OVERVIEW

1. A Search for Radio Transients and Variables in the Galactic Center Jennifer L. Neureuther, Scott D. Hyman (Sweet Briar College), T. Joseph W. Lazio, Michael E. Nord, & Namir E. Kassim (NRL) OVERVIEW VLA OBSERVATIONS TRANSIENTS We report on a search for variable and transient radio sources in the Galactic center using a number of high resolution, wide-field (~2.5°) 330 MHz VLA observations made during the 1990's and monthly during Spring and Summer 2002. Known classes of variable and transient radio sources include radio counterparts of X-ray sources and microquasars. The Galactic center (GC) is a promising region to search as the stellar densities are high, and neutron- and black hole binaries appear as X-ray transients concentrated toward the GC. Two previous radio transients detected toward the GC, A1742-28 (Davies et al. 1976) and the Galactic Center Transient (Zhao et al. 1992), had similar radio properties, but only the former was associated with an X-ray source. Previous radio surveys have suffered from confusion in the GC or have been limited to a single epoch. We have detected at least one new radio transient, GCRT J1746-2757, located only 1.1 degrees north of the Galactic center, and ~5 variable candidates. We discuss the implications of this work for constraining the Galactic center population of variables and transients. • GCRT J1746-2757 • (Hyman et al. 2002, Astron. J., 123, 1497) • Detected only on 1998 September 25 at 330 MHz. • Flux Density = 216 ± 20 mJy (20  detection). • Unresolved (< 9´´); Located 1.1º (150 pc) north of • Sgr A*. • Nonthermal; brightness temperature > 5 x 104 K. • No X-ray counterpart detected; source either Doppler • boosted in radio or a “fast” X-ray transient; or a member of a class of radio transients with no associated X-ray emission. • XTE J1748-288 • (Hjellming, Rupen, et al. 1998, Hyman et al. 2002) • 330 MHz detection in 1998 September and 1998 November. • High frequency monitoring from radio peak in 1998 June to present. • Classified as a black hole binary by Fender & Kuulkers 2001. • Of the ~30 Low-Mass X-ray Binary sources reported within our field-of-view, we detect only one radio transient counterpart (XTE J1748-288). Hjellming et al. find one or two other radio counterparts located near the edge of our field-of-view where our sensitivity is poor. • However, we do detect the following source in a quiescent state: • GRS 1734-292 (Marti et al. 1998) • X-ray burst in 1992 September. • Classified as Seyfert 1 galaxy based on redshift. • 330 MHz flux density constant at S ~ 150 mJy from 1989 – 1998. Below: Detection thresholds for transient detection as a function of timescale, . The blue line is our actual threshold for observations of ~ 5 hr duration. The red curve is our threshold for timescales < 5 hr. It is determined by adding simulated transient sources to the 1998 March observations. We are sensitive to detecting sources with flux densities in the region above the red and blue lines. Constant brightness temperature curves are shown based on source size upper limits (c). The temperature upper limit for an incoherent synchrotron emitter is 1012 K. SEARCH METHODOLOGY • TRANSIENTS • The 1998 epoch images are used as model images to compare to the images at other epochs. • Run the task UVSUB in AIPS to subtract CLEAN components of the model images from each epoch’s uv-data. • Image the residual data using 512 (55) fields for the higher (lower) resolution epochs. • Search each residual field for bright sources and "hole" sources corresponding to possible transients. • The 1989 image below clearly shows a "hole" at the location of the 1998 transient GCRT J1746-2757. Other sources with constant flux density are effectively removed from the field, although the source in the northeast corner is still visible in the 1989 image due to insufficient CLEANing of the 1998 image. CONCLUSIONS • VLA observations at 330 MHz of the GC allow for efficient transient searching because they provide high-resolution (<20´´), high dynamic range, large field of view (2.5º) images. • Low-frequency observations exploit the apparent steep-spectrum nature of these sources. • From 330 MHz VLA observations over the past decade and Summer 2002, we conclude that radio transients above ~100 mJy are either very infrequent (~ one every few years) or have timescales much shorter than a month. • Thus far, we have found only a few radio variable candidates out of ~250 sources detected, but our observations may yet reveal additional variables. The task is more difficult than detecting transients; e.g., a 100 mJy source visible in only one epoch is a more reliable detection than a 100 mJy variation in a 500 mJy source. The latter could be due to a number of systematic uncertainties. • The scarcity of radio transients/variables detected in this survey so far underscores the need for a far more extensive monitoring program, with more sensitive and frequent observations, in order to detect, monitor, and identify a large number of radio sources. POSSIBLE VARIABLES 1989 1998 1998 1989 • VARIABLES • Automated source detection program (AIPS task Search and Destroy SAD) used to find sources in combination image from the 1996 and 1998 epoch observations, produced reference database of ~250 sources (Nord et al. 2003). • Run SAD on each epoch image. Select measurements of sources corresponding to those in the reference database. This saves the time of "weeding" out false detections by eye for each epoch. Also, sources reliably detected on the combined 1996+1998 image lends credence to only marginal detections made on individual epoch images. • Confirm all sources whose flux densities differ by more than 5; source flux densities confirmed by hand. ~5 variable candidates detected thus far. • Sources that differ due to unmatched resolutions and/or confusion with extended emission are removed from the list of variable candidates. G0.490-1.043: The light curve shows consistent detections with one notable non-detection in March 1989 and possible variability in 2002. G358.638-1.161: The light curve shows a significant increase in March 1998 and a non-detection in August 1995. The NVSS detects the source at 1.4 GHz with flux density 21.1 ± 0.8 mJy. ACKNOWLEDGEMENTS: This research has been helped considerably by students at Sweet Briar College. Jennifer Neureuther, Suzanne Bollinger, Ashlee Bartleson, and Mariana Lazarova have participated under various SBC student research programs. S.D.H. thanks Grant Denn for further assistance. The Very Large Array of the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement with Associated Universities, Inc. Basic research in radio astronomy at NRL is supported by the Office of Naval Research, and at SBC by Research Corporation, the Jeffress Memorial Trust, and the National Science Foundation. Contact: shyman@sbc.edu