PI: Fabien Grise, University of Iowa, firstname.lastname@example.org
Address: Department of Physics and Astronomy, 751 Van Allen Hall, Iowa City, Iowa 52242, United States
CoI: Philip Kaaret, University of Iowa
CoI: Stephane Corbel, CEA
CoI: David Cseh, CEA
Title: A direct test on the nature of the optical emission in an ultraluminous X-ray source
Abstract: Based on a recent Swift/XRT monitoring of the ultraluminous X-ray source (ULX) NGC 5408 X-1, we discovered that the source experiences intense dips where its X-ray count rate drops by at least an order of magnitude compared to its long term average. The next predicted series of dips should occur between mid-January and mid-March. We propose to observe the source with Gemini during that period to measure its optical flux in three photometric bands that we will compare with recent results obtained with HST/WFC3. Studying the optical emission of the source while in this X-ray dipping phase will allow us to distinguish if the optical emission from the ULX comes mainly from the donor star or an irradiated accretion disk. This would be a major, new result in the field.
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