Cloud Sgr B2 Research Articles

AUSTRALIA TELESCOPE COMPACT ARRAY RADIO CONTINUUM 1384 AND 2368 MHz OBSERVATIONS OF SAGITTARIUS B

We present images of the Sagittarius (Sgr) B giant molecular cloud at 2368 and 1384 MHz obtained using new, multi-configuration Australia Telescope Compact Array (ATCA) observations. We have combined these observations with archival single-dish observations yielding images at resolutions of 47" by 14" and 27" by 8" at 1384 and 2368 MHz respectively. These observations were motivated by our theoretical work (Protheroe et al. 2008) indicating the possibility that synchrotron emission from secondary electrons and positrons created in hadronic cosmic ray (CR) collisions with the ambient matter of the Sgr B2 cloud could provide a detectable (and possibly linearly polarized) non-thermal radio signal. We find that the only detectable non-thermal emission from the Sgr B region is from a strong source to the south of Sgr B2, which we label Sgr B2 Southern Complex (SC). We find Sgr B2(SC) integrated flux densities of 1.2+/-0.2 Jy at 1384 MHz and 0.7+/-0.1 Jy at 2368 MHz for a source of FWHM size at 1384 MHz of ~54". Despite its non-thermal nature, the synchrotron emission from this source is unlikely to be dominantly due to secondary electrons and positrons. We use polarization data to place 5-sigma upper limits on the level of polarized intensity from the Sgr B2 cloud of 3.5 and 3 mJy/beam at 1384 and 2368 MHz respectively. We also use the angular distribution of the total intensity of archival 330 MHz VLA and the total intensity and polarized emission of our new 1384 MHz and 2368 MHz data to constrain the diffusion coefficient for transport of the parent hadronic CRs into the dense core of Sgr B2 to be no larger than about 1% of that in the Galactic disk. Finally, we have also used the data to perform a spectral and morphological study of the features of the Sgr B cloud and compare and contrast these to previous studies.

Ground-state ammonia and water in absorption towards Sgr B2

We have used the Odin submillimetre-wave satellite telescope to observe the ground state transitions of ortho-ammonia and ortho-water, including their 15N, 18O, and 17O isotopologues, towards Sgr B2. The extensive simultaneous velocity coverage of the observations, >500 km/s, ensures that we can probe the conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the Galactic centre, and the more diffuse gas in the Galactic disk clouds along the line-of-sight. We present ground-state NH3 absorption in seven distinct velocity features along the line-of-sight towards Sgr B2. We find a nearly linear correlation between the column densities of NH3 and CS, and a square-root relation to N2H+. The ammonia abundance in these diffuse Galactic disk clouds is estimated to be about (0.5-1)e-8, similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of H218O absorption in the 3 kpc arm, and the absence of such a feature in the H217O spectrum, we conclude that the water abundance is around 1e-7, compared to ~1e-8 for NH3. The Sgr B2 molecular cloud itself is seen in absorption in NH3, 15NH3, H2O, H218O, and H217O, with emission superimposed on the absorption in the main isotopologues. The non-LTE excitation of NH3 in the environment of Sgr B2 can be explained without invoking an unusually hot (500 K) molecular layer. A hot layer is similarly not required to explain the line profiles of the 1_{1,0}-1_{0,1} transition from H2O and its isotopologues. The relatively weak 15NH3 absorption in the Sgr B2 molecular cloud indicates a high [14N/15N] isotopic ratio >600. The abundance ratio of H218O and H217O is found to be relatively low, 2.5--3. These results together indicate that the dominant nucleosynthesis process in the Galactic centre is CNO hydrogen burning.

FADING HARD X-RAY EMISSION FROM THE GALACTIC CENTER MOLECULAR CLOUD Sgr B2

The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons. Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure. The measured decay time is compatible with the light crossing time of the molecular cloud core . Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago.

Interstellar HOCN in the Galactic center region

Aims. Our aim is to confirm the interstellar detection of cyanic acid, HOCN, in the Galactic center clouds. It has previously been tentatively detected only in Sgr B2(OH). Methods. We used a complete line survey of the hot cores Sgr B2(N) and (M) in the 3 mm range, complemented by additional observations carried out with the IRAM 30 m telescope at selected frequencies in the 2 mm band and towards four additional positions in the Sgr B2 cloud complex in the 2 and 3 mm bands. The spectral survey was analysed in the local thermodynamical equilibrium approximation (LTE) by modeling the emission of all identified molecules simultaneously. This allowed us to distinguish weak features of HOCN from the rich line spectrum observed in Sgr B2(N) and (M). Lines of the more stable (by 1.1 eV) isomer isocyanic acid, HNCO, in these sources, as well as those of HOCN and HNCO towards the other positions, were analysed in the LTE approximation as well. Results. Four transitions of HOCN were detected in a quiescent molecular cloud in the Galactic center at a position offset in (R.A., decl.) by (20'',100'') from the hot core source Sgr B2(M), confirming its previous tentative interstellar detection. Up to four transitions were detected toward five other positions in the Sgr B2 complex, including the hot cores Sgr B2(M), (S), and (N). A fairly constant abundance ratio of ~ 0.3 - 0.8 % for HOCN relative to HNCO was derived for the extended gas components, suggesting a common formation process of these isomers.

Origin of 6.4 keV Line Emission from Molecular Clouds in the Galactic Center

Abstract We analyze the 6.4 keV line and continuum emission from the molecular cloud Sgr B2 and the source HESS J1745$-$303, which is supposed to be a complex of molecular gas. From the HESS results it follows that Sgr A$^\ast$ is a source of high energy protons, which penetrate into molecular clouds producing there a TeV gamma-ray flux. We present arguments that Sgr A$^\ast$ may also produce a flux of subrelativistic protons which generate the 6.4 keV line and bremsstrahlung continuum emission from the clouds.

Time Variability of the Neutral Iron Lines from the Sagittarius B2 Region and Its Implication of a Past Outburst of Sagittarius A$^\ast$

Abstract We investigated long-term X-ray behavior from the Sgr B2 complex using archival data of the X-ray satellites Suzaku, XMM-Newton, Chandra, and ASCA. The observed region of the Sgr B2 complex included two prominent spots in the FeI K$\alpha$ line at 6.40 keV, a giant molecular cloud, M0.66$-$0.02, known as the “Sgr B2 cloud” and an unusual X-ray source, G0.570$-$0.018. Although these 6.40 keV spots have spatial extensions of a few parsec scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe XXV K$\alpha$ line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M0.66$-$0.02; the 6.40 keV line flux declined in 2001, and decreased to 60% in the time span 1994–2005. The other spot, G0.570$-$0.018, is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability ($\sim$10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A$^\ast$ was X-ray bright in the past 300 years, and exhibited a time variability with a period of a few years.

A Time-Variable X-Ray Echo: Indications of a Past Flare of the Galactic-Center Black Hole

Abstract A time-variability study of the neutral iron line flux at 6.40keV in the Sgr B2 region from data of Suzaku and Chandra is presented. The highly ionized iron line at 6.68keV is due to Galactic Center Diffuse X-rays, and is thus time invariable. By comparing the 6.68keV and 6.40keV line fluxes, we found that the 6.40keV flux from the Sgr B2 complex region is time variable; particularly the giant molecular cloud M 0.66$-$0.02, known as “Sgr B2 cloud” is highly variable. The variability of the 6.40keV line in intensity and spatial distribution strongly supports the scenario that the molecular clouds in the Sgr B2 region are X-ray Reflection Nebulae irradiated by the Galactic Center black hole Sgr A$^*$.

A confusion-limited spectral-line survey of Sgr B2(N) at 1, 2, and 3mm: Establishing the organic inventory in molecular clouds

AbstractWe present preliminary results of an spectral-line survey at 1, 2, and 3 mm of the galactic center cloud Sgr B2(N). With the current data, several simple prebiotic molecules have been conclusively identified, while several more complex molecules have not. When complete, this survey will provide an accurate database of the gas-phase organic inventory in Sgr B2(N).

Detection of Acetamide (CH 3 CONH 2 ): The Largest Interstellar Molecule with a Peptide Bond

Acetamide (CH3CONH2) has been detected in emission and absorption toward the star-forming region Sagittarius B2(N) with the 100 m Green Bank Telescope (GBT) by means of four A-species and four E-species rotational transitions. All transitions have energy levels less than 10 K. The Sgr B2(N) cloud is known to have a cold halo with clumps of gas at several different velocities. Absorption features are largely characterized by local standard of rest (LSR) velocities that are typical of the two star-forming cores with systemic LSR velocities of +64 and +82 km s-1. Continuum sources embedded within the star-forming cores give rise to the absorption from the molecular gas halo surrounding the cores. Emission features are seen at an approximate intermediate LSR velocity of +73 km s-1 that characterizes the widespread molecular halo that has a spatial scale of a few arcminutes. Two low-energy transitions of formamide (HCONH 2) were also observed with the GBT toward Sagittarius B2(N) since formamide is the potential parent molecule of acetamide; both molecules are the only interstellar species with an NH2 group bound to a CO group, the so-called peptide bond, that provides the linkage for the polymerization of amino acids. While the acetamide transitions observed appear to be confined to the cold (~8 K) halo region, only the 101-0 00 transition of formamide appears to be exclusively from the cold halo; the 312-313 transition of formamide is apparently contaminated with emission from the two hot cores. The relative abundance ratio of acetamide to formamide is estimated to be in the range of ~0.1 to ~0.5 in the cold halo. The exothermic neutral-radical reaction of formamide with methylene (CH2) may account for the synthesis of interstellar acetamide in the presence of shock phenomenon in this star-forming region.

The HNCO ring in the Sgr B2 region

The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b)=(0.7°,−0.07°), with a radius of ∼5 pc and a total mass of 1.0×105 to 1.6×106 M⊙. The expansion velocity of the Ring is estimated to be 30–40kms−1, which gives an expansion time scale of ∼1.5×105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring.

The Sgr B2 X‐Ray Echo of the Galactic Center Supernova Explosion that Produced Sgr A East

Using a combination of one-dimensional and three-dimensional hydrodynamic simulations, we have carried out the first in-depth analysis of the remnant's evolution and its various interactions: with the stellar winds flowing out from the inner ~2 pc; with the supermassive black hole, Sgr A*; and with the 50 km s-1 molecular cloud behind and to the east of the nucleus. We have found that, unlike previous estimates, a rather "standard" supernova explosion with energy ~1.5 × 1051 ergs would have been sufficient to create the remnant we see today and that the latter is probably only ~1700 yr old. We have found that the passage of the remnant across the black hole would have enhanced the accretion rate onto the central object by less than a factor of 2. Such a small increase cannot explain the current Fe fluorescence observed from the molecular cloud Sgr B2; this fluorescence would have required an increase in the luminosity of Sgr A* by 6 orders of magnitude several hundred years ago. Instead, we have uncovered what appears to be a more plausible scenario for this transient irradiation: the interaction between the expanding remnant and the 50 km s-1 molecular cloud. The first impact would have occurred about 1200 yr after the explosion, producing a 2-200 keV luminosity of ~1039 ergs s-1. During the intervening 300-400 yr, the dissipation of kinetic energy subsided considerably, leading to the much lower luminosity (~1036 ergs s-1 at 2-10 keV) we see today.

SGR B2 지역에 있는 HCO+분자운의 특성 연구

우리은하중심에 있는 Sgr B2 분자운 지역에 대하여 <TEX>$HCO^{+}1-0$</TEX> 천이선을 관측하여 이 지역의 역학적, 화학적 특성을 연구하였다. 이 지역에는 속도가 <TEX>${\sim}50$</TEX>과 <TEX>${\sim}100kms^{-1}$</TEX>인 가스 구름이 상호작용을 하고 있는 것으로 보이며, 이에 의하여 Sgr B2 분자운의 새로운 별-탄생이 촉진될 수 있다고 여겨진다. 속도성분이 <TEX>${\sim}50kms^{-1}$</TEX>인 지역에서, 뚜렷한 가스 집중을 보이는 'OF38 분자운'('Odenwald & Fazio FIR 38' Cloud)이 새로이 발견되었다. 이 분자운의 <TEX>$HCO^+$</TEX> 칼럼밀도는 <TEX>$N(HCO^+)=(2-5){\times}10^{14}cm^{-2}$</TEX>, 크기는 약 15pc이며, 총 질량은 약 <TEX>$10^6M_{\odot}$</TEX>으로 계산되었다. 그리고 이 Sgr B2 지역에는 속도 범위가 <TEX>$100kms^{-1}$</TEX>에 이르는 큰 규모의 터뷸런트한 컴포넌트가 넓게 퍼져 존재하며, 이 성분의 <TEX>$N(HCO^+)=1{\times}10^{13}cm^{-2}$</TEX>으로 상당한 양으로 존재함을 알 수 있다. 그러나 이 지역에서 관측되는 <TEX>$HCO^+$</TEX>의 생성반응은, 기존에 제안되었던 <TEX>$H^{+}_3$</TEX>와 CO의 반응보다는, 충격파의 영향에 의하여 증가하는 <TEX>$C^+$</TEX>와 OH의 반응에서 보다 효과적으로 생성될 것으로 보인다. 새로이 발견된 'OF38 분자운'의 물리 화학적 특성을 밝히는 일은 앞으로의 과제라고 생각한다. The <TEX>$HCO^+$</TEX> 1-0 transition line was observed toward the Sgr B2 region in our Galactic center. We found that there exist two large-scale velocity structures of <TEX>$v_{lsr}\;{\sim}50\;and\;{\sim}100kms^{-1}$</TEX>, which are thought to interact with each other. A new gas clump 'OF28 Cloud'('Odenwald & Fazio FIR 38' Cloud), showing different chemical and kinematical properties with the Sgr B2(M) cloud, was found in the <TEX>$50kms^{-1}$</TEX> gas component. Toward the core of this component, we derive the <TEX>$HCO^+$</TEX> total column density, <TEX>$N(HCO^+)=(2-5){\times}10^{14}cm^{-2}$</TEX> and the mass <TEX>$M=1{\times}10^6M_{\odot}$</TEX>, by estimating its size, <TEX>${\sim}15pc$</TEX>, from the half-power width of this component. We also found that there is a highly turbulent component in this region in the velocity range of about <TEX>$100kms^{-1}$</TEX>. The column density of this component is <TEX>$N(HCO^+)=1{\times}10^{13}cm^{-2}$</TEX>. The <TEX>$HCO^+$</TEX> in this region may form effectively by the reaction between <TEX>$C^+$</TEX> and OH, which are the elements whose abundances increase rapidly in shocked region.

Hard X-ray view of the past activity of Sgr A$^{\star}$ in a natural Compton mirror

We report the association of the recently discovered hard X-ray source IGR J17475-2822 with the giant molecular cloud Sgr B2 in the Galactic Center region. The broad band (3-200 keV) spectrum of the source constructed from data of different observatories strongly supports the idea that the X-ray emission of Sgr B2 is Compton scattered and reprocessed radiation emitted in the past by the Sgr A* source. We conclude that 300-400 years ago Sgr A* was a low luminosity (L ~ 1.5e39 erg/s at 2-200 keV) AGN with a characteristic hard X-ray spectrum (photon index Gamma ~ 1.8). We estimate the mass and iron abundance of the Sgr B2 scattering gas at 2e6 Msun (r/10pc)^2 and 1.9 solar, respectively (where r is the radius of the cloud).

Atomic Oxygen Abundance in Molecular Clouds: Absorption toward Sagittarius B2

We have obtained high-resolution (approximately 35 km/s) spectra toward the molecular cloud Sgr B2 at 63 micrometers, the wavelength of the ground-state fine-structure line of atomic oxygen (O(I)), using the ISO-LWS instrument. Four separate velocity components are seen in the deconvolved spectrum, in absorption against the dust continuum emission of Sgr B2. Three of these components, corresponding to foreground clouds, are used to study the O(I) content of the cool molecular gas along the line of sight. In principle, the atomic oxygen that produces a particular velocity component could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in the UV illuminated surface layer (PDR) of a cloud, and in an atomic (H(I)) gas halo. For each of the three foreground clouds, we estimate, and subtract from the observed O(I) column density, the oxygen content of the H(I) halo gas, by scaling from a published high-resolution 21 cm spectrum. We find that the remaining O(I) column density is correlated with the observed (13)CO column density. From the slope of this correlation, an average [O(I)]/[(13)CO] ratio of 270 +/- 120 (3-sigma) is derived, which corresponds to [O(I)]/[(13)CO] = 9 for a CO to (13)CO abundance ratio of 30. Assuming a (13)CO abundance of 1x10(exp -6) with respect to H nuclei, we derive an atomic oxygen abundance of 2.7x10(exp -4) in the dense gas phase, corresponding to a 15% oxygen depletion compared to the diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination of the PDR contribution to the O(I) column density. The PDR regions should contain O(I) but not (13)CO, and would thus be expected to produce an offset in the O(I)-(13)CO correlation. Our data do not show such an offset, suggesting that within our beam O(I) is spatially coexistent with the molecular gas, as traced by (13)CO. This may be a result of the inhomogeneous nature of the clouds.

Polarization of X-ray emission from the Sgr B2 cloud

The Sgr B2 giant molecular cloud is claimed to be an ‘X-ray reflection nebula’– the reprocessing site of a powerful flare of the Sgr A* source, which occurred a few hundred years ago. The shape of the X-ray spectrum and the strength of the iron fluorescent line support this hypothesis. We argue that the cleanest test of the origin of X-rays from Sgr B2 would be a detection of polarized emission from this source.

Evaluating the N/O Chemical Network: The Distribution of N2O and NO in the Sagittarius B2 Complex

Mapping observations of the J = 6 → 5 transition of N2O and the Π, J = 3/2 → 1/2 line of NO in the 2 mm band toward the core region of the Sagittarius B2 complex have been carried out using the Kitt Peak 12 m telescope. Emission from NO was found to be extended over a region 2' × 5' in size that includes the Sgr B2 (N), Sgr B2 (M), and Sgr B2 (OH) positions, very similar to the distribution found for HNO. In contrast, N2O emission was confined to a source approximately 1' in extent, slightly elongated in the north-south direction and centered on the Sgr B2 (N) core. A virtually identical distribution was found for the JKτ = 140 → 14-1 E transition of methanol, which lies 255 K above ground state and samples very hot gas. Excitation conditions are favorable for the J = 6 → 5 line of N2O over the entire NO region; hence, the confined nature of this species is a result of chemistry. The J = 3 → 2 and J = 9 → 8 lines of N2O at 75 and 226 GHz, respectively, were also detected at Sgr B2 (N). Combined with the J = 6 → 5 data, these transitions indicate a column density for this molecule of Ntot ~ 1.5 × 1015 cm-2 at this position and an abundance of f(N2O/H2) ~ 1.5 × 10-9. This fractional abundance is almost 2 orders of magnitude higher than predicted by low-temperature chemical models. The N2O observations suggest that this molecule is preferentially formed in high-temperature gas; a likely mechanism is the neutral-neutral reaction NO + NH → N2O + H, which has an appreciable rate only at T > 125 K. The column density of NO found over the Sgr B2 cloud was Ntot ~ (0.8-1.5) × 1016 cm-2, corresponding to a fractional abundance of f(NO/H2) ~ (0.8-1.5) × 10-8, which is about 1 order of magnitude less than model predictions. The similar distributions of NO and HNO suggest a chemical connection. It is likely that the major route to HNO is from NO via the ion-molecule process NO + HNO+ → NO+ + HNO, which occurs readily at low temperatures. The NO molecule thus appears to be the main precursor species in the N/O chemical network.

Radio Observations of Sgr B2

The 13CO (J = 1-0) map of the molecular cloud Sgr B2 reveals that the mass center of the molecular cloud nucleus does not coincide with that of compact HII regions which are likely to be the outcome of a shock on the cloud. We find evidence of cloud contraction probably resulting from cloud-cloud collision at subsonic speed.

ASCAObservations of the Sagittarius B2 Cloud: An X‐Ray Reflection Nebula

We present the ASCA results of imaging spectroscopy of the giant molecular cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a strong emission line at 6.4 keV, a low-energy cutoff below about 4 keV, and a pronounced edge structure at 7.1 keV. The X-ray image is extended, and its peak position is shifted from the core of the molecular cloud toward the Galactic center by about 1'-2'. The X-ray spectrum and the morphology are well reproduced by a scenario that X-rays from an external source located in the Galactic center direction are scattered by the molecular cloud Sgr B2 and come into our line of sight. Thus, Sgr B2 may be called an X-ray reflection nebula. Possible implications of the Galactic center activity related to this unique source are presented.

Carbon Dioxide in Star-forming Regions

We consider the gas-phase chemistry of CO2 molecules in active regions. We show that CO2 molecules evaporated from dust in hot cores cannot be efficiently destroyed and are in fact copiously produced in cooler gas. When CO2-rich ices are sputtered in strong MHD shock waves, the increase in atomic hydrogen, due to H2 dissociation by ion-neutral streaming, means that CO2 can be depleted by factors of approximately 500 from its injected abundance. We find that a critical shock speed exists at higher preshock densities below which CO2 molecules can be efficiently sputtered but survive in the postshock gas. These calculations offer an explanation for the low gas/solid CO2 ratios detected by the Infrared Space Observatory in star-forming cores as being due to shock destruction followed by partial reformation in warm gas. The presence of high abundances of CO2 in the strongly shocked Galactic center clouds Sgr B2 and Sgr A also find a tentative explanation in this scenario. Shock activity plays an important role in determining the chemistry of star-forming regions, and we suggest that most hot cores are in fact shocked cores.

Observation of FE-line emission from sagittarius B2 — Evidence for past activities of our galaxy

We present the ASCA results of imaging spectroscopy of the giant molecular cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a strong emission line at 6.4 keV, a low energy cutoff below 4 keV and a pronounced edge-structure at 7.1 keV. The X-ray image is extended and its peak position is shifted in the direction of the Galactic center by about 1–2 arcminute from the core of the molecular cloud. The X-ray spectrum and morphology are well reproduced by a scenario that X-rays from a source located at the Galactic center side are scattered by the molecular cloud Sgr B2, and come into our line of sight. Thus Sgr B2 may be called an X-ray reflection nebula.