u h soito fW9 ihahproaada and hypernova a with W49B of association the but eject the of parts Fe- where more explosion and spherical hotter a is with jet emission eastern the and structure jet-like e words: Key ftermat ecnld htteseai fa anisotrop an of scenario the that an conclude We of remnant. analysis the the of of results new present we asym ambiguity an of result the be may remnant the that indicates W49B of h ihs eprtr on nteesensd n h low the and an side Ca eastern Ar, the S, in Si, found from temperature lines highest emission the K strong by characterized emntdo t atr n yasappredclrstruc perpendicular sharp a by end eastern its on terminated tla aeil efcso h aatcsproaremnan supernova galactic the their and on remnants focus supernova we of material, study the stellar of framework the In Abstract oaepoin n ntercnrbto otechemical the Galaxy. to the contribution super- of their evolution the ejecta, on of the and dynamics explosions study the nova to on information tool is obtaining diagnostic SNRs thus powerful ejecta-dominated young, a the therefore of ISM ambient study large a The with cloud. remnant produced the of shock, interaction reflected the the by reverse with or the 1974), with (McKee interaction X-ray shock the to up by heated temperatures are ejecta emitting The speed explosion. supersonic SN at the associ- expelled during material is ejected emission the In X-ray with ated inhomogeneities. the ISM SNRs, the generated ejecta-dominated and the shock explosion wave SN blast com- the their the the during by between and influenced interaction (ISM) deeply plex medium are evolution interstellar and the emission for energy and Introduction 1. arXiv:astro-ph/0701475v1 16 Jan 2007 rpitsbitdt Elsevier to submitted Preprint uenv enns(Ns r oeflsucso mass of sources powerful are (SNRs) remnants Supernova b iatmnod cez iih dAtooih,Szoed Sezione Astronomiche, ed Fisiche Scienze di Dipartimento hsclitrrtto ftejtlk -a msinfr emission X-ray jet-like the of interpretation physical A .Miceli M. d eateto hsc n srnm,RtesUiest,1 University, Rutgers Astronomy, and Physics of Department -as S,IM uenv enns S:idvda obje individual ISM: remnants, supernova ISM: ISM, X-rays: a S/ANASriedAtohsqe I-M 18 E Sa CEA 7158, AIM-UMR d’Astrophysique, DSM/DAPNIA/Service e etrfrAtohsclSine,TeJhsHpisUnive Hopkins Johns The Sciences, Astrophysical for Center f aoaoyfrHg nryAtohsc,GdadSaeFli Space Goddard Astrophysics, Energy High for Laboratory c a NF-Osraoi srnmc iPlro izadlParl del Piazza Palermo, di Astronomico Osservatorio - INAF , b , c .Decourchelle A. a .Ballet J. enn W49B remnant γ − XMM-Newton a us,atog tl osbe sntdrcl support directly not is possible, still although burst, ray aem,Italy Palermo, ihta h etr n.Aohrpsil cnroassoci scenario possible Another one. western the than rich r neatn ihadnebl fabetmtra.T ov To material. ambient of belt dense a with interacting are a a 4B t opooyehbt nXrybih lnae ne elongated bright X-ray an exhibits morphology Its W49B. t srnma nvri` iPlro izadlParlament del Piazza Palermo, Universit`a di Astronomia, i .Bocchino F. cjtlk xlso xlisqientrlyorobserva our naturally quite explains explosion jet-like ic ueaindwt h ai hl.TeXrysetu fW49B of spectrum X-ray The shell. radio the with aligned ture s n ntewsensd.Teaayi ftercn observ recent the of analysis The side. western the in one est e hr savraino h eprtr ntermatwi remnant the in temperature the of variation a is There Fe. d ope neato ihteitrtla eimadteci the and medium interstellar the with interaction complex ercbplrepoinweeteeet r olmtdal collimated are ejecta the where explosion bipolar metric 6FeigusnRa,Psaaa,N 85-19 USA 08854-8109, NJ Piscataway, Road, Frelinghuysen 36 etwt hi e L aa sntspotdb very possible by of supported range not the is that data, and VLA evidence consis- clear new perfectly their although with estimate, tent this that shown have 4Bwt h trfrigrgo 4A(hc sa 11 at is (which W49A region of forming association star possible the a with with W49B consistent is limit upper This n osdrn h orcin yMfft n Reynolds and Moffett by corrections (1994), (1972), the al. et considering Radhakrishnan and of analysis absorption HI the e sqielre being large, quite is ues e niain htW9 scoe ou.I htfollows, what In us. use a to will are closer we there however, is although W49B 1992), that al. indications et few Gwinn to according kpc, n ieetosrain aebe efre ihthe with performed been have observations different and hlg.Isdistance Its phology. ai hl woedaee is diameter bright (whose a has shell It radio band. X-ray the in observed SNRs dominated t W49B ct: bevto n eprometmtso h asadenergy and mass the of estimates perform we and observation h -a msino 4Bhsbe ieystudied widely been has W49B of emission X-ray The h aatcSRW9 soeo h rgts ejecta- brightest the of one is W49B SNR galactic The st,30 hre t atmr D21218 MD Baltimore St, Charles 3400 rsity, h etr rebl,M 07,USA 20771, MD Greenbelt, Center, ght D c ly 19 i-u-vteCdx France Cedex, Gif-sur-Yvette 91191 clay, W .Hughes J. mno1 03 aem,Italy Palermo, 90134 1, amento 49 B ∼ p,btBoa n rln (2001) Troland and Brogan but kpc, 8 D D d W kpc. 8 = .Hwang U. 49 ∼ B kpc 8 ssilucran codn to According uncertain. still is ∼ msupernova om 4 ′ D < n etrfildmor- filled center a and ) e , f W .Petre R. db n evidence. any by ed 49 B tsteX-ray the ates < ,90134 1, o D inresults, tion roethis ercome uy2018 July 2 ∼ W f 49 2kpc. 12 B ations rcum- n a ong bula, val- th . is 4 past generations of X-ray satellites (Einstein Observatory, e. the higher the continuum temperature), the higher is Pye et al. 1984; EXOSAT, Smith et al. 1985, and ASCA, the local MPE. To produce this map, we considered the Fujimoto et al. 1995, Hwang et al. 2000). In particular, the XMM-Newton EPIC MOS1 and MOS2 event files in the analysis of the ASCA spectrum of W49B has shown that continuum energy band 4.4 6.2 keV, with a bin size of the X-ray emission of W49B can be described with two 6′′ (so as to collect more than− 10 counts per pixel every- thermal components of optically thin plasma with signifi- where in the remnant). For each pixel, we calculated the cant overabundances of Si, S, Ar, Ca, and Fe with respect median energy of the photons and then we smoothed the to the solar values (Hwang et al. 2000). map by using the following procedure: for each pixel k of In Miceli et al. (2006) (hereafter Paper I) we proposed −(i−k)2 the MPE map we define its weight as Wk = Ck e σ2 , two possible physical scenarios associated with W49B and where Ck is the number of photons detected in that pixel, both these interpretations are consistent with the observa- the i index runs over all the other pixels and σ = 12′′. tions subsequently analyzed by Keohane et al. (2006). The The smoothed median energy value at the i pixel is then observed X-ray and IR emission of W49B can be the result SMPEi = Σk (Wk MP Ek)/Σk (MP Ek). The smoothed of i) an asymmetric bipolar explosion (with the eastern jet MPE map of W49B is shown in Fig. 2 and confirms the being hotter and more Fe-rich than the western jet) or ii) presence of a higher continuum mean photon energy in the a spherical explosion in a highly inhomogeneous circum- eastern side of the remnant than in its western end. stellar and interstellar medium. Here we describe in detail these possible scenarios, we show new results of the analy- sis of the XMM-Newton data presented in Paper I, and we try to obtain a more complete picture of the remnant and to discriminate between the two interpretations.
2. The X-ray emitting plasma in W49B
As shown in Fig. 1, the supernova remnant W49B presents a complex X-ray morphology, which is character- ized by a bright centrally elongated structure. This struc- ture is terminated on the eastern side by a perpendicular sharp region and on the western side by a more diffuse, nearly aligned structure. The spatially resolved spectral analysis we performed in Paper I shows that the physical conditions of the plasma are not homogeneous throughout the remnant. All spectra are described well by two thermal components in collisional ionization equilibrium and lower temperatures are found in the western end of the remnant, while hotter regions are located in the center and at East. Everywhere in the remnant Si, S, Ar, Ca, and Fe are over- abundant with respect to their solar values and we also − +2 Fig. 1. XMM-Newton EPIC count-rate image of W49B in the 1 9 measured a high Ni overabundance (Ni/Ni⊙ = 10−1) in keV energy band. The image is vignetting-corrected, adaptively the central region of W49B. While the central and eastern smoothed (with signal-to-noise ratio 10) and background-subtracted. −3 −1 structure have similar abundances, the western region ex- The count-rate ranges between 0 and 7.2 × 10 s . We have su- hibits lower abundances. There is therefore an anisotropy perimposed the ranges of the best fit parameter obtained in Paper I for the spectral regions at the central-eastern side of the remnant in temperature and abundances, as shown in Fig. 1, where and for those at its western side. we report the ranges of the best-fit parameters we derived in Paper I in the spectral regions at the central-eastern As shown in Paper I, although both the X-ray emit- side of the remnant and in those at its western side. This ting components seem to be associated with overabundant anisotropy has been also confirmed by the analysis of the plasma, it is not possible to rule out the possibility for Chandra observation described in Keohane et al. (2006). the soft component to be associated with the shocked cir- To highlight this temperature anisotropy and to obtain cumstellar material (CSM) or interstellar medium. The information about the thermal structure of the source we hard component is, however, associated with the ejecta. present here the MOS median photon energy (MPE) con- As shown in Paper I, the Si He-like emission is clearly as- tinuum map, that is an image where each pixel holds the sociated with the soft component, whose contribution be- median energy of the detected MOS photons in the con- comes negligible in the energy band of the Si H-like emis- tinuum band 4.4 6.2 keV. This map provides informa- sion line complex. To investigate the spatial distribution of tion about the spatial− distribution of the spectral prop- these components and their relative weight, we produce an erties of the plasma, because the harder the spectrum (i. XMM-Newton EPIC image of the ratio Si He-like/Si H-like
2 Fig. 2. MOS mean photon energy map of the continuum emission (bin size= 6′′). In each pixel the local median photon energy of the photons detected by the MOS cameras in the continuum energy band 4.4 − 6.2 keV is reported. We superimposed, in black, 4 contour levels indicating where the average is performed over more than 10, 20, 30, and 40 counts (from the outer to the inner level, respectively). Pixels with less than 4 counts have been masked out. The color bar has a linear scale and ranges between 4.85 keV and 5.25 keV. emission (i. e. the count-rate image in the 1.77 1.9 keV en- perature (few 103 K) and density (2 3 103 cm−3) of ergy band divided by the count-rate image in the− 1.96 2.06 the molecular gas. Since in Paper I we− estimated× for the keV energy band). This map has been produced according− hotter X-ray emitting component a density of a few cm−3, to the data analysis procedure described in Paper I and is the H2 cloud is about three order of magnitude denser than shown in Fig. 3. The Si He-like/Si H-like ratio is enhanced the ejecta. The cloud then acts as a wall that prevents the in the outer parts of the remnant (with two maxima in the expansion in the East direction. Molecular H2 emission is northern and southeastern parts of W49B), while it is al- present also in the south western region of the shell, where most uniform in the jet-like central region and at its eastern we observe high Si He-like/Si H-like ratios and where copi- and western ends. If we associate the cool component (i. e. ous [Fe II] and radio emission are observed. the Si He-like emission) with the ISM (or with the CSM) and the Si H-like emission with the ejecta, this may indi- cate that the ejecta are mainly distributed along the elon- 3. The physical origin of W49B gated jet-like structure, while the shocked ambient mate- rial totally embeds the remnant. On the other hand, the Si The peculiar jet-like morphology of W49B and the evi- He-like/Si H-like ratio reaches its maxima in regions with dence for the presence of X-ray emitting ejecta suggest that low surface brightness, where the statistics are not good the remnant is the result of a highly aspherical bipolar ex- enough to make these features completely reliable. plosion. On the other hand, it is also possible that W49B At the eastern border of the remnant, where bright radio originated by a spherical explosion in a complex ambient emission has been observed (see, for example, Lacey et al. medium with enhanced density in a torus-like structure all 2001), a shocked molecular H2 cloud has been revealed by across W49B. In this case, we would observe only a small Keohane et al. (2006), who derived estimates of the tem- fraction of the ejecta, i. e. only the material expelled in
3 30.0
08:00.0
30.0
07:00.0
30.0
9:06:00.0 Declination
30.0
05:00.0
30.0
04:00.0
16.0 12.0 08.0 04.0 19:11:00.0 Right ascension
Fig. 3. XMM-Newton EPIC image of the count-rate in the 1.77 − 1.9 keV energy band divided by the count-rate in the 1.96 − 2.06 keV energy band. The image is vignetting-corrected, adaptively smoothed (with signal-to-noise ratio 25) and background-subtracted. The color bar ranges between 1 and 2.2. We have superimposed (in black) the X-ray contour levels derived from the 1 − 9 keV EPIC image at 25%, 50%, and 75% of the maximum, and the contour levels of the radio image at 327 MHz (obtained by Lacey et al. 2001) at 10% (yellow) and 50% (white) of the maximum. the plane of the torus that is interacting with the reverse shock and is heated up to X-ray emitting temperature. A Fig. 4. Schematic representation of the two possible physical scenarios schematic representation of the two scenarios is shown in associated with W49B. Fig. 4. As we have shown in Paper I, both these interpreta- tions are consistent with the spectral analysis results and in the case of a spherical explosion, we would observe only with the observed abundance pattern. the ejecta heated by the reflected shock generated by the We can estimate the mass of the X-ray emitting ejecta in impact of the main shock with the circumstellar torus. We both these scenarios. As shown in Paper I, the hard compo- can therefore assume that the ejecta are distributed into nent is clearly associated with the ejecta, therefore, for the the cylinder shown in the lower panel of Fig. 5 (with ra- ′ ′ mass estimates, we will refer only to this component. Our dius Rsph = 1.8 , height Hsph = 1.1 and volume Vsph mass estimates, however, would at maximum double by as- 4.1 1057 cm3 at 8 kpc). Taking the emission measure per∼ sociating also the soft component with the ejecta, since the unit× area (EM) of this region into account, we obtain a −3 emission measure of the soft component is similar to that density nsph = EM/2Rsph 2.6 cm and an X-ray p ∼ of the hot component (within a factor of two, see Table 3 emitting mass Msph 10 M⊙. It is also possible that the in Paper 1). Notice also that we assume the mass density ejecta do not occupy∼ all the cylindric volume, but only a to be ρ = µmH nH , with ne = nH and with µ =1.26. This small fraction of it. For example, we can assume the ejecta µ value corresponds to cosmic abundances, therefore, since to occupy a torus with outer radius Rsph and inner radius in the hard component we find significant metal overabun- gRsph with 0
2 vr 7 Ts = 10 K , (1) 839 km s−1 ×
where Ts is the post-shock temperature of the plasma. The spectral analysis performed in Paper I allowed us to derive the electron post-shock temperature Te 2.3 keV (see Ta- ∼ ble 4 in Paper I) which is always Ts. If we assume the ≤ ion temperature to be of the same order of Te, we obtain the lower limit vr > 1350 km/s. We remind the reader that this is the speed of the ejecta in the reference frame of the reverse shock and is therefore larger than the speed of the ejecta in the reference frame of the unperturbed ambient ISM. In fact, given the mass of the shocked ejecta, it is reasonable to conclude that the reverse shock is moving to- wards the center of the remnant. Notice that this is true also if we assume the ejecta to be heated by the reflected shock generated by the interaction of the main shock front with the dense molecular cloud. The vr value is remark- ably in agreement with the estimate of the X-ray forward shock velocity with respect to the unperturbed medium, vs 1200 km/s, derived by Keohane et al. (2006). Tak- ∼ ing our Mjet estimate into account, we then derive a ki- 50 netic energy Kej 1.2 10 erg. Notice that, according to Rakowski (2005),∼ it is× reasonable to assume that 0.1 < Te/Ts < 1 (for shock velocity < 5000 km/s). In the case Te =0.1 Ts, we then obtain the upper limit vr < 4300 km/s 51 and then Kej < 1.2 10 erg. The internal energy Ujet = × 49 50 (3/2)njetkTsVjet is in the range 3.4 10 3.4 10 erg × − × (for Te/Ts = 0.1 1) and is even lower. The total energy of the remnant 1 is− then, in any case, lower than 1.5 1051 erg. Although our estimates of mass and velocity are× quite rough, it would be necessary for the velocity of the ejecta to be one order of magnitude larger than vjet, or for the total mass to be two orders of magnitude larger than Mjet Fig. 5. Same as Fig. 1 where we have superimposed the cylindric to obtain 1052 erg. This result, together with the observed volumes used to estimate the mass of the X-ray emitting ejecta for a jet-like explosion (upper panel) and for a spherical explosion (lower abundance pattern, concur in excluding the possible asso- panel). ciation of W49B with a hypernova (and a GRB), although this association cannot be definitely ruled out especially Aspherical explosions are a common feature of hyper- given the emergence of the class of underenergetic GRBs 52 novae (i. e. supernova explosions with E 10 erg, for associated with SNe (Mazzali et al. 2006). a review on hypernovae see Nomoto et al.∼ 2003), that are 1 linked to γ ray bursts, GRB, (see Paper I and references In the case of the spherical explosion scenario we find Ksph ∼ − 50 51 49 50 therein). In Paper I we showed that the comparison between 2 × 10 − 2 × 10 erg and Usph ∼ 6 × 10 − 6 × 10 erg.
5 4. Conclusion Blondin, J. M., Mezzacappa, A., DeMarino, C., Feb. 2003. Stability of Standing Accretion Shocks, with an Eye to- The morphology and physical properties of the X-ray ward Core-Collapse Supernovae. ApJ 584, 971–980. emitting plasma in W49B have been extensively studied in Brogan, C. L., Troland, T. H., Apr. 2001. Very Large Array Paper I, where we have shown that two thermal compo- H I Zeeman Observations toward the W49 Complex. ApJ nents in collisional ionization equilibrium provide a good 550, 799–816. description of the XMM-Newton spectra. While the results Dwarkadas, V. V., Dec. 2006. Hydrodynamics of Supernova of the spectral analysis do not allow us to clearly distin- Evolution in the Winds of Massive Stars. Ap&SS, 537. guish between solar and enhanced abundances for the soft Fujimoto, R., Tanaka, Y., Inoue, H., Ishida, M., Itoh, component, the map of the Si He-like/ Si H-like emission M., Mitsuda, K., Sonobe, T., Tsunemi, H., Murakami, lines presented here suggests an identification of the soft H., Koyama, K., Hayashi, I., Ikeda, K., Rasmussen, A., component with CSM material. Ricker, G., Becker, C. M., Oct. 1995. ASCA Observation We have also shown indications that W49B is linked to an of the Supernova Remnant W49B. PASJ 47, L31–L35. aspherical supernova explosion in a wind cavity, bounded Gwinn, C. R., Moran, J. M., Reid, M. J., Jul. 1992. Distance on the eastern side of the remnant by the infrared emitting and kinematics of the W49N H2O maser outflow. ApJ molecular clouds. It is also possible that W49B is the result 393, 149–164. of a spherical SN in a pre-existing structure in the ambient Hwang, U., Petre, R., Hughes, J. P., Apr. 2000. The X-Ray environment, maybe generated by the strong stellar winds Line Emission from the Supernova Remnant W49B. ApJ of the progenitor star, which may have produced an aspher- 532, 970–979. ical reverse shock. However, our estimates of the ejected Iwamoto, K., Mazzali, P. A., Nomoto, K., Umeda, H., Naka- mass in the case of a spherical explosion yield to very high mura, T., Patat, F., Danziger, I. J., Young, T. R., Suzuki, values, while more realistic values can be obtained assum- T., Shigeyama, T., Augusteijn, T., Doublier, V., Gonza- ing a jet-like morphology of the ejecta. lez, J.-F., Boehnhardt, H., Brewer, J., Hainaut, O. R., There is a link between very energetic jet-like explosions Lidman, C., Leibundgut, B., Cappellaro, E., Turatto, M., (or hypernovae) and γ ray bursts, although a bipolar ex- Galama, T. J., Vreeswijk, P. M., Kouveliotou, C., van plosion does not necessarily− imply a GRB, because a high Paradijs, J., Pian, E., Palazzi, E., Frontera, F., 1998. A explosion energy (some 1052 erg) is also required. The abun- hypernova model for the supernova associated with the dance analysis presented in Paper I has shown that the as- gamma-ray burst of 25 April 1998. Nature 395, 672–674. sociation of W49B with a GRB is not supported by obser- Keohane, J. W., Reach, W. T., Rho, J., Jarrett, T. H., vational evidence and the estimates of the total energy of Sep. 2006. A Near-Infrared and X-ray Study of W49B: the ejecta presented here confirm this conclusion. A Wind Cavity Explosion. ArXiv Astrophysics e-prints. W49B seems then to be the result of an aspherical jet- Lacey, C. K., Lazio, T. J. W., Kassim, N. E., Duric, N., like supernova explosion explosion of normal energy. Notice Briggs, D. S., Dyer, K. K., Oct. 2001. Spatially Re- that, in this case, we would have significant differences be- solved Thermal Continuum Absorption against Super- tween the eastern arm of the jet, where high values of tem- nova Remnant W49B. ApJ 559, 954–962. perature and abundance have been found, and the west- Maeda, K., Nomoto, K., Dec. 2003. Bipolar Supernova ern arm. It is, however, difficult to ascertain whether this Explosions: Nucleosynthesis and Implications for Abun- East-West anisotropy is the result of an anisotropic explo- dances in Extremely Metal-Poor Stars. ApJ 598, 1163– sion (like, for example, that produced by the instability of 1200. the accretion shocks described by Blondin and Mezzacappa Mazzali, P. A., Deng, J., Nomoto, K., Sauer, D. N., Pian, E., 2006 and Blondin et al. 2003), or by the interaction of the Tominaga, N., Tanaka, M., Maeda, K., Filippenko, A. V., jets with a complex, inhomogeneous ambient medium. Aug. 2006. A neutron-star-driven X-ray flash associated with supernova SN 2006aj. Nature 442, 1018–1020. McKee, C. F., Mar. 1974. X-Ray Emission from an Inward- Propagating Shock in Young Supernova Remnants. ApJ 188, 335–340. Acknowledgements We thank the referees and J. Vink for McKee, C. F., Hollenbach, D. J., 1980. Interstellar shock useful suggestions. This work was partially supported by waves. ARA&A 18, 219–262. the Ministero dell’Istruzione, dell’Universit`ae della Ricerca Miceli, M., Decourchelle, A., Ballet, J., Bocchino, F., and by ASI-INAF contract I/023/05/0. Hughes, J. P., Hwang, U., Petre, R., Jul. 2006. The X- ray emission of the supernova remnant W49B observed with XMM-Newton. A&A 453, 567–578. Moffett, D. A., Reynolds, S. P., Dec. 1994. Multifrequency References studies of bright radio supernova remnants. 2: W49B. Blondin, J. M., Mezzacappa, A., May 2006. The Spherical ApJ 437, 705–726. Accretion Shock Instability in the Linear Regime. Apj Nomoto, K., Maeda, K., Umeda, H., Ohkubo, T., Deng, J., 642, 401–409. Mazzali, P., 2003. Hypernovae and their nucleosynthesis.
6 In: IAU Symposium. pp. 395–+. Pye, J. P., Thomas, N., Becker, R. H., Seward, F. D., Apr. 1984. Radio and X-ray maps of the supernova remnant W49B. MNRAS 207, 649–657. Radhakrishnan, V., Goss, W. M., Murray, J. D., Brooks, J. W., Jan. 1972. The Parkes Survey of 21- CENTIMETER Absorption in Discrete-Source Spectra. III. 21- Centimeter Absorption Measurements on 41 Galactic Sources North of Declination -48 degrees. ApJs 24, 49–+. Rakowski, C. E., 2005. Electron ion temperature equilibra- tion at collisionless shocks in supernova remnants. Ad- vances in Space Research 35, 1017–1026. Smith, A., Peacock, A., Jones, L. R., Pye, J. P., Sep. 1985. The X-ray spectrum of the supernova remnant W49B from EXOSAT. ApJ 296, 469–474.
7