Gamma rays and interstellar gas in the Cepheus region: A new gamma-ray source?

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  • Adv. Space Res. Vol. 10, No. 2, pp. (2)81(2)84, 1990 02731177/90 $0.00 +50Printed in Great Britain. 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 dAstrophysique, Centre dEtudes Nucleaires de Saclay,

    91191 Gif-sur-Yvette Cedex, France

    ABSTRACT

    A recent CO survey of the Cepheus Flare region (1000~l1410 , 80th~22)ha8 allowed adetailed study of the diffuse gamma-ray emission and the gas content of this nearbyregion. The comparison of the HI and CO observations with the COS-B gamma-ray data yieldsestimates 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 emission has been found and can be Interpreted as a new gamma-ray source.

    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/gas correlation using the HI andCO surveys of vast nearby clouds provides instead a precise measurement of the localgamma-ray emissivity. The analysis of the Orion and Oph-Sag regions already led,respectively, to a standard ezuissivity /2/ and an emissivity enhancement possibly relatedto the Loop I presence /3/. Today, the recent CO map of the whole Cepheus region allows anew local measurement, inside another molecular complex, as well as an independentcalibration of the N( H

    2 )/WCO conversion factor.

    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 massive molecular complex at about 300 pc and clouds of the Local Arm at 800-900 pc /4/.Data integrated over all velocities have been considered to compare to the gamma-raydistribution. The CO observations of 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 has been assumed void of molecules. N(HI) column density maps havebeen constructed from the observations of Weaver and WIlliams /6/ for bl0 (with a uniformspin temperature of 125 K) and of Heiles and Hablng /7/ for b10 (assuming an opticallythIn emission). Maps of the gamma-ray intensities in different energy ranges have beenderived from 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-source term. Thus, in each bin of the sky, the predicted counts wereCalculated as 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 dependent resolutionof the COS-B telescope. ct refers to the effective exposure in 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 2x20 bins for the three classical COS-Benergy ranges (70-150, 150-300, 300-5000 MeV). Further details on the complete analysis,for Instance with and without the source term, can be found in /9/.

  • (2)82 I. A. Grenier and F. Lebrun

    TABLE 1 Maxlmum-likelthood values for the parameters describing the diffuseemission in the 3 energy ranges, and the resulting ratio N(H2)/WCO = B/2A:Energy ~ A B C N(H2)/W~OMeV 10 ats~sr~ 10

    6cm2sr~K1km~1 106cm2sr~s~ mol.ci I~kth1 s

    70150 1.1 0.3 0.5 5.0 72. 10. 0.23 2.3

    150300 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 Cepheus direction lies within at most 1.5kpc, the small emissivity gradient found by Strong et al. /1/ is negligeable over thisscale length. So, we may assume a 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 and B = 2. (q/4it). N(H2)/WCO. The resultsfor A, B and C in the 3 energy ranges and their respective errors are listed In Table 1.

    A quick look at the COS-B database shows that the background Intensities derived In theCepheus direction in the three energy ranges are the same as in the rest of the galacticsurvey.The emissivity spectrum (A) of the atomic gas fully agrees with the values derived at thesolar circle from the study of the entire galactic plane below 10~ /1/. It is alsoconsistent with the SAS-2 observations at medium latitude outside the Loop I region /3/and with the emlssivity of the Orion-Monoceros complex above 300 MeV /2/. Other previousmeasurements also 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-Sag region and with a molecular gas contribution estimated from 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 measurements Indifferent regions and with various observations suggests that the gamma-ray emissivitlesfound between 70 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~tagreement with that determined on the scale of the Galaxy /1/ (2.3 0.3, 1mol.cm K~km~s)or In the Orion-Monoceros complex /2/. Such a consistency gives confidenceto apply this value to determine the masses of nearby clouds. At low energy, the assumptionthat the atomic and molecular media have equivalent emlsslvities and the N(H, )/WCOconversion factor found above 1~50 MeV (since it should be energy lndep~ndent) would implya value for B of 5.1 io.6 cm2sf K~km~1.An even higher value of 6.7 10 has been found inthe plane at the solar circle by Strong et al. /1/. Hence, although the B value found inCepheus below 150 MeV nearly equals 0, it Is not statistically significant.

    A NEW COS-B SOURCE

    As already mentioned, the observation of a localized gamma-ray excess with 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-ray intensity observed in 4x4bins versus the emission expected fromthe Interstellar medium. The latter was predicted from the gas emissivities measured i~th~ ~ola~ nelghbouhood~, 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-Bdatabase for this energy range (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 measured by 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 present excess can therefore be considered as a newcandidate for the final COS-B catalogue of sources.

  • Gamma Rays in Cepheus (2)83

    Observed Intensity (70 - 150 MeU)10~ci2 s1 Sr120

    predicted Intensity

    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 possible new gamma-ray source.

    At the best position and at low energy, the increase in likelihood between the models withand without the source corresponds to a 3.75~ effect /9/. Between 150 and 300 MeV, noimprovement was reached by adding the source, whereas above 300 Hey, the analysis showeda weak 0.6a excess at 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, because of the rather low 70-150 MeV emissivity measured for the molecular gasin Cepheus (i.e. B), the source excess has been confronted to a more normal gas emission.With the A, B and C parameters forced to the values mentioned for Figure 1, the likelihoodanalysis still led to a source significance of 3.5a.

    The source position could only be tested in the 70-150 MeV range where the COS-B resolutionis worse. This explains why the ia confidence region, centered on 1 = 1110 and b = 19.80,has a rather large radius of 1.8. The fluxes given in Table 2 Illustrate the unusualsoftness of the source. The late emergence of this excess is due to the lack of an adequateCO survey of the Cepheus clouds and to the wide angular aspect of the source caused by 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 and b 19.8 surprisingly very few possible counterparts were

    noticed. Besides the variable X ray star /W Cep, aEnergy Soj.irce fll.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-ray source.

  • (2)84 I. A. Grenier and F. Lebrun

    REFERENCES

    1. Strong A.W., Bloemen J. B. G.M., Dame T . M., Grenier I. A., Hermsen W., Lebrun F., NymanL.-A., Poliock A., Thaddeus P., Astr. Ap., in press (1988)

    2. Bloemen J.B.G.M., Caraveo P.A., Hermsen W., Lebrun F., Maddalena R.J., Strong A.W.,Thaddeus P., Astr. Ap., 139, 37 (1984a)

    3. Lebrun F., Paul J.A., Proc. XIXth Int. Cosmic Ray Conf., La Jolla, 1, 309 (1985)

    4. Grenler l.A., Lebrun F., Arnaud M., Dame T.M., Thaddeus P., submitted to Ap. J. (1988)

    5. Dame T.M., Ungerechts H., Cohen R.S., de Geus E.J., Grenier l.A., May J., Murphy D.C.,Nyman L.-A., Thaddeus P., Ap. J., 322, 706 (1987)

    6. weaver H. and Williams D.R.W., Astr. Ap. Suppi., 8, 1 (1973)

    7. Heiles C., Habing H.J., Astr. Ap. Suppi., 14, 1 (1974)

    8. Mayer-Hasseiwander H.A., Explanatory Supplement to the COS-B database (1985)

    9. Grenier l.A. and Lebrun F., submitted to Ap. J. (1988)

    10. Strong A.W., Bloemen J.B.G.M., HermsenW., Mayer-Hasselwander H.A., Proc. XIXth Int

    .

    Cosmic Ray Conf., 1, 317 (1985)

    11. Lebrun F., Ap. J., 306, 16 (1986)

    12. Bloemen J. B.G.M., Bennett K., Bignami C.F., Blitz, Caraveo P.A., Gottweld M.,Hermsen W., Lebrun F., Mayer-Hasselwander H.A., Strong A.W., Astr. Ap., 135, 12 (1984)

    13. Eadie W.T., Drijard D., James F. E., Roos M., Sadoulet B., Statistical Methods Inexperimental physics, North Holland, Amsterdam (1977)

    14. PoliockA.M.D., BennettK., Bignamia.F., Bloemenj.B.G.M., Buccherl.R., CaraveoP.A.,Hermsen w., Kanbach G., Lebrun F., Mayer-Hasseiwander H.A., Strong A.W., Astr. Ap.,146,352 (1985)

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