1. Discovery of New SNR Candidates in the Galactic Center Region with ASCA and Chandra Atsushi Senda 1, Hiroshi Murakami 2, Aya Bamba 1, Shin-ichiro Takagi 1, and Katsuji Koyama 1 1 Department of Physics, Kyoto University, Japan; 2 Institute of Space and Astronautical Science (ISAS), Japan E-mail: senda@cr.scphys.kyoto-u.ac.jp 1. Introduction The origin of the diffuse X-ray emission from the Galactic Center (GC) region has been an open issue over decades. Ginga and ASCA found the large-scale ( ～ 1°x 1°) thin-thermal plasma with strong lines from ionized iron (Koyama et al. 1989; 1996, Yamauchi et al. 1990). The physical properties of the large scale hot plasma are as below. ･ electron temperature kT ～ 10 keV ･ total thermal energy E ～ 10 54 ergs ･ an age of the plasma t ～ 10 5 year Black : Chandra ACIS Green, Red : ASCA GIS 2, GIS3 Blue : ROSAT PSPC G359.79-0.26 G359.77-0.09 G0.570-0.018G359.92-0.09 G359.79-0.26 and G359.77-0.09 G0.570-0.018 is discovered by ASCA (Sakano et al. 2002). Chandra observation resolved this source as extended, shell-like structure. Obtained X-ray spectrum is consistent with that obtained with ASCA, which exhibits extremely strong Fe-K line emission. The combined X-ray spectra are well reproduced by a thermal NEI model. The narrow band image of the Chandra GC Survey shows that soft diffuse emissions extend from the Galactic Center to the galactic southeast direction. The extended emission is relatively soft and clumpy distributed. From these clumps, we focused on the two bright clumps and named G359.79-0.26 and G359.77-0.09. These clumps were also detected with ASCA and ROSAT. X-ray spectra of each clumps exhibit K-line emissions from He-like and H-like ions such as Si, S, Ar, and Ca. The results of the spectral fitting with thermal NEI model show that their physical parameters (N H and metal abundances) are similar to each other. In addition, the 1-3 keV band image shows G359.79-0.26 and G359.77-0.09 are south and north part of the large ( ～ 30 pc) elliptical shell, respectively. These indicate that the two clumps have the same origin, an energetic explosion such as a supernova occurred at the center of the large shell. G359.77-0.09 G359.79-0.26 kT 1.31 0.82 [keV](1.03—1.79) (0.75—0.93) τ 0.5 64 [10 11 s cm -3 ] (0.1—1.6) (5.6 <) N H 5.8 4.9 [10 22 H cm -2 ] (5.1— 6.5) (4.6—5.2) Flux 2.2 2.3 [10 -12 ergs s -1 cm -2 ] Abundances[solar] Si0.6 (0.4—0.9) 0.4 (0.3—0.6) S 0.8 (0.5—2.1) 0.8 (0.5—1.0) Ar0.3 (< 1.0) 1.3 (0.4—2.2) Ca1.4 (< 3.3) 3.4 (1.4—5.6) χ2/d.o.f.153.9/109 197.6/163 Best fit results of the thermal VNEI model G359.79-0.26 G359.77-0.09 b = 0.0° 0.2° -0.2° l = 0.0°359.8°359.6° 15“ ～ 0.6 pc G0.570-0.018 kT [keV] 6.1 (3.1—26) τ [10 10 s -1 cm -3 ] 1.7 (1.3—2.7) N H [10 22 H cm -2 ]13.9 (10.7—17.2) Abundance [solar] 4.5 (>1.6) Flux [ergs s -1 cm -2 ] 8.2 (7.7—8.7) χ 2 /d.o.f. 41.2/40 Best fit results of the thermal NEI model VLA 6cm (Ho et al. 1985) G359.92-0.09 wisp Sgr A East EH wisp SE EH SW wisp kT 11.4 2.6 12.5 [keV] (3.4<) (1.9 <) (7.1—80.0) τ 1.4 2.1 190 [1011 s cm-3] (0.8—5.4) (0.01 <) (0.001 <) N H 6.0 18 35 [10 22 H cm -2 ] (3.7—8.3) (7.0—33) (29—40) Flux 1.5 0.4 0.4 [10 -12 ergs s -1 cm -2 ] χ 2 /d.o.f. 44.3/38 9.8/9 8.83/7 Best fit results of the thermal NEI model ・ E. M. = 1.1 x 10 57 cm -3 ・ E EH = 1.6 x 10 50 ergs ・ ne = 0.4 cm -3 ・ t = 3800 yr (from the sound velocity@10keV) 1.2 x 10 4 yr (from an ionization parameter) By applying Sedov self-similar model R = 0.4 pc E = 3.5 x 10 48 ergs kT = 6.1 keV n a = 5.1 cm-3 E.M.= 8.1 x 10 56 cm -3 t = 70 yrs ⇒ The swept up mass (M= na x 4/3πR 3 = 0.03Mo) and estimated initial energy are both extremely small compared with that of a usual SNR. ⇒ Only a tiny fraction of the explosion energy ( ～ 10 51 ergs) and ejected mass ( ～ a few Mo) are converted to thermal plasma. G0.570-0.018 is a young SNR which is not in an adiabatic phase but still in a free expansion phase. (Senda et al. 2002, ApJ) non-thermal sources 4. Discussion ･ GC plasma has at least two temperature thermal components kT ～ 10keV hard component ← G359.92-0.09, G0.570-0.018 kT ～ 1 keV soft component ← G359.77-0.09 and G359.79-0.26 ･ To explain the total luminosity of GC diffuse emission ( ～ 2 x 10 38 ergs/s) with the sum of SNRs, ～ 100 SNRs should be detected in an X-ray band, which are far grater than the number of the present detection. ⇒ Undetected SNRs ? Other origin ? = G359.79-0.26 + GC plasma G11.0+0.0 G11.2-0.3 (a) G25.5+0.0 (b) G26.6-0.1 AX J1841.0-0536 (c) G28.6-0.1 (d) (a)(b)(c)(d) In the Galactic plane, a number of Synchrotron X-ray dominant SNRs (SXSs) have been found with ASCA (Bamba et al. 2001, 2002) and Chandra (Ueno et al. 2002). In the GC region, no SXSs have ever been found, however, deep observations with Chandra or Newton will be found SXSs in the GC region. The origin of the thermal diffuse emission for the GC region ACIS Sgr B2 region 6.0-7.0 keV G0.570-0.018 Obs Name Coordinates Exporsure l [deg]b [deg] [ksec] Sgr A*359.94 -0.05 48.7 Sgr B2 0.59 -0.02 99.0 GCS13 0.00 -0.20 11.8 GCS14 0.00 0.00 10.8 GCS16359.80 -0.20 11.8 GCS17359.80 0.00 10.8 GCS19359.61 -0.20 10.8 GCS20359.61 0.00 10.8 Chandra Observation log 2. Observations and X-ray Images Chandra ACIS Galactic Center 3.0-8.0 keV b = 0.0° -0.2° l = 0.0°359.8° 359.6° G359.92-0.09 0.2° What is the origin of the diffuse X-ray emission near the GC? ･ Multiple Supernovae ( ～ 1000 SNe / 10 5 yr) ? ･ Energetic explosion of the massive black hole Sgr A* ? Recent results of Chandra confirmed that the most of X-ray flux (both continuum and Fe-K line emission) from the GC region is truly diffuse (Wang et al. 2002). On the other hand, surface brightness of the diffuse emission is found to be rather clumpy. The presence of the clumpy structures may favor the multiple-SNe scenario. In fact, new X-ray SNRs have been discovered by observations with Chandra (e.g. Maeda et al. 2002). In this paper, we investigate newly discovered clumpy structures near the GC to reveal the origin of the diffuse X-ray emission. 3. Individual diffuse sources 5. Summary 0.0° -0.1° 0.1° 0.7°0.6°0.5° Ho et al. (1985) reported that there is non- thermal radio shell located about 5’ south of the SNR Sgr A East, and named it G359.92-0.09 as a possible GC SNR candidate. Chandra observation reveals an X-ray excess fills an eastern half (EH) and southwest(SW) part of the radio shell G359.92-0.09. Northwest part shows no excess in the shell because of the contamination of the intense X-ray emission from Sgr A East. In the southwest edge of the shell, an X-ray bright filament are also discovered, which clearly corresponds with a non-thermal radio filament called “wisp”. X-ray spectra are extracted from three different region; Eastern half (EH), Southwest quadrant (SW), and wisp. An thermal NEI model yields an acceptable fit for a spectrum from each region. wisp G359.92-0.09 Sgr A East 58’ 02’ -29°00’ 04’ 20 s 17 h 42 m 30 s 40 s ACIS 3.0-8.0 keV Dec (1950) R.A. (1950) With Chandra data, we found diffuse emission from the GC region has clumpy structures. Some of these clumps show thermal spectra from high temperature ( ～ 10keV) plasma (G0.570-0.018 and G359.92- 0.09), some show that from lower temperature ( ～ 1keV) plasma (G359.77-0.09 and G359.79-0.26). Their shell-like morphology and energetics suggest that they are SNRs. G359.92-0.09 has a counterpart of a non-thermal radio shell. Total diffuse emission from the GC emission is grater than that of GC SNRs by 1—2 order of magnitude, hence a relation between the GC diffuse and SNRs is unclear yet. Recently, a number of SXSs are found in the Galactic Plane, while no SXS has found yet in the GC region. Images: ASCA GIS 0.7-10keV (coordinates are in Galactic) Spectra: GIS2+3 data fitted with power-law model G0.570-0.018 Γ=1.6Γ=1.8Γ=1.3Γ=2.1 Chandra ACIS Galactic Center 1.0-3.0 keV By applying Sedov self-similar model R = 16.5 pc E = 2.1 x 10 51 ergs kT = 0.8 keV n a = 0.3 cm -3 E. M. = 1.47 x 10 59 cm -3 t = 6900 yr t = 2.4x10 4 yr (from an ionization parameter) Supposed two clumps are the part of a large shell-like structure, its energetics are consistent with that of a typical SNR. Young (3800—12000 yr) SNR References Bamba, A. et al. 2001, PASJ, 53, L21 Bamba, A. et al. 2002, submitted to ApJ Coil, A. L, and Ho, P. T. P. 2000, ApJ, 533, 245 Koyama, K. et al. 1989, nature, 339, 603 Koyama, K. et al. 1996, PASJ, 48, 249 Ho, P. T. P. et al. 1985, ApJ, 288, 575 Maeda, Y. et al. 2002, ApJ, 570, 671 Murakami, H. 2002, Ph.D thesis, Kyoto Univ. Sakano, M. et al. 2002, ApJS, 138, 19 Senda, A et al. 2002, ApJ, 565, 1017 Ueno, M. et al. 2002, submitted to ApJ Wang, Q. D. et al. 2002, nature, 415, 148 Yamauchi, S. et al. 1990, ApJ, 1990, 365, 532