Subject: GCFLASH - Vol. 3, No. 1 (Aug 6, 1996) ======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@astro.umd.edu http://www.astro.umd.edu/~gcnews ======================================================================== Vol. 3, No. 1 Aug 6, 1996 Well, that's how a Newsletter should work: a few days ago we discussed here at UMD what the chances are for detecting radio pulsars in the GC and today we get the answer already. The second paper today shows that if you are looking for Machos you sometimes find more than you expected ( - but unfortunately no early life forms in the GC yet). Recently submitted papers: -------------------------- 1) Gravitational Microlensing and the Structure of the Inner Milky Way (Gyuk, ApJ) 2) Finding Radio Pulsars in and Beyond the Galactic Center (Cordes & Lazio, ApJ) ------------------------------------------------------------------------ Email : gyuk@oddjob.uchicago.edu Title : Gravitational Microlensing and the Structure of the Inner Milky Way Author(s): G. Gyuk Institute: Department of Physics, Enrico Fermi Institute, University of Chicago, 5630 Ellis Avenue, Chicago IL 60637-1433, NASA/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510-0500 Paper : submitted to ApJ Weblink : ftp://astro.uchicago.edu/pub/astro/gyuk/micromap EPrint : astro-ph/9607134 Abstract: We analyze the first-year MACHO collaboration observations of microlensing towards the Galactic center using a new direct likelihood technique that is sensitive to the distribution of the events on the sky. We consider the full set of 41 events, and calculate the direct likelihood against a simply-parameterized Galactic model consisting of either a gaussian or exponential bar and a double exponential disk. Optical depth maps are calculated taking into account the contribution of both disk lenses and sources. We show that based on the presently available data, a slope in the optical depth has been clearly detected (3 sigma) in Galactic latitude and that there are indications of a small slope in Galactic longitude. We discuss limits that can be set on the mass, angle and axis ratio of the Galactic bulge. We show that based on microlensing considerations alone, M_Bulge>1.5* 10^10 M_sol at the 90% confidence level and that the bulge inclination angle is less than 30 Deg also at the 90% confidence level. The mostly likely bar mass is M_Bulge=3.5*10^10 M_sol. Such a high mass would imply a low MACHO fraction for the halo. We consider disk parameters and show that there are two degeneracies between the effects of a disk and those of a bar on the optical depths. Finally, we discuss how to break these degeneracies and consider various strategies for future microlensing observations. ------------------------------------------------------------------------ Email : lazio@spacenet.tn.cornell.edu Title : Finding Radio Pulsars in and Beyond the Galactic Center Author(s): James M. Cordes & T. Joseph W. Lazio Institute: Dept. of Astronomy and NAIC, Cornell University, Ithaca, NY 14853-6801, USA; cordes@spacenet.tn.cornell.edu; lazio@spacenet.tn.cornell.edu Paper : to appear in ApJ Weblink : http://astrosun.tn.cornell.edu/SPIGOT/papers/pulsar/gc_psr.web/ EPrint : astro-ph/9608028 Abstract: Radio-wave scattering is enhanced dramatically for Galactic center sources in a region with radius >~ 15 arc min. Using scattering from Sgr A^* and other sources, we show that pulse broadening for pulsars in the Galactic center is at least 6.3 nu^-4 seconds (nu = radio frequency in GHz) and is most likely 50-200 times larger because the relevant scattering screen appears to be within the Galactic center region itself. Pulsars beyond--but viewed through--the Galactic center suffer even greater pulse broadening and are angularly broadened by up to ~ 2 arc min. Periodicity searches at radio frequencies are likely to find only long period pulsars and, then, only if optimized by using frequencies >~ 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is nu =~ 7.3 GHz (Delta_0.1 P sqrt(alpha))^-1/4 where Delta_0.1 is the distance of the scattering region from Sgr A^* in units of 0.1 kpc, P is the period (seconds), and alpha is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of detectable pulsars in the Galactic center may range from <= 1 to 100, with the larger values resulting from recent, vigorous starbursts. Such pulsars provide unique opportunities for probing the ionized gas, gravitational potential, and stellar population near Sgr A^*. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@astro.umd.edu) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@astro.umd.edu ========================================================================