------------------------------------------------------------------------ From: ZADEH@OSSENU.ASTRO.NWU.EDU Date: Mon, 2 Mar 1998 11:31:25 -0600 (CST) To: gcnews@aoc.nrao.edu Subject: RE: astro-ph/9802361 %astro-ph/9802361 \documentstyle[12pt,aasms4,epsf]{article} %\documentstyle[phase1,12pt,epsf]{article} %\documentstyle[11pt,aaspp4]{article} %\documentstyle[aas2pp4]{article} %\twelvepoint\doublespace \received{00 September 1997} \revised {January 1998} \accepted{00 1997} %\journalid{337}{15 January 1989} %\articleid{11}{14} \centerline{Submitted to the Editor of the Astrophysical Journal {\it Letters} on November12, 1997} %\slugcomment{Submitted to the Astrophysical Journal {\it Letters}} % Authors may supply running head information, if they wish to do so, although % this may be modified by the editorial offices. The left head contains a % list of authors, usually three allowed---otherwise use et al. The right % head is a modified title of up to roughly 44 characters. Running heads % are not printed. \lefthead{Yusef-Zadeh, Cotton and Reynolds} \righthead{A Young SNR Near the Galactic Center\?} % This is the end of the "preamble". Now we wish to start with the % real material for the paper, which we indicate with \begin{document}. % Following the \begin{document} command is the front matter for the % paper, viz., the title, author and address data, the abstract, and % any keywords or subject headings that are relevant. \def\kms{km s$^{-1}$} \def\Blos{B$_{\rm los}$} \begin{document} \title{G359.87+0.18: A Young SNR Candidate Near the Galactic Center?} \author{F. Yusef-Zadeh} \affil{Department of Physics and Astronomy, Northwestern University, Evanston, Il. 60208 (zadeh@nwu.edu)} \author{W. D. Cotton} \affil{National Radio Astronomy Observatory, Charlottesville, VA 22903 (bcotton@aoc.nrao.edu)} \author{S. P. Reynolds} \affil{Physics Department, North Carolina State University, Box 8202, Raleigh, NC 27695-8202 (steve\_reynolds@ncsu.edu)} \begin{abstract} Sub-arcsecond radio continuum observations of the Galactic center region at $\lambda$6 and 2cm reveal a 0.5$^{\prime\prime}$ diameter source with a shell-like morphology. This source is linearly polarized at a level of 16\% at $\lambda$6cm and has a steep nonthermal spectrum with spectral index 1.6 between $\lambda$6 and 2 cm. The distance to this source is not known but the large rotation measure value of 3000 rad m$^{-2}$ suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. %Asssuming that this small-diamater synchrotron source is %located in the inner Galaxy, we intrepret it as an excellent candidate %for a low luminosity young SNR possibly with an age as short as 10-20 %years. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic. \end{abstract} \keywords{galaxies: ISM---Galaxy: center ---ISM: individual (supernova remnant) --- ISM: magnetic fields} \vfill\eject \section{Introduction} Recent surveys of radio supernova remnants (SNRs) indicate a underrepresentation of small-diameter young SNRs in the Galaxy (Green 1994). This deficiency, which is in part due to the lack of high-resolution observations, is particularly noticeable for SNRs having diameters less than 1$^\prime$. There are only four such sources with diameters less than 4$^\prime$ in the catalogue of Green (1994). Because of its large concentration of dense molecular clouds and massive-star formation, the Galactic center may be considered as a good target for searching for small diameter SNRs. There is a possibility that SNRs in the Galactic center region may be preferentially smaller in diameter than elsewhere, due to confinement by a dense interstellar medium in this region (Gray 1994). However, a search for such objects suffers from considerable confusion due to bright and extended radio continuum features associated with HII regions, planetary nebulae, and nonthermal sources in the Galactic center. A radio continuum survey of the Galactic center center region in search of planetary nebulae detected a continuum source, since named G359.87+0.18, at $\lambda$20cm using the Westerbork Synthesis Radio Telescope with the beam size of 22$^{\prime\prime} \times 120^{\prime\prime}$ (Isaacman 1981). VLA observations of this source were reported with a resolution of $\approx10^{\prime\prime}$ by Yusef-Zadeh (1986). These observations were then followed up by Lazio (1997) who studied the scattering medium toward the Galactic center region and found that the source is not heavily scattered. Here, we report high-resolution observations of this radio continuum source, showing that it is resolved into two components. The brighter component is characterized by a nonthermal spectrum, linear polarization, and shell-like morphology. The sub-arcsecond diameter of this shell-like polarized source makes it a possible candidate for a young SNR lying about 15$^\prime$ from the Galactic center along the rotation axis of the Galaxy. \section{Observations} Radio continuum observations of compact radio sources in the Galactic center were carried out with the Very Large Array of the National Radio Astronomy Observatory\footnote{The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.} in its A configuration at $\lambda$2 and 6cm in May 1986. Each source was observed for about 5 minutes at each wavelength using 100 MHz of bandwidth. A more detailed account of observations and the results of all the observed compact sources will be given elsewhere. Here we present the results of one of these compact sources, known as source J as described by Yusef-Zadeh (1986) and Lazio (1997). Standard calibration of all four Stokes parameters was done in AIPS using 3C286 and 1720-130 as the flux and phase calibrators. The synthesized beam sizes are 0.98$''\times0.34$\arcsec and 0.36$''\times0.13$\arcsec at $\lambda$6 and 2cm, respectively. The $\lambda$20cm continuum emission reported here is based on A-array observations described by Yusef-Zadeh et al. (1994). The rms noise at this frequency is 1.4 mJy beam$^{-1}$ with a beam size of $2.68''\times0.94''$ (PA= --19$^\circ$) Because the phase center of this observation is offset considerably from the position of the source, bandwidth smearing increases the source size in the East-West direction to an estimated 0.8$''$. %\placefigure{fig1} %\placefigure{fig2} \section{Results } Figure 1 shows contours of the total intensity of G359.87+0.18 at $\lambda$6cm, with rms noise of 0.19 mJy. This source breaks up into two components A and B with the respective peak flux densities of 22.4 mJy and 6.5 mJy. Gaussians fitted to these components are at positions $\alpha (1950)= 17^h 41^m 26^s.45$, $\delta(1950)= -28^0 55' 55.7''$ for source A (l=359.872$^\circ$, b=0.178$^\circ$), and $\alpha (1950)= 17^h 41^m 26^s.21$, $\delta(1950)=-28^0 55' 57.5''$ for source B (l=359.871$^\circ$, b=0.178$^\circ$). The brighter source A shows a 5$\sigma$ elongated protrusion with a size of $\approx$0.5$''$ running in the east-west direction. The Gaussians fitted to sources A and B indicate that they are partially resolved at $\lambda$6cm. This is supported by the $\lambda$2cm data, with three times higher resolution than the 6cm data. Figure 2 shows the 2cm image with a resolution of 0.36$''\times0.13''$ and rms noise of 0.34 mJy. Source A is resolved into a shell source with a barrel-shaped appearance and a diameter of about 0.5$''$. No significant polarized emission is detected at $\lambda$2cm, with an upper limit to the degree of polarization of 28\%. Source B is also resolved into an unresolved compact source and a weak extended structure at a level of 0.8 mJy to the north of the compact source. The flux density of the compact component of source B peaks at a level of 2 mJy at the position of $\alpha (1950)= 17^h 41^m 26^s.21$, $\delta(1950)= -28^0 55' 57.5''$. More support for the extended nature of these sources comes from multi-wavelength observations indicating that sources A and B are not compact and are not affected by the scattering medium toward the Galactic center region (Lazio 1997). Figure 3 shows the polarized intensity image of source A at $\lambda$6cm with rms noise of 0.16 mJy, superimposed on the distribution of electric field vectors. Source A has a peak polarized flux density of 2.1 mJy and appears asymmetric. There are two polarized clumps having fractional polarization of 16.5\% and 6\% to the west and to the east, respectively. The polarized clumps coincide with the eastern and western edges of the total intensity image as shown in Figure 1. The true distribution of the magnetic field cannot be determined with the present data due to large Faraday rotation at $\lambda$6cm. The mean rotation measure (RM) toward source A is about +3000 rad m$^{-2}$, based on two closely spaced frequencies (4860.1 and 4885.1 MHz). The east-west sides of the barrel-shape structure noted in the total intensity image of Figure 2 coincide with the clumps of polarized emission as seen in Figure 3. This indicates that the lack of polarized emission from the position of the peak of the 6cm total intensity is not due to depolarization but rather due to the shell-like morphology of the source as displayed in Figure 2. Because of the shell-like morphology of the total intensity at $\lambda$2cm and polarized intensity at $\lambda$6cm, the lack of polarized emission between the clumps is unlikely to be due to the rotation of the plane of polarization across the synthesized beam or due to internal Faraday depolarization. We consider it more likely that the polarized flux shares the shell morphology of source A as discussed below. The Gaussian fitted peak flux densities at $\lambda$6 and 2cm with identical beam size of 0.98$''\times0.32''$ are 22.47 (6.48) and 3.53 (1.52) mJy/beam for source A(B), respectively. Using the rms noise of 0.19 and 0.34 mJy for the $\lambda$6 and 2cm images, the estimated values of spectral index $\alpha$, where F$_{\nu} \propto \nu^{-\alpha}$, between $\lambda$2 and 6cm for sources A and B are 1.6$\pm$ 0.2 and 1.3$\pm$0.5, respectively. The spectral index estimate of Source A is based on using identical {\it {uv}} coverage between 50 and 500 $k\lambda$ whereas that of source B is based on slightly different {\it uv} coverage, thus the spectral index estimate may be an upper limit because some of the $\lambda$2cm flux may have been resolved out. The spectral index between $\lambda$6 and 20cm, though uncertain due to unmatched resolutions and to bandwidth smearing, is about 1.19 when the fluxes from both A and B sources are added. Source diameter and spectral indices of this source have also been measured at a number of frequencies in order to study the characteristics of the the scattering screen toward the Galactic center, though with different spatial resolutions (Lazio 1997). This again suggests that sources A and B have steep spectral indices between 2 and 20cm. % Similar problems %make the spectral index measurements between $\lambda$6 and 20cm data %quite uncertain until more detailed observations are carried out. \section{Discussion} A number of studies indicate a ``missing'' population of small-diameter SNRs in the Galaxy (e.g. Green 1991). Such small-diameter SNR candidates have been identified in the past but a majority of them turned out to be thermal sources. The initial interpretations as SNR candidates were based primarily on shell-like morphology, a nonthermal interpretation of the high-frequency spectrum, and a lack of radio recombination line emission (Green 1986, Cowan et al. 1989, Subrahmanyan, R. 1993; Muizon et al. 1988; Reich et al. 1984). We believe that the strongest argument that distinguishes this source from thermal sources such as G25.5+0.2 and G70.68+1.20 is the evidence for linearly polarized emission from source A of G359.87+0.18. Thus even if enough flux is missing from the 2 cm data to allow a flat spectrum, the interpretation of the emission as nonthermal seems secure. However, the source's extremely small size makes detailed interpretation difficult. The RM towards source A is about 3000 rad m$^{-2}$, more than two orders of magnitude greater than RM's toward sources in the outer Galaxy. Such rotation measures have been measured for a number of sources located within a degree of the Galactic center (Inoue et al. 1984; Yusef-Zadeh \& Morris 1987; Gray et al. 1995; Yusef-Zadeh, Wardle and Parataran 1996), suggesting that the Faraday rotation occurs close to or beyond the Galactic center. A milliGauss magnetic field strength with an electron density of 0.03 cm$^{-3}$ in the inner 50 pc of the Galactic center can account for the observed RM (Koyama et al. 1986, 1996; Yamauchi et al. 1990; Yusef-Zadeh and Morris 1987). Thus the high RM indicates that G359.87+0.18 is no closer than the Galactic center; let us initially presume that it is at the Galactic center, at a distance of 8.5 kpc, where $1^{\prime\prime} = 0.04$ pc. Then the source angular radius of about $0.^{\prime\prime}25$ corresponds to a linear radius of only 0.01 pc. We first consider an interpretation as a radio SN or SNR. The mass contained in this tiny sphere is only $1.6 \times 10^{-7} n_0 \ M_{\odot}$, where $n_0$ is the mean atomic hydrogen density, so unless G359.87+0.18 is in an exceptionally high-density environment, even for the GC, it has not swept up an appreciable amount of mass, and should still be freely expanding. Then the age is only $10 \ v_8^{-1}$ yr, where $v_8$ is the ejection velocity in units of 1000 km s$^{-1}$, expected to be of order 5 for SN Ib or II, and 10 for SN Ia. A core-collapse supernova should have emitted a neutrino flux larger than that of SN1987A by $(55 \ {\rm kpc} / 8.5 \ {\rm kpc})^2$ or about 42, so might have been seen by any neutrino detectors operating during the early 1980's. Furthermore, the mean surface brightness of G359.87+0.18 Source A is 6200 Jy arcmin$^{-2}$, 45 times higher than that of Cas A, which is itself far brighter than any other Galactic SNR (see data in Green 1991). Thus a supernova-remnant interpretation is really inappropriate, and we should consider the possibility that G359.87+0.18 is a radio supernova. Typical radio supernovae are detected by the VLA with fluxes in the range of mJy. For example, van Dyk et al.~(1993) report on five bright radio supernovae. All have radio spectral indices of order 1 or less (with errors of order 0.2), much flatter than G359.87+0.18 Source A. Furthermore, all these have radio fluxes at 6 cm of tens of mJy, typically, comparable to that of G359.87+0.18, even though they are farther away by factors of 1000 or more. Other radio supernovae (see, e.g., Weiler et al.~1986) are similar in flux density and spectral index. So all the radio supernovae we know are more luminous than G359.87+0.18 by six or more orders of magnitude. (Of course, ones as faint as G359.87+0.18 could not be seen in external galaxies; but the point is that G359.87+0.18 does not resemble known radio supernovae.) Could G359.87+0.18 be a nova remnant? One radio-shell remnant of a classical nova is known (GK Per; Reynolds and Chevalier 1984, Seaquist et al.~1989). But a standard equipartition analysis of G359.87+0.18 (e.g., Pacholczyk 1970) gives a minimum energy in magnetic field and relativistic electrons of $4 \times 10^{44}$ erg, only a factor of 2 less than the total energy emitted in a classical nova outburst, and 50 times larger than that of GK Per. So G359.87+0.18 cannot be a nova remnant. (The equipartition energy varies with distance $d$ as $d^{17/7}$, so moving G359.87+0.18 farther away does not help, and the RM constraint prevents us from moving it closer.) However, the equipartition magnetic field one derives is not unreasonable, at about $6 (d/8.5 \ {\rm kpc})^{17/14} \ \mu$gauss, so that the synchrotron interpretation of the radio emission is sensible. Based on different spectra of sources A and B and their morphology, it is unlikely that these sources are related to each other and therefore unlikely that G359.87+0.18 is an extragalactic double radio source. However, it is possible that Source A alone is extragalactic. Its morphology would be unusual, and its steep spectral index very rare but not unprecedented. Such ``ultrasteep-spectrum'' radio sources have been interpreted as radio galaxies at redshifts of order 1. Additional problems with the extragalactic interpretation is related to the scattering studies carried out toward this source by Lazio (1997). This author concludes that this source is unlikely to be extragalactic and affected by a scattering screen seen toward the Galactic center (e.g. van Langevelde et al. 1992). This conclusion is based on the predicted value of the observed diameter of this source at 0.33 GHz being 25\% larger than the observed value. The predicted source diameter assumes minimal scattering toward this source and an intrinsic source size $\theta_i$=0 (Lazio 1997). %Additional evidence that %G359.87+0.18 is unlikely to be an extragalactic source is %the lack of any large time variability of the source %when different data sets % with different spatial resolutions are examined %(Isaacman 1981; Yusef-Zadeh %1986; Lazio 1997 and this paper) %In particular, the integrated flux %density at $\lambda$20cm is measured to be about 123 mJy which is %consistent with low-resolution observations by Isaacman (1981) and %Yusef-Zadeh (1986). %The flux density variations over a time scale of few years % would have placed a constraint on the %size of the source being of the order of light years, hence %the source must be at an extragalactic distance. We are forced to conclude that G359.87+0.18 is not well explained by any known class of nonthermal radio source. Further observations are clearly called for to resolve its nature. \acknowledgments F. Yusef-Zadeh's work was supported in part by NASA grant NAGW-2518. S.~Reynolds acknowledges support from NASA grants NAG 5-2844. We thank Dale Frail for useful discussion. %\vfill\eject \begin{references} \reference {c89} Cowan, J.J., Ekers, R.D., Goss, W.M., Sramek, R.A., Roberts, D.A. et al. 1985, MNRAS, 241, 613 \reference {gr94} Gray, A.D. 1994, MNRAS, 270, 861 \reference {g91} Green, D.A. 1994, in Proc.IAU Colloq. 145, Supernona and Supernova Remnents, eds: R. McCray, Z. Wang, Cambridge University Press \reference {g86} Green, D.A. 1986, MNRAS, 219, 39p \reference {g94} Green, D.A. 1991, PASP, 103, 209 \reference {i84}Inoue, M., Takahashi, T., Tabara, H., Kato, T. \& Tsuboi, M. 1984 PASJ, 36, 633 \reference {i81} Isaacman, R. 1981, \aaps, 43, 405. \reference {k89}Koyama, K., Awaki, H., Kunieda, H., Takano, S., Tawara, Y. et al. 1989, Nature, 339, 603 \reference {l97} Lazio, T.J.W. 1997, PhD thesis, Cornell University, p305 \reference {m88} de Muizon, M., Strom, R.G., Oort, M.J.A., Claas, J.J. \& Braun, R. 1988, A.A., 193, 248 \reference {p70} Pacholczyk, A.G. 1970, Radio Astrophysics (San Francisco: Freeman) \reference {r85} Reich, W. F\"urst, E., Altenhoff, P., Reich, P. \&Junkes, N. 1985, \aaps, 151, L10. \reference {r84} Reynolds, S.P., \& Chevalier, R.A. 1984, \apj, \apj, 281, L33 \reference {s89} Seaquist, E.R., Bode, M.F., Frail, D.A., Roberts, J.A., Evans, A., \& Albinson, J.S. \apj, 344, 805 \reference {s93} Subrahmanyan, R., Ekers, R.D., Wilson, W.E., Goss, W.M., \& Allen, D.A. 1993, MNRAS, 263, 868 \reference {v93} van Dyk, S.D., Sramek, R.A., Weiler, K.W., \& Panagia, N. 1993, \apj, 409, 162 \reference {v92} van Langevelde, H.J., Frail, D.A., Cordes, J.M., \& Diamond, P.J. 1992, \apj, 396, 686 \reference {w86} Weiler, K.W., Sramek, R.A., Panagia, N., van der Hulst, J.M., \& Salvati, M. 1986, \apj, 301, 790 \reference {y90}Yamauchi, S., Kawada, M., Koyama, K., Kunieda, H. \& Tawara, Y. 1990, \apj, 365, 532 \reference {y86} Yusef-Zadeh, F. 1986, PhD thesis, Columbia University, p191 \reference {y94} Yusef-Zadeh, F. Cotton, W., Wardle, M., Melia, F. and Roberts, D. 1994, \apj, 434, L63 \reference {ym87} Yusef-Zadeh, F. and Morris, M. 1987, \apj, 322, 721 \reference{ywp96} Yusef-Zadeh, F., Wardle, M. \& Parataran, P. 1997, \apj, 475, L119 \end{references} \vfill\eject \clearpage \begin{figure} %\plotone{fig1_J.ps} \figcaption {Contours of total intensity showing sources A (NE) and B (SW) with levels set at $\lambda$6cm set (-3, 3, 4, 5, 7, 9, 11, 13, 15, 17, 19, 22, 25, 30, 40, 50, 60, 90)$\times$ the rms noise which is 0.194 mJy beam$^{-1}$. The spatial resolution is 0.98$''\times$0.34$''$ (PA=-3.9$^0$).} \end{figure} \begin{figure} %\plotone{fig2_J.ps} \figcaption {Contours of total intensity showing sources A and B at $\lambda$2cm with levels set at (-3, -2, 2, 3, 4, 5, 6, 7, 8, 10) $\times$ the rms noise which is 0.286 mJy beam$^{-1}$. The spatial resolution is 0.36$''\times$0.13$''$ (PA=6.8$^0$).} \end{figure} \begin{figure} %\plotone{fig3_J.ps} \figcaption {Contours of polarized intensity of source A at $\lambda$6cm with levels set at (5, 6, 7, 8, 9, 10, 12) $\times$ the rms noise 0.164 mJy beam$^{-1}$ are superimposed on the distribution of electric field vectors represented by straight lines. The lengths of the straight lines are fixed. and the maximum fractional polarization is 16\%. The spatial resolution is identical to that of Figure 1.} \end{figure} \end{document} \clearpage % SAMPTBLS.TEX -- AASTEX sample plano tables. \end{document} %\documentstyle[apjpt4]{article} %\begin{document} One of the most unusual aspect of a Galactic SNR interpretation is the small angular size of this source. Assuming that source A is 8.5 kpc away and having a diameter of 1$''$, its linear size is 0.04 pc. Considering a supernova shock speed of 5000 \kms, the age of the SNR is estimated to be only 10 years which is unusually young. It is possible that the interstellar environment of the Galactic center slows the expansion or that the source is located outside the Galaxy. Follow-up observations will give a time baseline of more than ten years to measure the proper motion of the expanding shell. These measuremnets combined with $\lambda$21cm HI absorption should be able to test whether this source is a Galactic or an extragalactic radio source. ------- End of Forwarded Message Dear Dr. Dalgarno, In my next email, please find a re-revised version of a paper entitled ``G359.87+0.18: A Young SNR Candidate Near the Galactic Center?'' in collaboration with W. Cotton and S. Reynolds. The referee was concerned about the lack of time variability argument that we used to interpret the extragalactic nature of G359.87+0.18. We believe that the referee has a good point in that this is not a strong argument to constrain the extragalactic nature of this source. We have removed this discussion at the end of the paper. Future observations with identical set up to our 1986 data should measure the time variability and angular size change of this source. Both parameters should be used with less ambiguous in interpreting the galactic or extragalactic nature of this source. We have also incorporated the editorial changes that the referee has suggested into the paper. We believe that the paper is now ready for publications in the Astrophysical Journal Letters. The address for express delivery is: Northwestern University Dept of Physics and Astronomy 2145 Sheridan Road Evanston, IL 60208 Sincerely, F. Yusef-Zadeh ------------- End Forwarded Message -------------