording to Cyganowski et al. (2008) may suggest that itcould be a MYSO that drives outflows. For the sources thatare also detected in the 2MASS JHK-bands, we derive theirspectral energy distribution (SED) by fitting the fluxes using thetool developed by Robitaille et al. (2007), which is availableonline1. This was possible only for three Class II sources, withthe result that they are indeed young objects (∼105 years). Weconclude that this region is active in star formation, which canbe considered as an alternative or complementary possibilityto explain the very high energy emission. HESS J1858+020could be a similar case as SNR W28. Several molecularclumps and star-forming regions were found towards W28and proposed to be candidates to explain the very-high energyemission detected in the region (Aharonian et al. 2008b).Further sub-mm observations, high CO transitions and tracers of1 http://caravan.astro.wisc.edu/protostars/star formation will provide more accurate mass and density es-timates and allow us to search for perturbed gas by both SNRshock and star formation processes.4. SummaryIn this work we investigate the ISM towards the southern borderof the recently re-identified source G35.6-0.4 as a SNR, whichcoincides with the northern portion of the very high energysource HESS J1858+020. Analyzing the 13CO J = 1–0 emis-sion we find a molecular cloud in positional and kinematicalagreement with the SNR, which suggests that the moleculargas is aﬀected by the remnant shock. The discovered molecularcloud is composed by two clumps, each with a mass and den-sity of ∼5 × 103 M and ∼500 cm−3, respectively. Consideringthe gamma-ray flux observed towards HESS J1858+020, we es-timate that a molecular cloud with a density of at least 150 cm−3could explain the very high energy emission hadronically. Thuswe suggest that the interaction between the SNR G35.6-0.4and the molecular cloud is responsible for the γ-ray emission.Additionally, analyzing mid-IR emission, we find that the re-gion is active in star formation, which could be considered asan alternative or complementary possibility to explain the veryhigh energy emission. This SNR-molecular cloud-HESS sourcecomplex could be a similar case as the high-energy sources de-tected towards SNR W28. Further sub-mm observations willprovide more accurate mass and density estimates and allow usto search for perturbed gas by both SNR shock and star forma-tion processes.Acknowledgements. S.P. and E.G. are members of the Carrera del investigadorcientífico of CONICET, Argentina. This work was partially supported by theCONICET grant PIP 112-200801-02166, UBACYT A023 and ANPCYT PICT-2007-00902.ReferencesAharonian, F. A., & Atoyan, A. M. 1996, A&A, 309, 917Aharonian, F., Akhperjanian, A. G., Barres de Almeida, U., et al. 2008a, A&A,477, 353Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2008b, A&A, 481,401Allen, L. E., Calvet, N., D’Alessio, P., et al. 2004, ApJS, 154, 363Cyganowski, C. J., Whitney, B. A., Holden, E., et al. 2008, AJ, 136, 2391Falgarone, E., Lis, D. C., Phillips, T. G., et al. 1994, ApJ, 436, 728Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154, 10Gabici, S., Aharonian, F. A., & Blasi, P. 2007, Ap&SS, 309, 365Gabici, S., Aharonian, F. A., & Casanova, S. 2009, MNRAS, 396, 1629Green, D. A. 2009, MNRAS, 399, 177Hinton, J. 2008, J. Phys. Conf. Ser., 110, 062011Jackson, J. M., Rathborne, J. M., Shah, R. Y., et al. 2006, ApJS, 163, 145Robitaille, T. P., Whitney, B. A., Indebetouw, R., & Wood, K. 2007, ApJS, 169,328Simon, R., Jackson, J. M., Clemens, D. P., Bania, T. M., & Heyer, M. H. 2001,ApJ, 551, 747Stil, J. M., Taylor, A. R., Dickey, J. M., et al. 2006, AJ, 132, 1158Torres, D. F., Romero, G. E., Dame, T. M., Combi, J. A., & Butt, Y. M. 2003,Phys. Rep., 382, 303Werner, M. W., Roellig, T. L., Low, F. J., et al. 2004, ApJS, 154, 1Yamazaki, R., Kohri, K., Bamba, A., et al. 2006, MNRAS, 371, 1975Page 3 of 3

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Identifying the counterpart of HESS J1858+020

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