A New Population of X-ray Transients and ALMA Observation of Frontier Fields

Franz Bauer (Pont. U. Catolica)

During the recent Chandra campaign to extend the CDF-S from 4Ms to 7Ms, a fast X-ray transient was discovered. The wealth of ancillary data, plus subsequent optical/NIR limits, put interesting constraints on its origin, effectively ruling out a Galactic transient, as well as all known SNe and the vast majority of GRBs and TDEs. The remaining possibilities include a low-luminosity GRB and a beamed TDE involving an intermediate-mass black hole and a WD, although neither explains all of the observed properties. No similar event is seen in the Chandra Source Catalog, suggesting the event rate is low, but still plausibly factors of 5-20 higher than typical GRBs.

The Frontier Fields Legacy Program targets six strong-lensing clusters with deep HST and Spitzer imaging to detect and characterize the faint background galaxy population, particularly the first galaxies at z~6-10. However, given the wealth of sensitive ancillary data, we initiated an ALMA survey to produce shallow ~2'x2' band-6 maps of the Frontier Fields to study intrinsically faint, dusty star-forming galaxies (DSFGs) and place constraints on the star formation in a variety of interesting extragalactic source populations (including those at z>6). The first three fields (A2744, MACS0416, and MACs1149) have now been completed and yield uniform maps that pinpoint cool dust emission from powerful DSFGs at z>1. I will present a census of the detected objects thus far, as well as some early constraints our team is able to place on the average emission from undetected source populations.