Low-Frequency Radio Transients in the Galactic Center.pdf

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Low-Frequency Radio Transients in the Galactic Center.pdf

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Low-Frequency Radio Transients in the Galactic Center

a r X i v : a s t r o - p h / 0 1 1 0 1 5 1 v 1 5 O c t 2 0 0 1 Submitted to The Astronomical Journal Low-Frequency Radio Transients in the Galactic Center Scott D. Hyman Department of Physics, Sweet Briar College, Sweet Briar, VA 24595; hyman@sbc.edu T. Joseph W. Lazio, Namir E. Kassim Naval Research Laboratory, Code 7213, Washington, DC 20375-5320; lazio@rsd.nrl.navy.mil; kassim@rsd.nrl.navy.mil ABSTRACT We report the detection of a new radio transient source, GCRT J1746?2757, located only 1.?1 north of the Galactic center. Consistent with other radio transients toward the Galactic center, this source brightened and faded on a time scale of a few months. No X-ray counterpart was detected. We also report new 0.33 GHz measurements of the radio counterpart to the X-ray transient source, XTE J1748?288, previously detected and monitored at higher radio frequencies. We show that the spectrum of XTE J1748?288 steepened considerably during a period of a few months after its peak. We also discuss the need for a more efficient means of finding additional radio transients. Subject headings: Galaxy: center — radio continuum — stars: variable: other 1. Introduction Known classes of highly variable and transient radio sources include radio counterparts of X-ray sources and microquasars. Although there are many examples of variable radio sources discovered as a result of high-energy observations, there are surprisingly few radio surveys for highly variable or transient sources. A radio survey of the Galactic plane by Gregory Taylor (1981, 1986) discovered 4 variable sources including GT 0236+610, a Galactic X-ray binary, and 1 candidate transient. The MIT-Green Bank surveys (Langston et al. 1990; Griffith et al. 1990; Griffith et al. 1991) discovered a number of variable sources ( 40% variable). An on-going program at NRAO Green Bank monitors the Galactic plane at 8.4 and 14.4 GHz (Langston et al. 2000). The Galactic center (GC) is a promising region in which to search for