In this paper we briefly comment on the observational status of the possible
physical association between unidentified EGRET sources and supernova remnants
(SNRs) in our Galaxy. We draw upon recent results presented in the review by
Torres et al. (Physics Reports, 2003), concerning molecular gas in the vicinity
of all 19 SNRs found to be positionally coincident with EGRET sources at low
Galactic latitudes. In addition, we present new results regarding the supernova
remnant CTA~1. Our findings disfavor the possibility of a physical connection
with the nearby (in projection) EGRET source. There remains possible, however,
that the compact object produced in the supernova explosion be related with the
observed $\gamma$-ray flux.Comment: Presented for the proceedings of the II Workshop on Unidentified
  Gamma-Ray Sources, Hong Kong, June 1-4, 2004. To appear in Astrophysics and
  Space Science. Some changes to address referee's and readers' remarks.
  References added. Results unchange

D.F. Torres

Diego F. Torres

F.D. Seward

G. Sigl

G.E. Romero

Gustavo E. Romero

I. Grenier

J.J. Condon

K.T.S. Brazier

P. Slane

P.L. Nolan

R. Enomoto

S. Pineault

T.M. Dame

Thomas M. Dame

Y. Butt

Y. Uchiyama

English

arXiv

In this paper we briefly comment on the observational status of the possible physical association between unidentified EGRET sources and supernova remnants (SNRs) in our Galaxy. We draw upon recent results presented in the review by Torres et al. (2003), concerning molecular gas in the vicinity of all 19 SNRs found to be positionally coincident with EGRET sources at low Galactic latitudes. In addition, we present new results regarding the supernova remnant CTA 1. Our findings disfavor the possibility of a physical connection with the nearby (in projection) EGRET source. There remains possible, however, that the compact object produced in the supernova explosion be related with the observed ã-ray flux.Fil: Torres, Diego F.. Lawrence Livermore Laboratory; Estados UnidosFil: Dame, Thomas M.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Romero, Gustavo Esteban. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentin

Torres, Diego F.

Dame, Thomas M.

Romero, Gustavo Esteban

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

arXiv:astro-ph/0406335v2  27 Jul 2004Status of the connection between unidentified EGRETsources and supernova remnants: The case of CTA 1Diego F. Torres1, Thomas M. Dame2 & Gustavo E. Romero31Lawrence Livermore Laboratory, 7000 East Ave. L-413, Livermore, CA 94550,USA2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA02138, USA3Instituto Argentino de Radioastronomı́a, C.C.5, 1894 Villa Elisa, Buenos Aires,ArgentinaAbstract. In this paper we briefly comment on the observational status of thepossible physical association between unidentified EGRET sources and supernovaremnants (SNRs) in our Galaxy. We draw upon recent results presented in thereview by Torres et al. (Physics Reports, 2003), concerning molecular gas in thevicinity of all 19 SNRs found to be positionally coincident with EGRET sources atlow Galactic latitudes. In addition, we present new results regarding the supernovaremnant CTA 1. Our findings disfavor the possibility of a physical connection withthe nearby (in projection) EGRET source. There remains possible, however, that thecompact object produced in the supernova explosion be related with the observedγ-ray flux.Keywords: Supernova remnants – gamma rays: observations – interstellar medium1. A brief overview of the EGRET–SNR connectionWe have recently presented a case-by-case analysis of the positionalcorrelation between unidentified EGRET sources and supernova rem-nants (SNRs, Torres et al. 2003). There are 19 EGRET sources at lowGalactic latitudes (|b| < 10o) in spatial correlation with, mostly shell-type, SNRs. The Poisson probability for the 19 coincidences to be achance effect is 1.05×10−5 , i.e., there is an a priori 0.99998 probabilitythat at least one of the positional associations is physical. This expectedchance association was computed using a numerical code described byRomero et al. (1999), and Sigl et al. (2001). With the aim to identifythe most plausible candidates for an EGRET–SNR connection, we haveanalyzed, among other things, the variability status of the EGRETsources and the potential contribution from known radio pulsars to thehigh energy flux (Torres et al. 2003).In order to understand the origin of all the unidentified detections,their variability status is of paramount importance (Nolan 2003, Torreset al. 2001). Classic known models for γ-ray sources in our Galaxy, likea SNR–molecular cloud interaction (e.g. Aharonian & Atoyan 1996),c© 2017 Kluwer Academic Publishers. Printed in the Netherlands.snr-cta1-web.tex; 2/07/2017; 1:15; p.12 Torres, Dame, Romerowould produce non-variable detections during the timescale of EGRETobservations. To classify the variability status we take into account bothindices quoted in the literature, Torres et al.’s (2001) I and Tompkins’τ . They are different in nature, although results have proven to bestatistically correlated (Torres, Pessah & Romero 2001). The indexI is a relative classification of variability with respect to the pulsarpopulation. Sources with I > 2.5 are more than 3σ away from thestatistical variability of pulsars. Similarly, sources for which τ is at least0.6, are also more than 3σ away from the mean value of the τ upperlimit for pulsars, and are thus classified as variable. For comparison,note that the mean value of τ (I) for known AGNs is 0.9 (3.3). We findthat most EGRET sources in positional coincidence with SNRs, andcertainly those which represent the best cases for a physical association,are non-variable.In order to unambiguously identify a pulsar as the origin of the γ-rays from an EGRET source, γ-ray pulsations must be detected at thepulsar period. However the γ-fluxes are generally very low, and nearlycontemporaneous radio/X-ray and γ-ray observations are required tofold the few available photons with the correct ephemeris. Because thisis no longer possible owing to the demise of the Compton Gamma-Ray Observatory, new candidate associations must mostly be judgedby comparison with the properties of the six known EGRET pulsarsand those of the known best candidates. We have done so for everyknown radio pulsar found to be coincident with these 19 EGRETsources, and have isolated several interesting candidates for an EGRETsource–pulsar connection. These are mostly found with young pulsarspertaining to the recently released Parkes Multibeam Survey (see, e.g.Torres, Butt, & Camilo 2001, Torres & Nuza 2003). This scenario, then,is a competing one to the EGRET–SNR connection and has to be takeninto account in judging the plausibility of the latter.The need of a case-by-case analysis is then clearly established. Sev-eral cases for a physical association, based on the analysis of γ-raydata, the molecular environment of the SNR, and the analysis of anycoincident pulsar (if any), have been isolated and reviewed in Torres etal. (2003). Among the most notable cases for such a physical associationare G347.3-0.5, W66, W28 and IC443. The first of these, providingprobably the best evidence to date for a hadronic origin of the γ-raysource detected by EGRET, has received much attention recently (Buttet al. 2001, 2002, Enomoto 2002, Uchiyama et al. 2002). In addition,the case-by-case analysis shows that it is at least plausible, contraryto expectations, that EGRET has detected distant (more than 6 kpc)SNRs. There are 5 coinciding pairs of 3EG sources and SNRs for whichthe latter apparently lie at such high values of distance (disregardingsnr-cta1-web.tex; 2/07/2017; 1:15; p.2SNRs and Unidentified EGRET sources 3those related with SNRs spatially close to the galactic center). Forall these cases, we have uncovered the existence of nearby, large, insome cases giant, molecular clouds that could enhance the GeV signalthrough pion decay. It is possible that the physical relationship betweenthe 3EG source and the coincident SNR could provide for these pairs asubstantial part of the GeV emission observed. This does not preclude,however, composite origins for the total amount of the radiation de-tected, since some of these cases present other plausible scenarios (e.g.energetic pulsars in the EGRET field). AGILE observations, in advanceof GLAST, would greatly elucidate the origin for these 3EG sources,since even a factor of 2 of improvement in spatial resolution would beenough to reject the EGRET–SNR connection.In what follows we attempt to briefly analyze, similarly to whatwas done in the case-by-case analysis of Torres et al. (2003), the pairCTA 1/3EG J0010+7309, a supernova remnant lying at relatively highGalactic latitudes, and for which there has been little analysis on theirCO environment.2. CTA 1: General FeaturesCTA 1 is a shell-type SNR, but its shell is incomplete and broken-out towards the NW. This breakout phenomenon may be caused bymore rapid expansion of the blast wave shock into a lower densityregion toward the NW. HI observations supported this interpretation(Pineault et al. 1993, 1997). The latter papers place the SNR at 1.4±0.3kpc, the kinematic distance corresponding to the systemic velocity of apartial HI shell (−16 km/s) that the authors find to the NW. However,it is to be noticed that the systemic (unperturbed) velocity of a partiallyexpanding shell might be subject to a great uncertainty.Since CTA 1 is located at a relatively high latitude (b =10o) andit is nearby, it has a large angular size (90 arcmin), little foregroundor background confusion, and it can be observed at exceptionally highlinear resolution. The age of the SNR was estimated to be 104 yr byPineault et al. (1993), but it could be younger by a factor of 2 (Slaneet al. 1997). CTA 1 was subject of intense observational campaignsin the past years. There have been both ROSAT and ASCA X-rayobservations (Seward et al. 1995, Slane et al. 1997), as well as optical,infrared and radio studies (see Pineault et al. 1997 and Brazier et al.1998 for a review).CTA 1 belongs to the class of composite SNRs (SNRs with centralpulsar wind nebulae), which show a shell-type morphology in the radioband and are center-filled in X-rays. Five point sources were detectedsnr-cta1-web.tex; 2/07/2017; 1:15; p.34 Torres, Dame, Romerowith ROSAT, one of which was found to coincide with the EGRETsource (at the time of the analysis, 2EG J0008+7307), see Figure 3 ofBrazier et al. (1998). ASCA data suggested that this source, namedRX J0007.0+7302, has a non-thermal spectrum, and gave support tothe idea that it is the pulsar left from the supernova explosion (Slaneet al. 1997). Recent observations by Slane et al. (2004), using XMM,confirm that the X-ray spectrum of the source is consistent with thatof a neutron star and is well described by a power law with the ad-dition of a soft thermal component that may correspond to emissionfrom hot polar cap regions or to cooling emission from a light-elementatmosphere over the entire star. An extrapolation of the non-thermalspectrum of RX J0007.0+7302 to γ-ray energies would yield a fluxconsistent with that of EGRET source 3EG J0010+7309, supportingthe proposition that these sources could be related. A Chandra imageof the central X-ray source RX J0007.0+7303 (Halpern et al. 2004)reveals a point source, a compact nebula, and a bent jet, all of whichare characteristic of energetic, rotation-powered pulsars.Optical observations were carried out by Brazier et al. (1998), witha 2.12-m telescope, but no object was found within the positional errorbox of the X-ray source. This allowed an upper limit to be set on theoptical magnitude of any counterpart to the putative pulsar. Halpernet al. (2004) also obtained upper limits in the optical at the position ofthe point source, corresponding to an X-ray-to-optical flux ratio largerthan 100. Neither a VLA image at 1425 MHz nor a deep pulsar searchat 820 MHz using the NRAO Green Bank Telescope, revealed a radiopulsar counterpartThe γ-ray source 3EG J0010+7309 is non-variable under the I andτ schemes, and has a hard spectral index of 1.85±0.10, compatiblewith those of the Vela pulsar. This source was also detected in theSecond EGRET Catalog, but with a shifted position which made itcoincide with a nearby AGN – which was at the time proposed as apossible counterpart (Nolan et al. 1996). This AGN is not coincidentwith the 3EG source and can not be considered a plausible counter-part any longer. Based on positional coincidence, on the hard spectralindex, and on physical similarities between the Vela pulsar and RXJ0007.0+7302, Brazier et al. (1998) proposed that the 3EG sourceand this X-ray source were related. For an assumed 1 sr beaming, theobserved 100-2000 MeV flux corresponds to a luminosity of 4 × 1033erg s−1, compatible with other γ-ray pulsar detections. A competingexplanation to the γ-ray pulsar would be given if a massive and densemolecular cloud is found to be interacting with, or being overtaken by,the supernova remnant shock. The molecular environment of CTA 1 isanalyzed below to further assess this possibility.snr-cta1-web.tex; 2/07/2017; 1:15; p.4SNRs and Unidentified EGRET sources 5Figure 1. Molecular gas in the direction of the SNR CTA 1. (a) The false-color mapis CO integrated over the velocity range −15 to +5 km/s. The black contours are4.85 GHz intensity from the survey of Condon et al. (1994), which show CTA 1 asa semi-circular feature near the center of the map. The red (lighter) contours arethe 50%, 68%, 95%, and 99% confidence contours for the unidentified gamma raysource 3EG J0010+7309. (b) A CO latitude-velocity map integrated from 116o to120o in Galactic longitude. [[The figure is available on-line as a jpeg file]]As can be noted from the figure, the EGRET source seems too farfrom the SNR nearest rim. However, if some SNRs interact, as expected,with nearby massive clouds producing enhanced γ-ray emission throughhadronic/Bremsstrahlung interactions, cases in which there is just amarginal coincidence between the SNRs and the centers of the EGRETsources should be also considered, since the peak γ-ray emissivity willlikely be biased towards the adjacent cloud.3. CTA 1: Molecular environmentAs discussed in Dame et al. (1987), much of the local (closer thanPerseus Arm) CO emission in the second quadrant of the Galaxy bi-furcates into 2 velocity components, one centered near −12 km/s andthe other close to 0 km/s. Dame et al. (1987) argue that the “−12km/s clouds” are probably associated with a string of OB associationsat 800 to 1000 pc from Earth. The clouds with velocities near 0 km/swere associated with the so-called “Lindblad Ring” at 300 pc. Thelong thin cloud partially overlapping CTA 1 (see Figure 1) has a meanvelocity of ∼ −3 km/s, suggesting that it is part of the Lindblad Ringpopulation. In fact, these clouds appear to be part of a very large loopof local clouds (all in the range −5 to 0 km/s). Grenier et al. (1989)proposed that this loop surrounds a 4× 104 yr old Type I SNR 300 pcaway. Thus, the velocity of the cloud near to CTA 1 suggests that it ispart of this larger system, lying at only ∼300 pc.The total H2 mass (calculated from CO data) of the long thin cloudnear CTA1 (in the region l = 116o to 120o, b = 8o to 13o, v = −15 to5 km/s) is 4900 M⊙, assuming a distance of 300 pc. However, there isno evidence of interaction between the remnant and the cloud (what isconsistent with the different most plausible distances to each of theseobjects). Figure 1b attached, for example, shows a latitude–velocitymap integrated over the longitude extent of the cloud. There is littleindication of accelerated gas at the latitude/velocity of the remnant,b = 9o to 10o and ∼ −15 km/s, respectively. The cloud may well be,then, in the foreground, and unrelated to CTA 1. In addition to this,snr-cta1-web.tex; 2/07/2017; 1:15; p.56 Torres, Dame, Romerothere is no CO emission detected within the contours of the 3EG source,which also argues against a hadronic origin of the γ-ray source.Pineault et al.’s (1993) systemic velocity of −16 km/s for the HIshell is reasonable close to that of the “−12 km/s clouds”, which in turnare probably associated with the OB associations Cep OB2, Cep OB3,Cep OB4, and Per OB3 at ∼ 800 pc. This might be suggesting thatthe latter is the most likely distance for CTA 1, although it remainsuncertain. In any case, it seems that CTA 1 and 3EG J0010+7309might be physically related only through the compact object left by thesupernova explosion, RX J0007.0+7302. A better localization of the γ-ray source with AGILE and GLAST will certainly test this suggestion.For the moment, our CO analysis shows that a molecular cloud originof the γ-ray source 3EG J0010+7309 (for instance, through hadronicprocesses similar to the case of G347.3-0.5 proposed by Butt et al. 2001)is untenable.AcknowledgmentsThe work of DFT was performed under the auspices of the U.S. Depart-ment of Energy (NNSA) by University of California Lawrence Liver-more National Laboratory under contract No. W-7405-Eng-48. G.E.R.was supported by CONICET (under grant PIP No 0430/98), ANPCT(PICT 03-04881), as well as by Fundación Antorchas. We thank P.Slane for comments.ReferencesBrazier K.T.S., Reimer O., Kanbach G., & Carramiñana A. 1998, MNRAS 295, 819Butt Y., et al. 2001, ApJ 562, L167Butt Y., et al. 2002, Nature 418, 499Condon J.J., et al. 1994, AJ 107, 1829Dame T.M., et al. 1987, ApJ 322, 706Enomoto R., et al. 2002, Nature 416, 823Grenier I. et al. 1989, ApJ 347, 231Halpern J. P., Gotthelf E. V., Camilo F., Helfand D. J. & Ransom S. M. 2004,astro-ph/0404312Nolan P.L., et al. 1996, ApJ 459, 100Nolan P.L., Tompkins W.F., Grenier I.A., & Michelson P.F. 2003, ApJ 597, 615Pineault S., et al. 1993, AJ 105, 1060Pineault S., et al. 1997, A&A 324, 1152Slane P., Zimmerman E. R., Hughes J. P., Seward F. D., Gaensler B. M. & ClarkeM. J. 2004, ApJ 601, 1045Romero G.E., Benaglia P., & Torres D.F. 1999, A&A 348, 868Seward F.D., Schmidt B., & Slane P. 1995, ApJ 453, 284snr-cta1-web.tex; 2/07/2017; 1:15; p.6SNRs and Unidentified EGRET sources 7Sigl G., et al. 2001, Phys. Rev. D63, 081302Slane P., et al. 1997, ApJ 485, 221Torres D.F., et al. 2001, A&A 370, 468Torres D.F., Pessah M.E., & Romero G.E. 2001c, Astronomische Nach. 322, 223Torres D.F., Butt Y.M. & Camilo F. 2001, ApJ 560, L155Torres D.F., & Nuza S. E. 2003, ApJ 583, L25Torres D.F., et al. 2003, Physics Reports 382, 303Uchiyama Y., Takahashi T., & Aharonian F.A. 2002a, PASJ54, L73snr-cta1-web.tex; 2/07/2017; 1:15; p.7snr-cta1-web.tex; 2/07/2017; 1:15; p.8This figure "cta1.jpg" is available in "jpg" format from:http://arxiv.org/ps/astro-ph/0406335v2

Astrophysics and Space Science

Status of the Connection between Unidentified Egret Sources and Supernova Remnants: The Case of Cta 1

Crossref

CONICET Digital

arXiv:astro-ph/0406335v2  27 Jul 2004Status of the connection between unidentified EGRETsources and supernova remnants: The case of CTA 1Diego F. Torres1, Thomas M. Dame2 & Gustavo E. Romero31Lawrence Livermore Laboratory, 7000 East Ave. L-413, Livermore, CA 94550,USA2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA02138, USA3Instituto Argentino de Radioastronomı´a, C.C.5, 1894 Villa Elisa, Buenos Aires,ArgentinaAbstract. In this paper we briefly comment on the observational status of thepossible physical association between unidentified EGRET sources and supernovaremnants (SNRs) in our Galaxy. We draw upon recent results presented in thereview by Torres et al. (Physics Reports, 2003), concerning molecular gas in thevicinity of all 19 SNRs found to be positionally coincident with EGRET sources atlow Galactic latitudes. In addition, we present new results regarding the supernovaremnant CTA 1. Our findings disfavor the possibility of a physical connection withthe nearby (in projection) EGRET source. There remains possible, however, that thecompact object produced in the supernova explosion be related with the observedγ-ray flux.Keywords: Supernova remnants – gamma rays: observations – interstellar medium1. A brief overview of the EGRET–SNR connectionWe have recently presented a case-by-case analysis of the positionalcorrelation between unidentified EGRET sources and supernova rem-nants (SNRs, Torres et al. 2003). There are 19 EGRET sources at lowGalactic latitudes (|b| < 10o) in spatial correlation with, mostly shell-type, SNRs. The Poisson probability for the 19 coincidences to be achance effect is 1.05×10−5 , i.e., there is an a priori 0.99998 probabilitythat at least one of the positional associations is physical. This expectedchance association was computed using a numerical code described byRomero et al. (1999), and Sigl et al. (2001). With the aim to identifythe most plausible candidates for an EGRET–SNR connection, we haveanalyzed, among other things, the variability status of the EGRETsources and the potential contribution from known radio pulsars to thehigh energy flux (Torres et al. 2003).In order to understand the origin of all the unidentified detections,their variability status is of paramount importance (Nolan 2003, Torreset al. 2001). Classic known models for γ-ray sources in our Galaxy, likea SNR–molecular cloud interaction (e.g. Aharonian & Atoyan 1996),c© 2017 Kluwer Academic Publishers. Printed in the Netherlands.snr-cta1-web.tex; 2/07/2017; 1:15; p.12 Torres, Dame, Romerowould produce non-variable detections during the timescale of EGRETobservations. To classify the variability status we take into account bothindices quoted in the literature, Torres et al.’s (2001) I and Tompkins’τ . They are different in nature, although results have proven to bestatistically correlated (Torres, Pessah & Romero 2001). The indexI is a relative classification of variability with respect to the pulsarpopulation. Sources with I > 2.5 are more than 3σ away from thestatistical variability of pulsars. Similarly, sources for which τ is at least0.6, are also more than 3σ away from the mean value of the τ upperlimit for pulsars, and are thus classified as variable. For comparison,note that the mean value of τ (I) for known AGNs is 0.9 (3.3). We findthat most EGRET sources in positional coincidence with SNRs, andcertainly those which represent the best cases for a physical association,are non-variable.In order to unambiguously identify a pulsar as the origin of the γ-rays from an EGRET source, γ-ray pulsations must be detected at thepulsar period. However the γ-fluxes are generally very low, and nearlycontemporaneous radio/X-ray and γ-ray observations are required tofold the few available photons with the correct ephemeris. Because thisis no longer possible owing to the demise of the Compton Gamma-Ray Observatory, new candidate associations must mostly be judgedby comparison with the properties of the six known EGRET pulsarsand those of the known best candidates. We have done so for everyknown radio pulsar found to be coincident with these 19 EGRETsources, and have isolated several interesting candidates for an EGRETsource–pulsar connection. These are mostly found with young pulsarspertaining to the recently released Parkes Multibeam Survey (see, e.g.Torres, Butt, & Camilo 2001, Torres & Nuza 2003). This scenario, then,is a competing one to the EGRET–SNR connection and has to be takeninto account in judging the plausibility of the latter.The need of a case-by-case analysis is then clearly established. Sev-eral cases for a physical association, based on the analysis of γ-raydata, the molecular environment of the SNR, and the analysis of anycoincident pulsar (if any), have been isolated and reviewed in Torres etal. (2003). Among the most notable cases for such a physical associationare G347.3-0.5, W66, W28 and IC443. The first of these, providingprobably the best evidence to date for a hadronic origin of the γ-raysource detected by EGRET, has received much attention recently (Buttet al. 2001, 2002, Enomoto 2002, Uchiyama et al. 2002). In addition,the case-by-case analysis shows that it is at least plausible, contraryto expectations, that EGRET has detected distant (more than 6 kpc)SNRs. There are 5 coinciding pairs of 3EG sources and SNRs for whichthe latter apparently lie at such high values of distance (disregardingsnr-cta1-web.tex; 2/07/2017; 1:15; p.2SNRs and Unidentified EGRET sources 3those related with SNRs spatially close to the galactic center). Forall these cases, we have uncovered the existence of nearby, large, insome cases giant, molecular clouds that could enhance the GeV signalthrough pion decay. It is possible that the physical relationship betweenthe 3EG source and the coincident SNR could provide for these pairs asubstantial part of the GeV emission observed. This does not preclude,however, composite origins for the total amount of the radiation de-tected, since some of these cases present other plausible scenarios (e.g.energetic pulsars in the EGRET field). AGILE observations, in advanceof GLAST, would greatly elucidate the origin for these 3EG sources,since even a factor of 2 of improvement in spatial resolution would beenough to reject the EGRET–SNR connection.In what follows we attempt to briefly analyze, similarly to whatwas done in the case-by-case analysis of Torres et al. (2003), the pairCTA 1/3EG J0010+7309, a supernova remnant lying at relatively highGalactic latitudes, and for which there has been little analysis on theirCO environment.2. CTA 1: General FeaturesCTA 1 is a shell-type SNR, but its shell is incomplete and broken-out towards the NW. This breakout phenomenon may be caused bymore rapid expansion of the blast wave shock into a lower densityregion toward the NW. HI observations supported this interpretation(Pineault et al. 1993, 1997). The latter papers place the SNR at 1.4±0.3kpc, the kinematic distance corresponding to the systemic velocity of apartial HI shell (−16 km/s) that the authors find to the NW. However,it is to be noticed that the systemic (unperturbed) velocity of a partiallyexpanding shell might be subject to a great uncertainty.Since CTA 1 is located at a relatively high latitude (b =10o) andit is nearby, it has a large angular size (90 arcmin), little foregroundor background confusion, and it can be observed at exceptionally highlinear resolution. The age of the SNR was estimated to be 104 yr byPineault et al. (1993), but it could be younger by a factor of 2 (Slaneet al. 1997). CTA 1 was subject of intense observational campaignsin the past years. There have been both ROSAT and ASCA X-rayobservations (Seward et al. 1995, Slane et al. 1997), as well as optical,infrared and radio studies (see Pineault et al. 1997 and Brazier et al.1998 for a review).CTA 1 belongs to the class of composite SNRs (SNRs with centralpulsar wind nebulae), which show a shell-type morphology in the radioband and are center-filled in X-rays. Five point sources were detectedsnr-cta1-web.tex; 2/07/2017; 1:15; p.34 Torres, Dame, Romerowith ROSAT, one of which was found to coincide with the EGRETsource (at the time of the analysis, 2EG J0008+7307), see Figure 3 ofBrazier et al. (1998). ASCA data suggested that this source, namedRX J0007.0+7302, has a non-thermal spectrum, and gave support tothe idea that it is the pulsar left from the supernova explosion (Slaneet al. 1997). Recent observations by Slane et al. (2004), using XMM,confirm that the X-ray spectrum of the source is consistent with thatof a neutron star and is well described by a power law with the ad-dition of a soft thermal component that may correspond to emissionfrom hot polar cap regions or to cooling emission from a light-elementatmosphere over the entire star. An extrapolation of the non-thermalspectrum of RX J0007.0+7302 to γ-ray energies would yield a fluxconsistent with that of EGRET source 3EG J0010+7309, supportingthe proposition that these sources could be related. A Chandra imageof the central X-ray source RX J0007.0+7303 (Halpern et al. 2004)reveals a point source, a compact nebula, and a bent jet, all of whichare characteristic of energetic, rotation-powered pulsars.Optical observations were carried out by Brazier et al. (1998), witha 2.12-m telescope, but no object was found within the positional errorbox of the X-ray source. This allowed an upper limit to be set on theoptical magnitude of any counterpart to the putative pulsar. Halpernet al. (2004) also obtained upper limits in the optical at the position ofthe point source, corresponding to an X-ray-to-optical flux ratio largerthan 100. Neither a VLA image at 1425 MHz nor a deep pulsar searchat 820 MHz using the NRAO Green Bank Telescope, revealed a radiopulsar counterpartThe γ-ray source 3EG J0010+7309 is non-variable under the I andτ schemes, and has a hard spectral index of 1.85±0.10, compatiblewith those of the Vela pulsar. This source was also detected in theSecond EGRET Catalog, but with a shifted position which made itcoincide with a nearby AGN – which was at the time proposed as apossible counterpart (Nolan et al. 1996). This AGN is not coincidentwith the 3EG source and can not be considered a plausible counter-part any longer. Based on positional coincidence, on the hard spectralindex, and on physical similarities between the Vela pulsar and RXJ0007.0+7302, Brazier et al. (1998) proposed that the 3EG sourceand this X-ray source were related. For an assumed 1 sr beaming, theobserved 100-2000 MeV flux corresponds to a luminosity of 4 × 1033erg s−1, compatible with other γ-ray pulsar detections. A competingexplanation to the γ-ray pulsar would be given if a massive and densemolecular cloud is found to be interacting with, or being overtaken by,the supernova remnant shock. The molecular environment of CTA 1 isanalyzed below to further assess this possibility.snr-cta1-web.tex; 2/07/2017; 1:15; p.4SNRs and Unidentified EGRET sources 5Figure 1. Molecular gas in the direction of the SNR CTA 1. (a) The false-color mapis CO integrated over the velocity range −15 to +5 km/s. The black contours are4.85 GHz intensity from the survey of Condon et al. (1994), which show CTA 1 asa semi-circular feature near the center of the map. The red (lighter) contours arethe 50%, 68%, 95%, and 99% confidence contours for the unidentified gamma raysource 3EG J0010+7309. (b) A CO latitude-velocity map integrated from 116o to120o in Galactic longitude. [[The figure is available on-line as a jpeg file]]As can be noted from the figure, the EGRET source seems too farfrom the SNR nearest rim. However, if some SNRs interact, as expected,with nearby massive clouds producing enhanced γ-ray emission throughhadronic/Bremsstrahlung interactions, cases in which there is just amarginal coincidence between the SNRs and the centers of the EGRETsources should be also considered, since the peak γ-ray emissivity willlikely be biased towards the adjacent cloud.3. CTA 1: Molecular environmentAs discussed in Dame et al. (1987), much of the local (closer thanPerseus Arm) CO emission in the second quadrant of the Galaxy bi-furcates into 2 velocity components, one centered near −12 km/s andthe other close to 0 km/s. Dame et al. (1987) argue that the “−12km/s clouds” are probably associated with a string of OB associationsat 800 to 1000 pc from Earth. The clouds with velocities near 0 km/swere associated with the so-called “Lindblad Ring” at 300 pc. Thelong thin cloud partially overlapping CTA 1 (see Figure 1) has a meanvelocity of ∼ −3 km/s, suggesting that it is part of the Lindblad Ringpopulation. In fact, these clouds appear to be part of a very large loopof local clouds (all in the range −5 to 0 km/s). Grenier et al. (1989)proposed that this loop surrounds a 4× 104 yr old Type I SNR 300 pcaway. Thus, the velocity of the cloud near to CTA 1 suggests that it ispart of this larger system, lying at only ∼300 pc.The total H2 mass (calculated from CO data) of the long thin cloudnear CTA1 (in the region l = 116o to 120o, b = 8o to 13o, v = −15 to5 km/s) is 4900 M⊙, assuming a distance of 300 pc. However, there isno evidence of interaction between the remnant and the cloud (what isconsistent with the different most plausible distances to each of theseobjects). Figure 1b attached, for example, shows a latitude–velocitymap integrated over the longitude extent of the cloud. There is littleindication of accelerated gas at the latitude/velocity of the remnant,b = 9o to 10o and ∼ −15 km/s, respectively. The cloud may well be,then, in the foreground, and unrelated to CTA 1. In addition to this,snr-cta1-web.tex; 2/07/2017; 1:15; p.56 Torres, Dame, Romerothere is no CO emission detected within the contours of the 3EG source,which also argues against a hadronic origin of the γ-ray source.Pineault et al.’s (1993) systemic velocity of −16 km/s for the HIshell is reasonable close to that of the “−12 km/s clouds”, which in turnare probably associated with the OB associations Cep OB2, Cep OB3,Cep OB4, and Per OB3 at ∼ 800 pc. This might be suggesting thatthe latter is the most likely distance for CTA 1, although it remainsuncertain. In any case, it seems that CTA 1 and 3EG J0010+7309might be physically related only through the compact object left by thesupernova explosion, RX J0007.0+7302. A better localization of the γ-ray source with AGILE and GLAST will certainly test this suggestion.For the moment, our CO analysis shows that a molecular cloud originof the γ-ray source 3EG J0010+7309 (for instance, through hadronicprocesses similar to the case of G347.3-0.5 proposed by Butt et al. 2001)is untenable.AcknowledgmentsThe work of DFT was performed under the auspices of the U.S. Depart-ment of Energy (NNSA) by University of California Lawrence Liver-more National Laboratory under contract No. W-7405-Eng-48. G.E.R.was supported by CONICET (under grant PIP No 0430/98), ANPCT(PICT 03-04881), as well as by Fundacio´n Antorchas. We thank P.Slane for comments.ReferencesBrazier K.T.S., Reimer O., Kanbach G., & Carramin˜ana A. 1998, MNRAS 295, 819Butt Y., et al. 2001, ApJ 562, L167Butt Y., et al. 2002, Nature 418, 499Condon J.J., et al. 1994, AJ 107, 1829Dame T.M., et al. 1987, ApJ 322, 706Enomoto R., et al. 2002, Nature 416, 823Grenier I. et al. 1989, ApJ 347, 231Halpern J. P., Gotthelf E. V., Camilo F., Helfand D. J. & Ransom S. M. 2004,astro-ph/0404312Nolan P.L., et al. 1996, ApJ 459, 100Nolan P.L., Tompkins W.F., Grenier I.A., & Michelson P.F. 2003, ApJ 597, 615Pineault S., et al. 1993, AJ 105, 1060Pineault S., et al. 1997, A&A 324, 1152Slane P., Zimmerman E. R., Hughes J. P., Seward F. D., Gaensler B. M. & ClarkeM. J. 2004, ApJ 601, 1045Romero G.E., Benaglia P., & Torres D.F. 1999, A&A 348, 868Seward F.D., Schmidt B., & Slane P. 1995, ApJ 453, 284snr-cta1-web.tex; 2/07/2017; 1:15; p.6SNRs and Unidentified EGRET sources 7Sigl G., et al. 2001, Phys. Rev. D63, 081302Slane P., et al. 1997, ApJ 485, 221Torres D.F., et al. 2001, A&A 370, 468Torres D.F., Pessah M.E., & Romero G.E. 2001c, Astronomische Nach. 322, 223Torres D.F., Butt Y.M. & Camilo F. 2001, ApJ 560, L155Torres D.F., & Nuza S. E. 2003, ApJ 583, L25Torres D.F., et al. 2003, Physics Reports 382, 303Uchiyama Y., Takahashi T., & Aharonian F.A. 2002a, PASJ54, L73snr-cta1-web.tex; 2/07/2017; 1:15; p.7snr-cta1-web.tex; 2/07/2017; 1:15; p.8This figure "cta1.jpg" is available in "jpg" format from:http://arxiv.org/ps/astro-ph/0406335v2

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Status of the connection between unidentified EGRET sources and
  supernova remnants: The case of CTA 1

Status of the connection between unidentified EGRET sources and supernova remnants: The case of CTA 1

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