Quasisoft X-Ray Sources: White Dwarfs? Neutron Stars? Black Holes?
Abstract
Two of the most exciting areas of current research in astrophysics are black holes and Type Ia supernovae. We propose archival work that has the potential to shed light on both areas. The focus of our research is a newly-established class of x-ray sources called Quasisoft X-ray Sources (QSSs). Although they comprise a significant fraction of the x- ray sources in galaxies of all types, including M31, it has proved difficult to identify members of this class in the Milky Way or Magellanic Clouds. We have developed methods to find these sources, and have begun to meet with success in the application of our methods. The three-year project we propose will allow us to identify QSSs. We will then use the full range of archived data to determine which QSS candidates are highly luminous, and which are members of less luminous classes, such as quiescent low-mass x-ray binaries (qLMXBs), or even isolated neutron stars. Many will be nearby x-ray active stars, or else distant AGN, whose discovery will also be of interest to a range of researchers. In the end, we will have a subset of intriguing physical systems, some of which may be accreting black holes and some of which may be unusual states of neutron stars or even of nuclear-burning white dwarfs. The systems identified through this ADAP program will be targets of future observing programs, from space and from the ground. The information we derive from NASA archived data will provide insight into important astrophysical questions. Do intermediate-mass black holes (IMBHs) exist? It has only been during the past 15 years or so that accreting compact objects that were considered as black hole candidates have been promoted to black holes. This achievement required years of observations of candidates in the Milky Way and Magellanic Clouds. The discovery of ultraluminous X- ray source in external galaxies suggests that there are black holes with masses larger than the 10-30 solar masses typical of the known black holes. To establish that these systems are intermediate-mass black holes would be difficult: they are so far from us that it is challenging to collect enough photons from them. The solution to this problem may come from QSSs, which have temperatures and luminosities consistent with what is expected from the inner disk of a black hole of somewhat higher than stellar mass. To test whether QSSs are accreting black holes, we need to discover some closer to us. This is what we propose to do. What are the progenitors of Type Ia supernovae? Nuclear-burning white dwarfs (NBWDs) are implicated in all progenitor models, during some stages of the progenitor's evolution. While NBWDs could be observable as supersoft X-ray sources (SSSs), sources with temperatures a few times lower than that of QSSs, we have found that there are too few SSSs in other galaxies to support the hypothesis that Type Ia SNe progenitors pass through a long supersoft phase. We now understand that the key issue in testing models of the progenitors is understanding the range of observable properties of NBWDs. Even though there are too few QSSs to make up the deficit, identifying and studying those QSSs which may be NBWDs is important. For every luminous soft source we identify, there will be several less luminous sources, some of which could be accreting or isolated neutron stars. We have been able to find such a small fraction of the neutron stars in our Galaxy that the search for dim neutron stars is important. Given the rich trove of science to be generated by the archival work we propose, this is a quintessential NASA program: we will use archived data from many missions to make progress toward achieving NASA's science objectives. Our work will help to answer the question: Do intermediate-mass black holes exist? It will also shed light on the nature of the progenitors of Type Ia supernovae, thereby helping to reduce the systematic uncertainties now inherent in an important cosmological tool.
- Publication:
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NASA ADAP Proposal
- Pub Date:
- 2011
- Bibcode:
- 2011adap.prop..210D