X-ray Reflection Nebula Research Articles

New Constraints on Cosmic Particle Populations at the Galactic Center Using X-Ray Observations of the Molecular Cloud Sagittarius B2

The Sagittarius B2 (Sgr B2) molecular cloud complex is an X-ray reflection nebula whose nonthermal X-ray emissions have continued to decrease since 2001 as it reprocesses one or more past energetic outbursts from the supermassive black hole Sagittarius A* at the Galactic Center. The X-ray reflection model explains the observed time variability of Sgr B2 and provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, X-rays from Sgr B2 are also of interest as a probe of low-energy (sub-GeV) cosmic rays, which may be responsible for an increasing relative fraction of the nonthermal emission as the contribution from X-ray reflection decreases. Here, we present the most recent NuSTAR and XMM-Newton observations of Sgr B2, from 2018, and we emphasize the Kα fluorescence line of neutral Fe. These 2018 observations reveal small-scale variations within lower-density portions of the complex, including brightening features, yet still enable upper limits on X-rays from low-energy cosmic-ray interactions in Sgr B2. We present Fe Kα line fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic-ray interactions throughout the cloud.

Probing the clumping structure of giant molecular clouds through the spectrum, polarisation and morphology of X-ray reflection nebulae

We suggest a method for probing global properties of clump populations in Giant Molecular Clouds (GMCs) in the case where these act as X-ray reflection nebulae (XRNe), based on the study of the clumping's overall effect on the reflected X-ray signal, in particular on the Fe K-alpha line's shoulder. We consider the particular case of Sgr B2, one of the brightest and most massive XRN in our Galaxy. We parametrise the gas distribution inside the cloud using a simple clumping model, with the slope of the clump mass function (alpha), the minimum clump mass (m_{min}), the fraction of the cloud's mass contained in clumps (f_{DGMF}), and the mass-size relation of individual clumps as free parameters, and investigate how these affect the reflected X-ray spectrum. In the case of very dense clumps, similar to those presently observed in Sgr B2, these occupy a small volume of the cloud and present a small projected area to the incoming X-ray radiation. We find that these contribute negligibly to the scattered X-rays. Clump populations with volume filling factors of > 10^{-3}, do leave observational signatures, that are sensitive to the clump model parameters, in the reflected spectrum and polarisation. Future high-resolution X-ray observations could therefore complement the traditional optical and radio observations of these GMCs, and prove to be a powerful probe in the study of their internal structure. Finally, clumps in GMCs should be visible both as bright spots and regions of heavy absorption in high resolution X-ray observations. We therefore further study the time-evolution of the X-ray morphology, under illumination by a transient source, as a probe of the 3d distribution and column density of individual clumps by future X-ray observatories.

HARD X-RAY MORPHOLOGICAL AND SPECTRAL STUDIES OF THE GALACTIC CENTER MOLECULAR CLOUD SGR B2: CONSTRAINING PAST SGR A⋆ FLARING ACTIVITY

Galactic Center (GC) molecular cloud Sgr B2 is the best manifestation of an X-ray reflection nebula (XRN) reprocessing a past giant outburst from the supermassive black hole Sgr A*. Alternatively, Sgr B2 could be illuminated by low-energy cosmic ray electrons (LECRe) or protons (LECRp). In 2013, NuSTAR for the first time resolved Sgr B2 hard X-ray emission on sub-arcminute scales. Two prominent features are detected above 10 keV - a newly emerging cloud G0.66-0.13 and the central 90" radius region containing two compact cores Sgr B2(M) and Sgr B2(N) surrounded by diffuse emission. It is inconclusive whether the remaining level of Sgr B2 emission is still decreasing or has reached a constant background level. A decreasing Fe K$\alpha$ emission can be best explained by XRN while a constant background emission can be best explained by LECRp. In the XRN scenario, the 3-79 keV Sgr B2 spectrum can well constrain the past Sgr A* outburst, resulting in an outburst spectrum with a peak luminosity of $L_{3-79\rm~keV} \sim 5\times10^{38} \rm~erg~s^{-1}$ derived from the maximum Compton-scattered continuum and the Fe K$\alpha$ emission consistently. The XRN scenario is preferred by the fast variability of G0.66-0.13, which could be a molecular clump located in the Sgr B2 envelope reflecting the same Sgr A* outburst. In the LECRp scenario, we derived the required CR ion power $dW/dt=(1-4)\times10^{39}\rm~erg~s^{-1}$ and the CR ionization rate $\zeta_{H}=(6-10)\times 10^{-15}\rm~H^{-1}~s^{-1}$. The Sgr B2 background level X-ray emission will be a powerful tool to constrain GC CR population.

X-Ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A*

This paper presents Suzaku results obtained for the Sagittarius (Sgr) C region using the concept of X-ray reflection nebulae (XRNe) as the echo of past flares from a super-massive black hole, Sgr A*. The Sgr C complex is composed of several molecular clouds proximately located in projected distance. The X-ray spectra of Sgr C were analyzed based on the view that XRNe are located inside the Galactic center plasma X-ray emission with an oval distribution around Sgr A*. We found that XRNe are largely separated in the line-of-sight position, and are associated with molecular clouds in different velocity ranges detected by radio observations. We also applied the same analysis to Sgr B XRNe, and completed a long-term light curve for Sgr A* occurring in the past. As a new finding, we determined that Sgr A* has already experienced periods of high luminosity, $ \sim$ 500 years ago, which is longer than the previously reported value. Our results are consistent with a scenario that Sgr A* was continuously active with sporadic flux variabilities of $ L_{\rm X}$$ =$ (1–3) $ \times$ 10$ ^{39}$ erg s$ ^{-1}$ in the past 50 to 500 years. The average past luminosity was approximately 4–6 orders of magnitude higher than that presently observed. In addition, two short-term flares of 5–10 years have been found. Thus, the past X-ray flare should not be a single short-term flare, but can be interpreted as multiple flares superposed on a long-term high state.

An X-Ray Face-On View of the Sagittarius B Molecular Clouds Observed with Suzaku

Abstract We present a new methodology to derive the positions of the Sagittarius (Sgr) B molecular clouds (MCs) along the line of sight, as an application study of the Galactic center diffuse X-rays (GCDX). The GCDX are composed of hot plasma emission of about 7 keV and 1 keV temperatures, and non-thermal continuum emission including the 6.4 keV line from neutral irons. The former, the Galactic center plasma emission (GCPE), is uniformly distributed over 1$^\circ$ in longitude, while the latter is clumpy emission produced by Thomson scattering and fluorescence from MCs irradiated by external X-rays (the X-ray reflection nebula emission: XRNE). We examined the Suzaku X-ray spectra of the GCPE and XRNE near to the Sgr B MC complex, and found that the spectra suffer from two different absorptions of $N_{\rm H}$ (Abs1) $\geq$10$^{23}$H cm$^{-2}$ and $N_{\rm H}$ (Abs2) $\simeq$6 $\times$ 10$^{22}$H cm$^{-2}$. Abs1 is proportional to the 6.4 keV-line flux, and hence is due to the MCs, while Abs2 is typical of interstellar absorption toward the Galactic center. Assuming that the GCPE plasma is spherically-extended around Sgr A$^\ast$ with a uniform density and the same angular distribution of the two temperature components, we quantitatively estimated the line-of-sight positions of the MCs from the flux ratio the GCPE spectrum suffered by Abs1 and that with no Abs1. The results suggest that the Sgr B MCs are located at the near side of Sgr A$^\ast$ in the GCPE.

SiO EMISSION AS A TRACER OF X-RAY DOMINATED CHEMISTRY IN THE GALACTIC CENTER

We present emission maps of the Sgr A molecular cloud complex at the Galactic center (GC) in the J = 2 → 1 line of SiO observed with the IRAM 30 m telescope at Pico Veleta. Comparing our SiO(2–1) data cube with that of CS(1–0) emission with similar angular and velocity resolution, we find a correlation between the SiO/CS line intensity ratio and the equivalent width of the Fe Kα fluorescence line at 6.4 keV. We discuss the SiO abundance enhancement in terms of the two most plausible scenarios for the origin of the 6.4 keV Fe line: X-ray reflection nebula (XRN) and low-energy cosmic rays (LECRs). Both scenarios could explain the enhancement in the SiO/CS intensity ratio with the intensity of the 6.4 keV Fe line, but both present difficulties. The XRN scenario requires a population of very small grains to produce the SiO abundance enhancement, together with a past episode of bright X-ray emission from some source in the GC, possibly the central supermassive black hole, SgrA∗,~300 yr ago. The LECR scenario needs higher gas column densities to produce the observed 6.4 keV Fe line intensities than those derived from our observations. It is possible to explain the SiO abundance enhancement if the LECRs originate in supernovae and their associated shocks produce the SiO abundance enhancement. However, the LECR scenario cannot account for the time variability of the 6.4 keV Fe line, which can be naturally explained by the XRN scenario.

X-RAY OBSERVATION OF VERY HIGH ENERGY GAMMA-RAY SOURCE, HESS J1745–303, WITHSUZAKU

Suzaku observations of a TeV unidentified (unID) source, HESS J1745-303, are presented. A possible excess of neutral iron line emission is discovered, and is likely associated with the main part of HESS J1745-303, named "region A". It may be an X-ray reflection nebula where the X-rays from previous Galactic Center (GC) activity are reflected by a molecular cloud. This result further strengthens the assumption that the molecular cloud which is spatially coincident with region A of HESS J1745-303 is located in the GC region. The TeV emission from molecular clouds is reminiscent of the diffuse TeV gamma-rays from the GC giant molecular clouds, and it could have the same emission mechanism. With deep exposure mapping observations by Suzaku, a tight upper-limit on the 2-10 keV continuum diffuse emission from region A is obtained, as 2.1x10^-13ergs s^-1cm^-2. The flux ratio between 1-10 TeV and 2-10 keV is larger than 4. Possible scenarios to reproduce wide-band spectra from keV to TeV are examined. Thermal X-rays from nearby two old supernova remnants, G359.0-0.9 and G359.1-0.5, are detected, and their emission properties are well determined in the present study with deep exposure.

X-Ray Reflection Nebulae with Large Equivalent Widths of the Neutral Iron K$\alpha$ Line in the Sagittarius C Region

Abstract This paper reports on the first results of a Suzaku observation in the Sgr C region. We detected four diffuse clumps with strong line emission at 6.4 keV, K$\alpha$ from neutral or low-ionized Fe. One of them, M359.38$-$0.00, was newly discovered with Suzaku. The X-ray spectra of the two bright clumps, M359.43$-$0.07 and M359.47$-$0.15, after subtracting the galactic center diffuse X-ray emission (GCDX), exhibit a strong K$\alpha$ line from Fe i with large equivalent widths ($EW$s) of 2.0–2.2 keV and a clear K$\beta$ of Fe i. The GCDX in the Sgr C region is composed of the 6.4 keV- and 6.7 keV-associated components. These were phenomenologically decomposed by taking relations between the $EW$s of the 6.4 keV and 6.7 keV lines. Then, the former $EW$s against the associated continuum in bright clump regions were estimated to be 2.4$^{+2.3}_{-0.7}$keV. Since the two different approaches give similar large $EW$s of 2 keV, we strongly suggest that the 6.4 keV clumps in the Sgr C region are due to X-ray reflection/fluorescence (the X-ray reflection nebulae).

Correlation between SiO and X-ray emission in the galactic center

We present emission maps (spatial resolution ∼40’’) of the Sgr A molecular cloud complex at the Galactic Center (GC) in the J = 2→1 SiO line, observed with the IRAM-30m telescope at Pico Veleta. We have compared our SiO(2-1) data cube with CS(1-0) maps, and we have found a correlation between the SiO/CS intensity ratio and the equivalent width of the X-ray Fe line at 6.4 keV. We discuss the SiO abundance enhancement in the two most plausible scenarios for the origin of the Fe line, which is, at the moment, under debate: fluorescence in an X-ray Reflection Nebula (XRN), or impact by Low-Energy Cosmic Rays (LECRs) followed by electronic relaxation. Both could explain the enhancement in the SiO/CS intensity ratio with the intensity of the Fe line, but both scenarios present difficulties: the first one requires a population of very small grains to produce the enhancement in the SiO/CS intensity ratio, and the second needs higher column densities than the ones derived from observations in the region.

Suzaku Spectroscopy of an X-Ray Reflection Nebula and a New Supernova Remnant Candidate in the SgrB1 Region

Abstract We made a 100 ks observation of the Sagittarius (Sgr) B1 region at $(l,\ b)=$ (0D.5, $-$0D.1) near to the Galactic center (GC) with the Suzaku/XIS. Emission lines of SXV, FeI, FeXXV, and FeXXVI were clearly detected in the spectrum. We found that the FeXXV and FeXXVI line emissions smoothly distribute over the SgrB1 and B2 regions connecting from the GC. This result suggests that the GC hot plasma extends at least up to the SgrB region with constant temperature. There are two diffuse X-ray sources in the observed region. One of the two (G0.42$-$0.04) is newly discovered, and exhibits a strong SXV K$\alpha $ emission line, suggesting a candidate for a supernova remnant located in the GC region. The other one (M0.51$-$0.10), having a prominent FeI K$\alpha $ emission line and a strongly absorbed continuum, is likely to be an X-ray reflection nebula. There is no near source bright enough to irradiate M0.51$-$0.10. However, the FeI K$\alpha $ emission can be explained if SgrA* was $\sim10^6$ times brighter 300years ago, the light travel time for 100 pc to M0.51$-$0.10, than it is at present.

Discoveries of Diffuse Iron Line Sources from the Sgr B Region

Abstract The radio complex Sgr B region was observed with the X-ray Imaging Spectrometer (XIS) on board Suzaku. This region exhibits diffuse iron lines at 6.4, 6.7, and 6.9 keV, which are $\mathrm{K}\alpha$ lines of Fe I (neutral iron), Fe XXV (He-like iron), and Fe XXVI (H-like iron), respectively. The high energy resolving power of the XIS provided separate maps of the K-shell transition lines from Fe I (6.4 keV) and Fe XXV (6.7 keV). Although the 6.7 keV line is smoothly distributed over the Sgr B region, a local excess is found near $(l, b) = ({0\rlap {.}{}^{\mathrm {\circ }}61}, {0\rlap {.}{}^{\mathrm {\circ }}01})$, possibly due to a new SNR. The plasma temperature is $kT\sim 3 \,\mathrm{keV}$ and the age is estimated to be younger than $7 \times 10^{3} \,\mathrm{yr}$. The 6.4 keV image is clumpy with local excesses near Sgr B2 and at $(l, b) = ({0\rlap {.}{}^{\mathrm {\circ }}74},- {0\rlap {.}{}^{\mathrm {\circ }}09})$. Like Sgr B2, this excess may be another candidate of an X-ray reflection nebula.

Waiting in the Wings: Reflected X‐Ray Emission from the Homunculus Nebula

We report the first detection of X-ray emission associated with the Homunculus Nebula which surrounds the supermassive star eta Carinae. The emission is characterized by a temperature in excess of 100 MK, and is consistent with scattering of the time-delayed X-ray flux associated with the star. The nebular emission is bright in the northwestern lobe and near the central regions of the Homunculus, and fainter in the southeastern lobe. We also report the detection of an unusually broad Fe K fluorescent line, which may indicate fluorescent scattering off the wind of a companion star or some other high velocity outflow. The X-ray Homunculus is the nearest member of the small class of Galactic X-ray reflection nebulae, and the only one in which both the emitting and reflecting sources are distinguishable.

High Energy Activities of Our Galactic Center and Its Environment

The Japanese X-ray satellite Ginga discovered a largely extended hot plasma around the Galactic center (GC). possibly due to a violent explosion in the GC region within the past 10 5 years [K. Koyama, H. Awaki, H. Kunieda, S. Takano and Y. Tawara, Nature 339 (1989), 603]. Subsequently, the first hard X-ray imaging satellite, ASCA found strong 6.4 keV lines associated with the molecular clouds in the GC region [K. Koyama, Y. Maeda, T. Sonobe, T. Takeshima, Y. Tanaka and S. Yamauchi, Publ. Astron. Soc. Jpn. 48 (1996), 249]. The spectrum and morphology are explained by the fluorescent irradiated by strong X-rays (hence: X-ray reflection nebula) from the GC side. Possible scenario of the X-ray reflection nebula is that the Galactic black hole, Sgr A* was X-ray luminous of Lx ∼ 10 3 9 - 4 0 erg s - 1 in the past of ∼ 100-300 years ago Recent Chandra observations on the GC have confirmed these results and furthermore, with its unprecedented spatial resolution, have resolved a number of non-thermal X-ray filaments as well as the 6.4 keV line clouds [H. Murakami, K. Koyama, M. Sakano, M. Tsujimoto and Y. Maeda, Astrophys. J. 534 (2000), 283; H. Murakami, K. Koyama, M. Tsujimoto, Y. Maeda and M. Sakano, Astrophys. J. 550 (2001), 297: H. Murakami, K. Koyama and Y. Maeda. Astrophys. J. 558 (2001), 687; M. Sakano, R. S. Warwick. A. Decourchelle and P. Predehl, Mon. Not. R. Astron. Soc. 340 (2003), 747: Q. D. Wing, E. V. Gotthelf and C. C. Lang, Nature 415 (2002), 148; Q. D. Wang, F. Lu and C. C. Laiig, Astrophys. J. 581 (2002), 1148]. One of the most exciting discoveries is a jet-like Structure emanating near from Sgr A* [A. Bamba, R. Yamazaki, M. Ueno and K. Koyama, Astrophys. J. 589 (2003), 827]. We infer that the Galactic center jets were created some hundreds years ago by sudden mass serge onto the Galactic black hole. This might also be the origin of the X-ray reflection nebula.

Reflected X-ray Emissions on Giant Molecular Clouds ? Evidence of the Past Activities of Sgr A*

We have found strong 6.4-keV line emissions from the giant molecular clouds in the Galactic center region: Sgr B2, Sgr C, and M0.11–0.08 (at the Radio Arc region). The high angular resolution of Chandra reveals that the 6.4-keV line emissions are indeed coincident with the clouds, and shifted towards the Galactic center. The X-ray spectra have very strong 6.4-keV lines with equivalent widths ≥1 keV and are attenuated by larger column densities of interstellar gas. These characteristics imply that the massive molecular clouds are irradiated by an external X-ray source in the direction of the Galactic center and emit fluorescent and scattered X-rays. These clouds are new category of X-ray source: “X-ray Reflection Nebula”. The reflected X-ray flux reveals the recent luminosity history of the primary irradiating source, which may be the massive black hole Sgr A*, according to the light travel time to each cloud. Making use of the radio determinations of the cloud masses, we find that Sgr A* was as luminous as 1039 erg s−1 a few hundred years ago, and has gradually decreased to present value.

XMM‐Newton observation of the galactic centre – evidence against the X‐ray reflection nebulae model?

AbstractWe present preliminary results of a 50 ksec XMM‐Newton observation of the Galactic Centre. Due to the excellent spectral resolution of XMM‐Newton combined with its high throughput at energies even beyond 7 keV, the iron line complex could be resolved into individual components (neutral, ionized, both in Kα and Kβ) in the spectra of a number of sources. In the spectrum of Sgr A East, we find an overabundance of iron, although less than reported in a recent analysis of Chandra data. No flare from Sgr A*was detected. The large scale diffuse emission seen already by Einstein, ROSAT, and ASCA extending into the north‐east direction is most probably of thermal origin. This region extends from Sgra A East also towards both directions perpendicular to the Galactic Plane (“outflow”) and contains some blobs of plasma with a higher temperature. Particularly interesting is the map of the 6.4 keV fluorescence line of neutral iron which shows a very peculiar filamentary structure. Spectral modelling of these filaments is obviously not in agreement with the X‐ray reflection nebulae (XRN) model commonly discussed for the origin of the fluorescence line, primarily because we do not see a significant iron absorption edge at 7.1 keV.

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.

ASCADiscovery of Diffuse 6.4 keV Emission near the Sagittarius C Complex: A New X‐Ray Reflection Nebula

We present an ASCA discovery of diffuse hard X-ray emission from the Sgr C complex with its peak in the vicinity of the molecular cloud core. The X-ray spectrum is characterized by a strong 6.4 keV line and large absorption. These properties suggest that Sgr C is a new X-ray reflection nebula that emits fluorescent and scattered X-rays via irradiation from an external X-ray source. We found no adequately bright source in the immediate Sgr C vicinity to fully account for the fluorescence. The irradiating source may be the Galactic nucleus Sgr A*, which was brighter in the past than it is now, as is suggested from observations of the first X-ray reflection nebula Sgr B2.

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.

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.

An X-ray reflection nebula Sgr B2 – a new category of X-ray astronomy

Astronomische NachrichtenVolume 320, Issue 4-5 p. 325-325 Original Paper An X-ray reflection nebula Sgr B2 – a new category of X-ray astronomy H. Murakami, H. Murakami Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorK. Koyama, K. Koyama Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorY. Maeda, Y. Maeda Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, U.S.A.Search for more papers by this authorM. Sakano, M. Sakano Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorM. Tsujimoto, M. Tsujimoto Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this author H. Murakami, H. Murakami Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorK. Koyama, K. Koyama Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorY. Maeda, Y. Maeda Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, U.S.A.Search for more papers by this authorM. Sakano, M. Sakano Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this authorM. Tsujimoto, M. Tsujimoto Department of Physics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto, JapanSearch for more papers by this author First published: 03 July 2001 https://doi.org/10.1002/1521-3994(199908)320:4/5<325::AID-ASNA325>3.0.CO;2-%23Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume320, Issue4-5August 1999Pages 325-325 RelatedInformation