Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
aa r X i v : . [ a s t r o - ph ] M a r Astronomy&Astrophysicsmanuscript no. HESS˙RGB0152 c (cid:13)
ESO 2018October 30, 2018 L etter to the E ditor Discovery of VHE γ -rays from the high-frequency-peakedBL Lac object RGB J0152 + F. Aharonian , , A.G. Akhperjanian , U. Barres de Almeida ⋆ , A.R. Bazer-Bachi , B. Behera , M. Beilicke ,W. Benbow , K. Bernl¨ohr , , C. Boisson , V. Borrel , I. Braun , E. Brion , J. Brucker , R. B ¨uhler , T. Bulik ,I. B ¨usching , T. Boutelier , S. Carrigan , P.M. Chadwick , R.C.G. Chaves , L.-M. Chounet , A.C. Clapson ,G. Coignet , R. Cornils , L. Costamante , , M. Dalton , B. Degrange , H.J. Dickinson , A. Djannati-Ata¨ı ,W. Domainko , L.O’C. Drury , F. Dubois , G. Dubus , J. Dyks , K. Egberts , D. Emmanoulopoulos , P. Espigat ,C. Farnier , F. Feinstein , A. Fiasson , A. F ¨orster , G. Fontaine , M. F ¨ußling , S. Gabici , Y.A. Gallant ,B. Giebels , J.-F. Glicenstein , B. Gl¨uck , P. Goret , C. Hadjichristidis , D. Hauser , M. Hauser , G. Heinzelmann ,G. Henri , G. Hermann , J.A. Hinton , A. Ho ff mann , W. Hofmann , M. Holleran , S. Hoppe , D. Horns ,A. Jacholkowska , O.C. de Jager , I. Jung , K. Katarzy´nski , S. Kaufmann , E. Kendziorra , M. Kerschhaggl ,D. Khangulyan , B. Kh´elifi , D. Keogh , Nu. Komin , K. Kosack , G. Lamanna , I.J. Latham , J.-P. Lenain ,T. Lohse , J.-M. Martin , O. Martineau-Huynh , A. Marcowith , C. Masterson , D. Maurin , T.J.L. McComb ,R. Moderski , E. Moulin , M. Naumann-Godo , M. de Naurois , D. Nedbal , D. Nekrassov , S.J. Nolan , S. Ohm ,J.-P. Olive , E. de O ˜na Wilhelmi , K.J. Orford , J.L. Osborne , M. Ostrowski , M. Panter , G. Pedaletti ,G. Pelletier , P.-O. Petrucci , S. Pita , G. P ¨uhlhofer , M. Punch , A. Quirrenbach , B.C. Raubenheimer ,M. Raue , S.M. Rayner , M. Renaud , F. Rieger , J. Ripken , L. Rob , S. Rosier-Lees , G. Rowell , B. Rudak ,J. Ruppel , V. Sahakian , A. Santangelo , R. Schlickeiser , F.M. Sch¨ock , R. Schr¨oder , U. Schwanke ,S. Schwarzburg , S. Schwemmer , A. Shalchi , H. Sol , D. Spangler , Ł. Stawarz , R. Steenkamp ,C. Stegmann , G. Superina , P.H. Tam , J.-P. Tavernet , R. Terrier , C. van Eldik , G. Vasileiadis , C. Venter ,J.-P. Vialle , P. Vincent , M. Vivier , H.J. V ¨olk , F. Volpe , , S.J. Wagner , M. Ward , A.A. Zdziarski , andA. Zech (A ffi liations can be found after the references) Received 19 February 2008 / Accepted 26 February 2008
ABSTRACT
Aims.
The BL Lac object RGB J0152 +
017 ( z = . >
100 GeV) γ -ray source, due to its highX-ray and radio fluxes. Our aim is to understand the radiative processes by investigating the observed emission and its production mechanismusing the High Energy Stereoscopic System (H.E.S.S.) experiment. Methods.
We report recent observations of the BL Lac source RGB J0152 +
017 made in late October and November 2007 with the H.E.S.S.array consisting of four imaging atmospheric Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and
RXTE satellites, in the optical band with the ATOM telescope, and in the radio band with the Nanc¸ay Radio Telescope.
Results.
A signal of 173 γ -ray photons corresponding to a statistical significance of 6.6 σ was found in the data. The energy spectrum of thesource can be described by a powerlaw with a spectral index of Γ = . ± . stat ± . syst . The integral flux above 300 GeV corresponds to ∼ Conclusions.
RGB J0152 +
017 is discovered as a source of VHE γ -rays by H.E.S.S. The location of its synchrotron peak, as derived from theSED in Swift data, allows clear classification as a high-frequency-peaked BL Lac (HBL).
Key words. galaxies: BL Lacertae objects: individual: RGB J0152 +
017 – gamma rays: observations – galaxies: BL Lacertae objects: general –galaxies: active
1. Introduction
First detected as a radio source (Becker et al. 1991) by theNRAO Green Bank Telescope and in the Parkes-MIT-NRAO
Send o ff print requests to : J.-P. Lenain, D. Nedbale-mail: [email protected],[email protected] ⋆ supported by CAPES Foundation, Ministry of Education of Brazil surveys (Gri ffi th et al. 1995), RGB J0152 +
017 was later iden-tified as a BL Lac object by Laurent-Muehleisen et al. (1998),who located it at z = . +
017 in X-rays in the
ROSAT -Green Bank (RGB)sample. The host is an elliptical galaxy with luminosity M R = − . F. Aharonian et al.: Discovery of VHE γ -rays from the high-frequency-peaked BL Lac object RGB J0152 + X-ray fluxes, making it a good candidate for VHE emission(Costamante & Ghisellini 2002), motivating its observation bythe H.E.S.S. experiment.The broad-band SED of BL Lac objects is typically char-acterised by a double-peaked structure, usually attributed tosynchrotron radiation in the radio-to-X-ray domain and inverseCompton scattering in the γ -ray domain, which is frequentlyexplained by SSC models (see, e.g., Aharonian et al. 2005).However, since the flux of BL Lac objects can be highly vari-able (e.g. Krawczynski et al. 2000), stationary versions of thesemodels are only relevant for contemporaneous multi-wavelengthobservations of a non-flaring state. The contemporaneous ra-dio, optical, X-ray, and VHE observations presented here do notshow any significant variability, and thus enable the first SSCmodelling of the emission of RGB J0152 +
2. H.E.S.S. observations and results
RGB J0152 +
017 was observed by the H.E.S.S. array consistingof four imaging atmospheric Cherenkov telescopes, located inthe Khomas Highland, Namibia (Aharonian et al. 2006a). Theobservations were performed from October 30 to November 14,2007. The data were taken in wobble mode, where the telescopespoint in a direction typically at an o ff set of 0.5 ◦ from the nom-inal target position (Aharonian et al. 2006a). After applying se-lection cuts to the data to reject periods a ff ected by poor weatherconditions and hardware problems, the total live-time used foranalysis amounts to 14.7 h. The mean zenith angle of the obser-vations is 26 . ◦ .The data are calibrated according to Aharonian et al. (2004).Energies are reconstructed taking the e ff ective optical e ffi ciencyevolution into account (Aharonian et al. 2006a). The separa-tion of γ -ray-like events from cosmic-ray background eventswas made using the Hillas moment-analysis technique (Hillas1985). Signal extraction was performed using standard cuts (Aharonian et al. 2006a). The on-source events were taken froma circular region around the target with a radius of θ = . ◦ . The background was estimated using reflected regions (Aharonian et al. 2006a) located at the same o ff set from the cen-tre of the observed field as the on-source region.A signal of 173 γ -ray events is found from the directionof RGB J0152 + σ according to Li & Ma (1983). The preliminary detec-tion was reported by Nedbal et al. (2007). A two-dimensionalGaussian fit of the excess yields a position α J = h m . s ± . s stat ± . s syst , δ J = ◦ ′ . ′′ ± ′′ stat ± ′′ syst . Themeasured position is compatible with the nominal position ofRGB J0152 +
017 ( α J = h m . s δ J = ◦ ′ . ′′ σ level. Given this spatial coincidence, we identify thesource of γ -rays with RGB J0152 + + ff erential spectrum. Thespectrum was derived using standard cuts with an energy thresh-old of 300 GeV. Another set of cuts, the spectrum cuts de-scribed in Aharonian et al. (2006b), were used to lower the en-ergy threshold and improve the photon statistics (factor ∼ standard cuts ). Both give consistent results (seeinlay in Fig. 2 and caption). Because of the better statistics andenergy range, we use the spectrum derived using spectrum cuts in the following. Between the threshold of 240 GeV and 3.8 TeV,this di ff erential spectrum is described well ( χ / d.o.f. = / ] [degrees θ E x ce ss e v e n t s -50050100150 Fig. 1.
Angular distribution of excess events. The dot-dashed lineshows the angular distance cut used for extracting the signal. Theexcess distribution is consistent with the H.E.S.S. point spreadfunction as derived from Monte Carlo simulations (solid line).
Energy ( TeV )1 10 ) - T e V - s - d N / d E ( c m -14 -13 -12 -11 -10 Γ ) - T e V - s - ( T e V ) ( c m Φ -12 × Spectrum cutsStandard cuts
Fig. 2. Di ff erential spectrum of RGB J0152 + spectrum cuts (black closed circles) is comparedwith the one obtained by the standard cuts (blue open circles).The black line shows the best fit by a powerlaw function ofthe former. The three points with the highest photon energyrepresent upper limits at 99% confidence level, calculated us-ing Feldman & Cousins (1998). All error bars are only statis-tical. The fit parameters of a powerlaw fit are Γ = . ± . stat ± . syst and Φ (1TeV) = (5 . ± . stat ± . syst ) × − cm − s − TeV − for the spectrum cuts , and Γ = . ± . stat ± . syst and Φ (1TeV) = (4 . ± . × − cm − s − TeV − for the standard cuts . The insert shows 1 and 2 σ confidence levels ofthe fit parameters.by a power law d N / d E = Φ ( E / − Γ with a photon in-dex Γ = . ± . stat ± . syst and normalisation at 1 TeVof Φ (1TeV) = (5 . ± . stat ± . syst ) × − cm − s − TeV − .The 99% confidence level upper limits for the highest threebins shown in Fig. 2 were calculated using Feldman & Cousins(1998).The integral flux above 300 GeV is I = (2 . ± . stat ± . syst ) × − cm − s − , which corresponds to ∼
2% of the fluxof the Crab nebula above the same threshold as determined byAharonian et al. (2006a). Figure 3 shows the nightly evolution of . Aharonian et al.: Discovery of VHE γ -rays from the high-frequency-peaked BL Lac object RGB J0152 +
017 3
MJD54405 54410 54415 ) - s - I( > G e V ) ( c m -50510 -12 × Fig. 3.
Mean nightly integral flux from RGB J0152 +
017 above300 GeV. Only the statistical errors are shown. Upper limits at99% confidence level are calculated when no signal is found(grey points). The dashed line shows a fit of a constant to thedata points with χ / d . o . f . of 17 . /
12. The fit was performed us-ing all nights.the γ -ray flux above 300 GeV. There is no significant variabilitybetween nights in the lightcurve. The χ / d . o . f . of the fit to aconstant is 17 . /
12, corresponding to a χ probability of 14%.All results were checked with independent analysis proce-dures and calibration chain giving consistent results.
3. Multi-wavelength observations with Swift, RXTE,ATOM, and the Nanc¸ ay Radio Telescope
Swift and
RXTE
Target of opportunity (ToO) observations of RGB J0152 + Swift and
RXTE on November 13, 14, and15, 2007 triggered by the H.E.S.S. discovery.The
Swift / XRT (Burrows et al. 2005) data (5.44 ks) weretaken in photon-counting mode. The spectra were extracted with xselect v2.4 from a circular region with a radius of 20 pixels(0 . ′ ) around the position of RGB J0152 + xrtmkarf v0.5.6 and the response matrices were taken fromthe Swift package of the calibration database caldb v3.4.1 .Due to the low count rate of 0 . / s, any pileup e ff ect on thespectrum is negligible. We find no significant variability dur-ing any of the pointings or between the three subsequent days;hence, individual spectra were combined to achieve better pho-ton statistics. The spectral analysis was performed using the tool Xspec v11.3.2 . A broken powerlaw ( Γ = . ± . , Γ = . ± . , E break = . ± .
12 keV) with a Galactic absorp-tion of 2 . × cm − (LAB Survey, Kalberla et al. 2005)is a good description ( χ / d . o . f . = / F . − ∼ . × − erg cm − s − and F −
10 keV ∼ . × − erg cm − s − .Simultaneous observations at higher X-ray photon energieswere obtained with the RXTE / PCA (Jahoda et al. 1996). Onlydata of PCU2 and the top layer were taken to obtain the bestsignal-to-noise ratio. After filtering out the influence of the SouthAtlantic Anomaly, tracking o ff sets, and the electron contamina-tion, an exposure of 3.2 ks remains. Given the low count rate of0 . / s, the “faint background model” provided by the RXTE
Guest Observer Facility was used to generate the backgroundspectrum with the script pcabackest v3.1 . The response ma-trices were created with pcarsp v10.1 . Again no significantvariations were found between the three observations, and indi- vidual spectra were combined to achieve better photon statistics.The PCA spectrum can be described by an absorbed single pow-erlaw with photon index
Γ = . ± .
08 ( χ / d . o . f . = / Swift data. The resulting flux F −
10 keV ∼ . × − erg cm − s − exceeds the one obtained simultaneously with Swift by a factorof 2.5. We attribute this mostly to contamination by the nearbygalaxy cluster Abell 267 (44 . ′ o ff set from RGB J0152 +
017 butstill in the field of view of the non-imaging PCA).A detailed decomposition is beyond the scope of this paper,so we exclude
RXTE data from broadband modelling. The
RXTE data-set confirms the absence of variability during November2007, also in the energy range up to 10 keV. For the SED mod-elling, the average spectrum is treated as an upper limit. Furtherobservations with
RXTE in December 2007 also show no indica-tion of variability.
Optical observations were taken using the ATOM telescope(Hauser et al. 2004) at the H.E.S.S site from November 10, 2007.No significant variability was detected during the nights betweenNovember 10 and November 20; R-band fluxes binned nightlyshow an RMS of 0.02 mag.Absolute flux values were found using di ff erential photome-try against stars calibrated by K. Nilsson (priv. comm.). We mea-sured a total flux of m R = . ± .
01 mag (host galaxy + core)in an aperture of 4 ′′ radius. The host galaxy was subtracted withgalaxy parameters given in Nilsson et al. (2003), and aperturecorrection given in Eq. (4) of Young (1976). The core flux in theR-band (640 nm) was found to be 0.62 ± The Nanc¸ay Radio Telescope (NRT) is a meridian transittelescope with a main spherical mirror of 300 m ×
35 m(Theureau et al. 2007). The low-frequency receiver, covering theband 1.8–3.5 GHz was used, with the NRT standard filterbankbackend.The NRT observations were obtained in two contiguousbands of 12.5 MHz bandwidth, centred at 2679 and 2691 MHz(average frequency: 2685 MHz). Two linear polarisation re-ceivers were used during the 22 60-second drift scan observa-tions on the source on November 12 and 14, 2007. The data havebeen processed with the standard NRT software packages NAPSand SIR. All bands and polarisations have been averaged, givingan RMS noise of 2.2 mJy. The source 3C 295 was observed forcalibration, on November 11, 13, and 15, 2007.Taking into account a flux density for this source of 12 . ± .
06 Jy using the spectral fit published by Ott et al. (1994),we derived a flux density of 56 ± +
4. Discussion
Figure 4 shows the SED of RGB J0152 +
017 with the data fromNanc¸ay, ATOM,
Swift / XRT,
RXTE / PCA, and H.E.S.S. Eventhough some data are not strictly simultaneous, no significantvariability is found in the X-ray and optical bands throughoutthe periods covered; hence, a common modelling of the contem-poraneous X-ray and VHE data appears justified.
F. Aharonian et al.: Discovery of VHE γ -rays from the high-frequency-peaked BL Lac object RGB J0152 + Fig. 4.
The spectral energy distribution ofRGB J0152 + red filled circles and upper lim-its ), and contemporaneous RXTE ( blue opentriangles ), Swift / XRT (corrected for Galacticabsorption, magenta filled circles ), opti-cal host galaxy-subtracted (ATOM) and ra-dio (Nanc¸ay) observations ( large red filledsquares ). The black crosses are archival data.The blue open points in the optical R-bandcorrespond to the total and the core fluxesfrom Nilsson et al. (2003). A blob-in-jet syn-chrotron self-Compton model (see text) ap-plied to RGB J0152 +
017 is also shown, de-scribing the soft X-ray and VHE parts of theSED, with a simple synchrotron model shownat low frequencies to describe the extendedpart of the jet. The contribution of the dom-inating host galaxy is shown in the opticalband. The dashed line above the solid line atVHE shows the source spectrum after correct-ing for EBL absorption. The left- and right-hand side inlays detail portions of the ob-served X-ray and VHE spectrum, respectively.The optical part of the SED is mainly due to the hostgalaxy, which is detected and resolved in optical wavelengths(Nilsson et al. 2003). A template of the spectrum of such an el-liptic galaxy is shown in the SED, as inferred from the codePEGASE (Fioc & Rocca-Volmerange 1997). The host-galaxy-subtracted data point from the ATOM telescope might includeseveral additional contributions—from an accretion disk, an ex-tended jet (see below), or a central stellar population—so that itis considered as an upper limit in the following SSC model. Amodel including the optical ATOM data with possible additionalcontributions is beyond the scope of this paper.We applied a non-thermal leptonic SSC model(Katarzy´nski et al. 2001) to account for the contemporane-ous observations by
Swift in X-rays and by H.E.S.S. in theVHE band. The radio data are assumed to originate in anextended region, described by a separate synchrotron modelfor the extended jet (Katarzy´nski et al. 2001) to explain thelow-frequency part of the SED (as in, e.g., Aharonian et al.2005, 2008).We should emphasise that the aim of applying this modelin this work is not to present a definitive interpretation for thissource, but rather to show that a standard SSC model is able toaccount for the VHE and
Swift
X-ray observations.For the SSC model, we describe the system as a small ho-mogeneous spherical, emitting region (blob) of radius R withinthe jet, filled with a tangled magnetic field B and propagatingwith a Doppler factor δ = (cid:2) Γ (1 − β cos θ ) (cid:3) − . Here Γ is the bulkLorentz factor of the emitting plasma blob, β = v / c , and θ isthe angle of the velocity vector, with respect to the line-of-sight.The electron energy distribution (EED) is described by a brokenpowerlaw, with indices n and n , between Lorentz factors γ min and γ max , with a break at γ break and density normalisation K .The model also accounts for the absorption by the extra-galactic background light (EBL) with the parameters given inPrimack et al. (2005). RGB J0152 +
017 is too nearby ( z = . H =
70 km s − Mpc − , giving a lumi-nosity distance of d L = . × cm for RGB J0152 + K = . × cm − , γ min =
1, and γ max = × . The break energy is assumedat γ break = . × and is consistent with the Swift / XRT spec-trum, while providing good agreement with the H.E.S.S. data.We assume the canonical index n = . n = . δ = R = . × cm, and B = .
10 G.For the extended jet, the data are described well by R jet = cm, δ jet = K jet =
70 cm − , B jet = .
05 G, and γ break , jet = at the base of the jet, and L jet =
50 pc (all the parameters aredetailed in Katarzy´nski et al. 2001).Assuming additional contributions in the optical band, themulti-wavelength SED can thus be explained with a standardshock-acceleration process. The parameters derived from themodel are similar to previous results for this type of source (see,e.g., Ghisellini et al. 2002).From the current Nanc¸ay radio data and the
Swift
X-ray data,we obtain a broad-band spectral index α rx ∼ .
56 between theradio and the X-ray domains. The obtained SED, the correspond-ing location of the synchrotron peak, and the flux and shape ofthe
Swift spectrum lead us to conclude that RGB J0152 +
017 canclearly be classified as an HBL object at the time of H.E.S.S.observations.
5. Conclusion
The HBL RGB J0152 +
017 was detected in VHE at energies >
300 GeV with the H.E.S.S. experiment. The contemporane-ous
Swift , RXTE , Nanc¸ay, ATOM, and H.E.S.S. data allow themulti-wavelength SED for RGB J0152 +
017 to be derived forthe first time , and to clearly confirm its HBL nature at the timeof the H.E.S.S. observations. In general, large variations of the . Aharonian et al.: Discovery of VHE γ -rays from the high-frequency-peaked BL Lac object RGB J0152 +
017 5
VHE flux are expected in TeV blazars, making further monitor-ing of this source to detect high states of the VHE flux (flares)desirable.
Acknowledgements.
The support of the Namibian authorities and of theUniversity of Namibia in facilitating the construction and operation of H.E.S.S.is gratefully acknowledged, as is the support by the German Ministryfor Education and Research (BMBF), the Max Planck Society, the FrenchMinistry for Research, the CNRS-IN2P3 and the Astroparticle InterdisciplinaryProgramme of the CNRS, the U.K. Science and Technology Facilities Council(STFC), the IPNP of the Charles University, the Polish Ministry of Science andHigher Education, the South African Department of Science and Technology andNational Research Foundation, and by the University of Namibia. We appreciatethe excellent work of the technical support sta ff in Berlin, Durham, Hamburg,Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and op-eration of the equipment.This research made use of the NASA / IPAC Extragalactic Database (NED).The authors thank the
RXTE team for their prompt response to our ToO requestand the professional interactions that followed. The authors acknowledge the useof the publicly available
Swift data, as well as the public HEASARC softwarepackages. This work uses data obtained at the Nanc¸ay Radio Telescope. Theauthors also thank Dr. Mira V´eron-Cetty for fruitful discussions.
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