Researchers at the University of California, Santa Cruz, and their collaborators at other institutions anticipate new insights into the mysteries of high-energy astrophysics as plans for an innovative x-ray telescope progress toward final approval by NASA. If all goes well with a technical study approved by NASA for this year, the telescope should be orbiting Earth by the end of the decade and taking the first focused high-energy x-ray pictures of matter falling into black holes and shooting out of exploding stars.
Named the Nuclear Spectroscopic Telescope Array (NuSTAR), the project has just been pegged by NASA for detailed study in the competitive Small Explorer Program, which seeks out new technologies and new proposals for space missions that can be launched at low cost. Pending a final NASA review next year, NuSTAR will be scheduled for launch in 2009.
"This is a real explorer mission. There has never been a sharp focusing telescope in this part of the spectrum before, so there is a very broad range of exploration science that needs to be done," said Stephen Thorsett, professor and chair of astronomy and astrophysics at UC Santa Cruz.
Thorsett chairs the science team of 11 co-investigators from seven institutions who have been planning the science goals for NuSTAR. Stan Woosley, also a professor of astronomy and astrophysics at UCSC, is another of the co-investigators. Supermassive black holes, gamma-ray jets, and the birth of new elements in supernovae are the three main areas of investigation planned for NuSTAR. But Thorsett said the most interesting results are likely to be the unexpected ones.
"We're almost certain to be surprised. Even in the first calibration observations, when we plan to look at local neutron stars, I expect the results will provide a thesis worth of work for a graduate student," he said.
According to Fiona Harrison, a California Institute of Technology astrophysicist and principal investigator of the NuSTAR project, an April high-altitude balloon flight in New Mexico should help to demonstrate whether the telescope's advanced sensors, invented and built at Caltech, are ready for space. The balloon test will mark the first time that focused pictures at high-energy ("hard") x-ray wavelengths will have been returned from high altitudes.
Focusing hard x-rays the way an optical telescope focuses light is technically very challenging, but well worth the effort, Thorsett said. Hard x-rays tend to penetrate the gas and dust of galaxies much better than the soft x-rays observed by existing x-ray satellites. NuSTAR will be 1,000 times more capable of finding new black holes than anything previously launched into space.
"Black holes that have large amounts of matter falling into them shine very brightly in this part of the spectrum, and the signal can get out of the cores of galaxies because hard x-rays are very penetrating. So this telescope will give us a very direct way to see and measure the growth of supermassive black holes, which we think are very common in the cores of galaxies," Thorsett said.
Studying black holes is one of three basic science goals for NuSTAR. The telescope will be used to take a census of black holes at all scales, and will also measure the "accretion rate" at which matter has fallen into them over time.
Another goal is to detect and measure radiation emitted in the aftermath of stellar explosions, or supernovae. The remnants of supernovae that exploded centuries ago still emit signals that can shed new light on how elements are formed and ejected in supernovae. NuSTAR will be especially good at observing the radioactive decay of titanium to calcium, which tends to be produced in the region of a supernova where material either is ejected forever from the explosion or falls back inward to form a compact remnant of some sort. NuSTAR will provide a direct test of theories developed by Woosley and others to explain how the elements are formed.
"We can look at these newborn elements in supernova remnants and compare the observations with the predictions from computer models," Thorsett said.
The third goal is to observe the highly energetic jets that stream out of certain black holes at nearly the speed of light. Coupled with observations from the Gamma-Ray Large-Area Space Telescope (GLAST), NuSTAR will provide data to help scientists explain this still-enigmatic but powerful phenomenon.
UCSC scientists are also involved in the GLAST project, which will use detectors designed and built by UCSC physicists in the Santa Cruz Institute for Particle Physics.
"NuSTAR is another step in the development of a strong program in high-energy astrophysics at UCSC. Together with our partner institutions in the Bay Area and southern California, and other national and international partners, we are looking forward to a very active future for high-energy astrophysics research," Thorsett said.
In addition to UCSC and Caltech, the other participating organizations and universities in the NuSTAR project are the Jet Propulsion Laboratory (managed by Caltech for NASA), Columbia University, Stanford Linear Accelerator (SLAC), the Lawrence Livermore National Laboratory, Sonoma State University, and the Danish Space Research Institute. NuSTAR's spacecraft will be built by General Dynamics Spectrum Astro.
The Small Explorer Program is designed to provide frequent, low-cost access to space for physics and astronomy missions with small to midsized spacecraft. NASA has successfully launched six SMEX missions since 1992. The missions include the Reuven Ramaty High Energy Solar Spectroscopic Imager, launched in February 2002, and the Galaxy Evolution Explorer, launched in April 2003 and led by Caltech physics professor Chris Martin.
The selected proposals were among 29 SMEX and eight mission-of-opportunity proposals submitted to NASA in May 2003. They were in response to an Explorer Program Announcement of Opportunity issued in February 2003. NASA selected six proposals in November 2003 for detailed feasibility studies. Of these, NASA has now selected the International Boundary Explorer for launch in 2008 and NuSTAR for a possible launch in 2009.
NASA's Goddard Space Flight Center, Greenbelt, Md., manages the Explorer Program for the Science Mission Directorate.