TRANSIENT EVENTS IN ARCHIVAL VERY LARGE ARRAY OBSERVATIONS OF THE GALACTIC CENTER

The Galactic center has some of the highest stellar densities in the Galaxy and a range of interstellar scattering properties, which may aid in the detection of new radio-selected transient events. Here, we describe a search for radio transients in the Galactic center, using over 200 hr of archival data from the Very Large Array at 5 and 8.4 GHz. Every observation of Sgr A* from 1985 to 2005 has been searched using an automated processing and detection pipeline sensitive to transients with timescales between 30 s and 5 minutes with a typical detection threshold of ∼100 mJy. Eight possible candidates pass tests to filter false-positives from radio-frequency interference, calibration errors, and imaging artifacts. Two events are identified as promising candidates based on the smoothness of their light curves. Despite the high quality of their light curves, these detections remain suspect due to evidence of incomplete subtraction of the complex structure in the Galactic center, and apparent contingency of one detection on reduction routines. Events of this intensity (∼100 mJy) and duration (∼100 s) are not obviously associated with known astrophysical sources, and no counterparts are found in data at other wavelengths. We consider potential sources, including Galactic center pulsars, dwarf stars, sources like GCRT J1745−3009, and bursts from X-ray binaries. None can fully explain the observed transients, suggesting either a new astrophysical source or a subtle imaging artifact. More sensitive multiwavelength studies are necessary to characterize these events, which, if real, occur with a rate of in the Galactic center.

[1]  S. Burke-Spolaor,et al.  A MILLISECOND INTERFEROMETRIC SEARCH FOR FAST RADIO BURSTS WITH THE VERY LARGE ARRAY , 2014, 1412.7536.

[2]  R. P. Eatough,et al.  A strong magnetic field around the supermassive black hole at the centre of the Galaxy , 2013, Nature.

[3]  S. Johnston,et al.  Radio properties of the magnetar near sagittarius A* from observations with the Australia telescope compact array , 2013, 1305.3036.

[4]  Kristin K. Madsen,et al.  NuSTAR DISCOVERY OF A 3.76 s TRANSIENT MAGNETAR NEAR SAGITTARIUS A* , 2013, 1305.1945.

[5]  E. O. Ofek,et al.  SENSITIVE SEARCH FOR RADIO VARIABLES AND TRANSIENTS IN THE EXTENDED CHANDRA DEEP FIELD SOUTH , 2013, 1303.6282.

[6]  E. Berger,et al.  QUASI-QUIESCENT RADIO EMISSION FROM THE FIRST RADIO-EMITTING T DWARF , 2013, 1301.2321.

[7]  R. Genzel,et al.  NEW OBSERVATIONS OF THE GAS CLOUD G2 IN THE GALACTIC CENTER , 2012, Proceedings of the International Astronomical Union.

[8]  G. Langston,et al.  A GIANT SAMPLE OF GIANT PULSES FROM THE CRAB PULSAR , 2012, 1210.0452.

[9]  E. O. Ofek,et al.  A REVISED VIEW OF THE TRANSIENT RADIO SKY , 2011, 1111.0007.

[10]  J. Cordes,et al.  MULTIWAVELENGTH CONSTRAINTS ON PULSAR POPULATIONS IN THE GALACTIC CENTER , 2011, 1111.4216.

[11]  Eric Gaidos,et al.  AN ALL-SKY CATALOG OF BRIGHT M DWARFS , 2011, 1108.2719.

[12]  S. Lowe,et al.  STATISTICAL STUDIES OF GIANT PULSE EMISSION FROM THE CRAB PULSAR , 2011, 1108.2307.

[13]  Baltimore,et al.  VARIABLE AND TRANSIENT RADIO SOURCES IN THE FIRST SURVEY , 2011, 1107.5901.

[14]  J. Hessels,et al.  An automated archival VLA transients survey , 2011, 1103.0511.

[15]  P. McMillan,et al.  Mass models of the Milky Way , 2011, 1102.4340.

[16]  J. Cordes,et al.  The Dynamic Radio Sky , 2004, Proceedings of the International Astronomical Union.

[17]  Geoffrey C. Bower,et al.  A Search for Radio Transients in VLA Archival Images of the 3C 286 Field , 2010 .

[18]  S. D. Hyman,et al.  CIRCULARLY POLARIZED EMISSION FROM THE TRANSIENT BURSTING RADIO SOURCE GCRT J1745 − 3009 , 2010, 1001.5394.

[19]  A. Eckart,et al.  The nuclear star cluster of the Milky Way: proper motions and mass , 2009, 0902.3892.

[20]  W. N. Brandt,et al.  A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER , 2008, 0809.1105.

[21]  T. Joseph W. Lazio,et al.  GCRT J1742-3001: A NEW RADIO TRANSIENT TOWARD THE GALACTIC CENTER , 2008, 0811.1972.

[22]  D. Kaplan,et al.  A Search for the Near-Infrared Counterpart to GCRT J1745–3009 , 2008, 0807.1507.

[23]  G. Hallinan,et al.  Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs , 2008, 0805.4010.

[24]  N. Bhat,et al.  Bright Giant Pulses from the Crab Nebula Pulsar: Statistical Properties, Pulse Broadening, and Scattering Due to the Nebula , 2008, 0801.0334.

[25]  T. Bastian,et al.  Ultrahigh Time Resolution Observations of Radio Bursts on AD Leonis , 2007, 0710.5881.

[26]  T. Hankins,et al.  Radio Emission Signatures in the Crab Pulsar , 2007, 0708.2505.

[27]  Alexei V. Filippenko,et al.  Submillijansky Transients in Archival Radio Observations , 2007, 0705.3158.

[28]  J. G. Doyle,et al.  Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf , 2007, 0705.2054.

[29]  T. Lazio,et al.  A Faint, Steep-Spectrum Burst from the Radio Transient GCRT J1745–3009 , 2007, astro-ph/0701098.

[30]  D. Lorimer,et al.  A Periodically Active Pulsar Giving Insight into Magnetospheric Physics , 2006, Science.

[31]  E. Berger,et al.  Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths , 2006, astro-ph/0603176.

[32]  R. N. Manchester,et al.  Transient radio bursts from rotating neutron stars , 2005, Nature.

[33]  T. Bastian,et al.  Wide-Band Spectroscopy of Two Radio Bursts on AD Leonis , 2005, astro-ph/0509815.

[34]  W. Goss,et al.  Radio Emission from the Double-Pulsar System J0737–3039 Revisited , 2005, astro-ph/0509889.

[35]  T. Lazio,et al.  A New Radio Detection of the Transient Bursting Source GCRT J1745–3009 , 2005, astro-ph/0508264.

[36]  W. M. Goss,et al.  A Radio Transient 0.1 Parsecs from Sagittarius A* , 2005, astro-ph/0507221.

[37]  R. Manchester,et al.  The Australia Telescope National Facility Pulsar Catalogue , 2005 .

[38]  W. N. Brandt,et al.  A Remarkable Low-Mass X-Ray Binary within 0.1 Parsecs of the Galactic Center , 2005, astro-ph/0503572.

[39]  T. Lazio,et al.  A powerful bursting radio source towards the Galactic Centre , 2005, Nature.

[40]  A. Burgasser,et al.  Quiescent Radio Emission from Southern Late-Type M Dwarfs and a Spectacular Radio Flare from the M8 Dwarf DENIS 1048–3956 , 2005, astro-ph/0502365.

[41]  MIT,et al.  An Overabundance of Transient X-Ray Binaries within 1 Parsec of the Galactic Center , 2004, astro-ph/0412492.

[42]  R. Manchester,et al.  The Australia Telescope National Facility Pulsar Catalogue , 2003, astro-ph/0309219.

[43]  A universal radio-X-ray correlation in low/hard state black hole binaries , 2003, astro-ph/0305231.

[44]  E. Berger,et al.  Flaring up All Over—Radio Activity in Rapidly Rotating Late M and L Dwarfs , 2001, astro-ph/0111317.

[45]  J. Schmitt,et al.  Microwave plasma emission of a flare on AD Leo , 2001, astro-ph/0106369.

[46]  J. Reynolds,et al.  The ATCA intraday variability survey of extragalactic radio sources , 2001, astro-ph/0103506.

[47]  D. Frail,et al.  Discovery of Radio Emission from the Brown Dwarf LP944-20 , 2001, astro-ph/0102301.

[48]  R. P. Fender E. Kuulkers,et al.  On the peak radio and X-ray emission from neutron star and black hole candidate X-ray transients , 2001, astro-ph/0101155.

[49]  Simon T. Garrington,et al.  Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819–254=SAX J1819.3–2525) , 2000 .

[50]  D. Stinebring,et al.  Five Years of Pulsar Flux Density Monitoring: Refractive Scintillation and the Interstellar Medium , 2000 .

[51]  Andrew Gould,et al.  A Cluster of Black Holes at the Galactic Center , 2000, astro-ph/0003269.

[52]  A. Castro-Tirado,et al.  The Ulysses Supplement to the Granat/WATCH Catalog of Cosmic Gamma-Ray Bursts , 2000, astro-ph/0001019.

[53]  W. Lewin,et al.  On the Nature of XTE J0421+560/CI Camelopardalis , 1999, astro-ph/9907124.

[54]  T. Lazio,et al.  Hyperstrong Radio-Wave Scattering in the Galactic Center. I. A Survey for Extragalactic Sources Seen through the Galactic Center , 1998, astro-ph/9804156.

[55]  J. Cordes,et al.  Hyperstrong Radio-Wave Scattering in the Galactic Center. II. A Likelihood Analysis of Free Electrons in the Galactic Center , 1998, astro-ph/9804157.

[56]  J. Cordes,et al.  Giant pulses from the Crab pulsar: A joint radio and gamma-ray study , 1995 .

[57]  D. Lorimer,et al.  Multifrequency flux density measurements of 280 pulsars , 1995 .

[58]  Anthony C. S. Readhead,et al.  Equipartition brightness temperature and the inverse Compton catastrophe , 1994 .

[59]  Robert H. Becker,et al.  A 5-GHz VLA Survey of the Galactic Plane (Becker+ 1994) , 1994 .

[60]  D. Frail,et al.  A Transient Radio Source near the Center of the Milky Way Galaxy , 1992, Science.

[61]  M. Kundu,et al.  A VLA Survey of Nearby Flare Stars , 1989 .

[62]  F. Schwab,et al.  Relaxing the isoplanatism assumption in self-calibration; applications to low-frequency radio interferometry , 1984 .

[63]  E. Fenimore,et al.  Gamma-burst observations from the Pioneer Venus Orbiter , 1981 .

[64]  F. J. Wymer,et al.  The Pioneer Venus Orbiter Gamma Burst Detector , 1980, IEEE Transactions on Geoscience and Remote Sensing.

[65]  V. Trimble.  THE DISTANCE TO THE CRAB NEBULA AND NP 0532 , 1973 .

[66]  Kenneth I. Kellermann,et al.  The Spectra of Opaque Radio Sources , 1969 .