The Rocket Experiment Demonstration of a Soft X-ray Polarimeter

The primary goal of this project is to demonstrate a soft X-ray polarimeter concept. All components have been verified to perform as desired, so a flight system would verify system performance as a basis for a more capable orbital mission. The secondary goal is to expand the experiences of several early career researchers. The scientific objective of the Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX Polarimeter, name used with permission of Major League Baseball) is to make the first measurement of the linear X-ray polarization in the soft X-ray band (E < 1 keV). The first flight of the REDSoX Polarimeter would target Mk 421, which is commonly modeled as a highly relativistic jet aimed nearly along the line of sight. Such sources are likely to be polarized at a level of 20% or more due to strong magnetization of jet plasma, so the goal is to obtain a significant detection even if it is as low as 20%. An isolated neutron star (INS) with a strong magnetic field would be the target for a future flight. Due to effects of vacuum birefringence predicted in quantum electrodynamics, we expect 80-100% polarization from such stars, which should be detectable in a sounding rocket flight. We employ multilayer-coated mirrors as Bragg reflectors at the Brewster angle. By matching to the dispersion of a spectrometer, one may take advantage of high multilayer reflectivities and achieve polarization modulation factors over 90%. Using replicated mirrors from MSFC and gratings made at MIT, we construct a spectrometer that disperses to three laterally graded multilayer mirrors (LGMLs). The lateral grading changes the wavelength of the Bragg peak for 45 degree reflections linearly across the mirror, matching the dispersion of the spectrometer. By dividing the entrance aperture into six equal sectors, pairs of blazed gratings from opposite sectors are oriented to disperse to the same LGML. The position angles for the LGMLs are 120 degrees to each other. CCD detectors then measure the intensities of the dispersed spectra after reflection and polarizing by the LGMLs, giving the three Stokes parameters needed to determine the source polarization. We will rely on components whose performance has been verified in the laboratory or in space. The mirror fabrication team at MSFC has significant experience with flight systems and five mandrels to be used already exist. LGMLs have been in development under NASA APRA funding for the past few years and are sufficient for this project. Prototype gratings for the project have been fabricated at MIT and have been demonstrated to have the desired performance. We have constructed a source of polarized X-rays that operates at a wide range of energies with a selectable polarization angle in the lab for testing prototype components of our proposed instrument; we used the system to test gratings with polarized X-rays at the C-K line and measured grating efficiencies and blaze angles at several energies. A grating alignment method has been demonstrated for the Arcus project and a nearly identical system for the polarimeter has been fabricated, to be used to refine the grating assembly and alignment procedure. The REDSoX Polarimeter would undergo a one time roll by 60 degrees about the optical axis in flight in order to assess and remove possible systematic effects. Our technological approach has significant promise for future missions that would operate in the 0.18-0.4 keV band, with the potential to extend the bandpass to 1 keV. This sounding rocket program would provide a demonstration that a multilayer-based polarimetry approach can work, providing a basis for an orbital mission.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground-based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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