Proposal #116

Proposal: Our team is running a long-term program of observing polars (i.e. highly-magnetized CVs) with NASA’s NuSTAR X-ray telescope. Since polars are highly variable (with ~50% duty cycle), we would like to schedule our X-ray observations when our targets are bright in the X-ray band. We plan to tailor their high X-ray flux states based on optical magnitude measurements. We would like to propose that AAVSO astronomers observe each of our 5 polar targets bi-weekly until we trigger NuSTAR observations. We plan to run this program until the end of May 2020.

Goals: Our NuSTAR observations of polars in our neighborhood are the first crucial steps to resolve one of the long-standing problems in high-energy astrophysics - the source populations that comprise the Galactic Ridge X-ray emission. Nearly three decades ago, an X-ray haze that extends out from the Galactic Center for ~60 degrees in longitude and a few degrees in latitude was discovered and named as the Galactic ridge X-ray emission (GRXE). A series of very deep Chandra and XMM-Newton observations revealed that the GRXE consists of thousands of unresolved point sources largely dominated by magnetic cataclysmic variables (CVs), the exact X-ray populations of the GRXE are still unknown. Our NuSTAR observations will
test our hypothesis that the GRXE is dominated by a large number of polars. More specifically, we will characterize X-ray spectra of polars using NuSTAR’s broad-band, high-resolution X-ray spectroscopy and compare their plasma temperature (from shocked accretion column) and white dwarf masses with those of the GRXE.

Targets: We selected a group of five bright polars which are visible by AAVSO members in the United States: ST Lmi, VV Pup, V2301 Oph, UZ For and EP Dra. Based on their historical data, we roughly estimated that the thresholds for high states are 15.4V, 14.9V, 15.7V, 18.0V, 18.5V respectively magnitude in the V band. Using the future optical magnitude data obtained by AAVSO members, we will make a decision on whether we should trigger X-ray observations or not. In case any of these targets are not observable by the AAVSO or if you have any seasonable restrictions, please let us know.

Observation strategy: Since polars are highly variable (with ~50% duty cycle), we would like to schedule our X-ray observations when our targets are bright in the X-ray band. Since the optical and X-ray fluxes of magnetic CVs are positively correlated, we would like to ask the AAVSO astronomers to observe our targets in the optical band bi-weekly, provided that they are visible. Once our targets are found to be bright in the optical band, we will trigger their NuSTAR observations within 1-2 weeks. We do not need any further optical monitoring of our target once its NuSTAR observation is performed. A similar monitoring program with AAVSO has been performed to trigger a NuSTAR observation of AM Her successfully. Our NuSTAR observation program will run until the end of May 2020.

Kaya Mori (PI), Charles Hailey, Ninad Nirgudkar (Columbia University) and Gavin Ramsay (Armagh observatory)