Adv. SpaceRes.Vol. 10, No. 2, pp. (2)81—(2)84, 1990 0273—1177/90 $0.00 +50Printedin GreatBritain. All rights reserved. Copyright© 1989 COSPAR

GAMMA RAYS AND INTERSTELLAR GASIN THE CEPHEUS REGION: A NEWGAMMA-RAY SOURCE?

I. A. Grenier and F. Lebrun

Service d’Astrophysique,Centre d’EtudesNucleairesde Saclay,

91191 Gif-sur-YvetteCedex, France

ABSTRACT

A recent CO survey of the CepheusFlare region (1000~l�1410, 80th~22°)ha8 allowed adetailed study of the diffuse gamma-ray emission and the gas content of this nearbyregion. The comparisonof the HI and CO observationswith the COS-B gamma-raydatayieldsestimates of the N (H,)/WCO ratio In this molecular complex and of the emissivity spectrumof the gas between‘70 MeV and 5 GeV. A significant (3.9a) point-like excess above thediffuse emissionhasbeenfound and canbe Interpretedas a new gamma-raysource.

INTRODUCTION

At low latitude, the bulk of the galactic gamma-ray emission observed by COS-B Is wellreproduced using the gas content of the disc and a small radial gradient of the gasemissivity In the galactic plane which most likely reflects the cosmic ray distribution,e.g. /1/. At medium latitude, the study of the gamma-ray/gascorrelation using the HI andCO surveys of vast nearby clouds provides instead a precise measurementof the localgamma-ray emissivity. The analysis of the Orion and Oph-Sag regions already led,respectively, to a standard ezuissivity /2/ and an emissivity enhancementpossibly relatedto the Loop I presence/3/. Today, the recent CO map of the whole Cepheusregion allows anew local measurement, inside another molecular complex, as well as an independentcalibration of the N( H

2 )/WCO conversionfactor.

DATA AND ANALYSIS

The analysis has been performed between 99.5°and 140.5~in longitude, from 8° to 22° inlatitude. In this direction, the interstellar matter breaks up in two velocity components:a massivemolecularcomplex at about 300 pc and clouds of the Local Arm at 800-900pc /4/.Data integrated over all velocities have been considered to compare to the gamma-raydistribution. The CO observationsof this region by Orenier et al. /4/ have been mergedat low latitude to the Columbia 2nd Quadrant CO Survey /5/, while towards higherlatitudes, the medium hasbeenassumedvoid of molecules. N(HI) column density mapshavebeen constructedfrom the observationsof Weaverand WIlliams /6/ for b�l0° (with a uniformspin temperatureof 125 K) and of Heiles and Hablng /7/ for b�10°(assumingan opticallythIn emission). Maps of the gamma-ray intensities in different energy ranges have beenderivedfrom the Final COS-B Database/8/.

A likelihood analysis has been applied to test the correlation between the diffuse ganuuarays and a linear combination of the atomic and molecular gas plus an isotropicbackground. The latter should also include the faint and flat inverse-Compton componentIn these directions. The discovery of a localized gamma-ray excess has further led to theaddition of a point-sourceterm. Thus, in each bin of the sky, the predicted counts wereCalculatedas follows:

= l~t[ A. N(HI) + B. WCO + C + F.1SOURC ],b) ] (1)

where the tilde indicates the convolution of the maps with the energy dependentresolutionof the COS-B telescope. c�t refers to the effective exposurein the energy range. Withoutother discrete gamma-ray sources in the field, A and B are related to the gamma-rayemissivity per H atom in atomic and molecular form, respectively. F and ~ representthe flux and angular distribution of the tested source according to the ~OS-B point-spread-function. As a compromise between the need for detailed maps and good photonstatistics, the analysis has been performed in 2°x20 bins for the three classical COS-Benergy ranges (70-150, 150-300, 300-5000 MeV). Further details on the completeanalysis,for Instancewith andwithout the sourceterm, canbe found in /9/.

(2)82 I. A. Grenierand F. Lebrun

TABLE 1 Maxlmum-likelthoodvaluesfor the parametersdescribingthe diffuseemissionin the 3 energyranges,and the resulting ratio N(H2)/WCO = B/2A:

Energy ~ A B C N(H2)/W~O

MeV 10 ats~sr~ 106cm2sr~K1km~1 106cm2sr~s~ mol.ciü’ I~’kth1 s

70—150 1.1 ±0.3 0.5 ± 5.0 72. ± 10. 0.23 ±2.3

150—300 0.70±0.18 3.25±1.75 22. ± 5. 2.32± 1.38

300-5000 0.67±0.17 3.0 ± 1.4 21.0± 4.5 2.24± 1.19

RESULTS ON THE DIFFUSE EMISSION

Since the bulk of the Interstellar matter in the Cepheusdirection lies within at most 1.5kpc, the small emissivity gradient found by Strong et al. /1/ is negligeable over thisscale length. So, we may assumea uniform distribution of the cosmic rays in the wholeregion. Assuming further that the same cosmic-ray flux pervades the diffuse atomic anddenser molecular media, the A and B parameters directly yield estimates of the gasexn.issivlty q and of the N(H

2)/WCO ratio: A = q/4it andB = 2. (q/4it). N(H2)/WCO. The resultsfor A, B and C in the 3 energy ranges and their respectiveerrors are listed In Table 1.

A quick look at the COS-B databaseshows that the background Intensities derived In theCepheusdirection in the three energy ranges are the same as in the rest of the galacticsurvey.The emissivity spectrum (A) of the atomic gas fully agreeswith the values derived at thesolar circle from the study of the entire galactic plane below 10~ /1/. It is alsoconsistent with the SAS-2 observationsat medium latitude outside the Loop I region /3/and with the emlssivity of the Orion-Monoceroscomplex above 300 MeV /2/. Other previousmeasurementsalso yielded consistent results although they should be taken with some care.For Instance, a similar spectrum was derived at medium latitude /10/, including thedeviant Oph-Sagregion and with a moleculargas contribution estimatedfrom galaxy countsthat were not yet corrected for the field-stars bias /11/. And another similar spectrumwas found in the outer galactic plane within 2.5 kpc while neglecting the molecularcontent of the medium /12/. Therefore, the overall consistency between measurementsIndifferent regions and with various observationssuggests that the gamma-ray emissivitlesfound between70 MeV and 5 GeV for the atomic gas lying within about 1 kpc from the Sunare now reliable.

Above 150 MeV, our B value implies an N(H, )/WCO ratio of (2.3 ±1.2) 10~mol.cm2K~km~

in exce~lei~tagreementwith that determined on the scaleof the Galaxy /1/ (2.3 ±0.3, 1mol.cm K~km~s)or In the Orion-Monoceroscomplex /2/. Such a consistencygives confidenceto apply this value to determinethe massesof nearby clouds. At low energy, the assumptionthat the atomic and molecular media have equivalent emlsslvities and the N(H, )/WCOconversionfactor found above 1~50MeV (since it should be energy lndep~ndent)would implya value for B of 5.1 io.6 cm2sf K~km~1.An evenhigher value of 6.7 10 has beenfound inthe plane at the solar circle by Strong et al. /1/. Hence, although the B value found inCepheusbelow 150 MeV nearly equals0, it Is not statistically significant.

A NEW COS-B SOURCE

As already mentioned, the observation of a localized gamma-rayexcesswith respect to theinterstellar emission stimulated the addition of a point- source term In the model to testagainst the observations. This soft excess Is ifiustrated in Figure 1 which shows the70-150 MeV gamma-rayintensity observedin 4°x4°bins versus the emission expectedfromthe Interstellar medium. The latter was predicted from the gas emissivities measuredi~th~~ola~ nelghbouçhood~,i

1ea~th1e plane /1/ and at higher latitudes /10/ (A= 1.05 10at~s~tsr~,B= 5 10~~cm sr K

1km~), and froi~t~e~Dackground Intensity given in the COS-Bdatabasefor this energyrange(C = 72 10~cm s~Sf ).

At low energy, the addition of a source term, with both its luminosity and position set asfree parameters, led to a significant improvement In the quality of the fit. For the bestparameters, this amelioration coresponds to an increase in likelihood between the modelswith and without a source which Is measuredby a log-likelihood ratio of 13.0 (l~= -2* ln{ L (without) IL (with) } where L is the maximum-likelihood value for a given model, see/13/). This value is to be compared to the statistical threshold of 12.0 whIch wasconsidered by Pollock et al. /14/ as a “reasonably weighty indication of the presenceof a source in the COS-B survey”. The presentexcesscan therefore be consideredas a newcandidatefor the final COS-B catalogueof sources.

GammaRays in Cepheus (2)83

Observed Intensity (70 - 150 MeU)

‘10~ci2 s1 Sr120

predictedIntensity

4 6 8 10 i2 14Fig. 1. Comparison of the 70-150 MeV intensities observed in 40x40 bins with thosepredicted from the gas content and the average gamma-ray emissivities measured in thesolar vicinity (plus the Instrumental noise). The highest data point reflects the presenceof a possiblenew gamma-raysource.

At the best position and at low energy, the increasein likelihood betweenthe models withand without the source correspondsto a 3.75~effect /9/. Between 150 and 300 MeV, noimprovementwas reachedby addingthe source,whereasabove300 Hey, the analysisshoweda weak 0.6a excessat this position. The full significance of the source emission from 70MeV to 5 GeV therefore amounts to 3.9a. Other statistical tests have been performed. Inparticular, becauseof the rather low 70-150 MeV emissivity measuredfor the molecular gasin Cepheus(i.e. B), the sourceexcesshasbeenconfronted to a more “normal” gasemission.With the A, B and C parametersforced to the valuesmentionedfor Figure 1, the likelihoodanalysisstill led to a sourcesignificanceof 3.5a.

The source position could only be testedin the 70-150 MeV rangewhere the COS-B resolutionis worse. This explains why the ia confidence region, centeredon 1 = 1110 and b = 19.80,has a rather large radius of 1.8°. The fluxes given in Table 2 Illustrate the unusualsoftnessof the source. The late emergenceof this excessis due to the lack of an adequateCO surveyof the Cepheusclouds andto the wide angularaspectof the sourcecausedby itsunusual soft spectrum. Indeed, while the present discovery corresponds to the 5thbrightest source in the 70-150 MeV sky, the wide point-spread-function reduces its

statistical significance to the limit ofdetectabIlity.

TABLE 2 MaxImum-likelihood fluxes Checking various catalogues of X ray sourcesin the 3 energy ranges of the (Uhuru, HEAO-A1 and A2), supernova remnants,proposed gamma-ray source at pulsars, HII regions and radiosources,1= 1110 andb 19.8° surprisingly very few possible counterparts were

noticed. Besidesthe variable X ray star ‘/W Cep, aEnergy Soj.ircefll.lx more Interesting object appears to be theMeV 10~cm2s~ radlogalaxy 3C427.1 at 1= 111.04~,b= 19.28°,which

is known to have two bright radio lobes and a70-150 2.3 ±0.7 redshlft of 0.572. But the Interesting spatial

coincidence Is not a sufficient argument to150-300 < 0.18 identify the source with this object. The next

generation of telescopes, such as Gamma-I and300-5000 0.07±0.09 Sigma, will be needed to confirm, locate and maybe

identify this new gamma-raysource.

(2)84 I. A. GrenierandF. Lebrun

REFERENCES

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