HaloSat Overview. Philip Kaaret August 17, 2016

Transcription

1 HaloSat Overview Philip Kaaret August 17, 2016

2 Outline Scientific Motivation Missing Baryon Problem Mission Goal and Science Requirements Impediments Mission Level 1 Requirements Technical Solution 6U CubeSat X-Ray Detector Sensor Assembly Functional Diagram Mechanical Design Operations Simulated Spectrum HaloSat 2

3 Scientific Motivation: Missing Baryon Problem Observations of the early universe indicate that normal matter or baryons make up 5% of the total mass/energy of the universe. The cosmic microwave background shows the universe at an age of 400,000 years. All the baryons were in the same form, plasma at 3000 K. Today, there are baryons in many different forms: cold gas, stars (in galaxies), warm gas at various temperatures. Census of baryons in the universe today shows: Temperature determines what radiation baryons emit. Missing baryons are likely warm or hot. Baryons Today Image from WMAP Cold gas Missing Galaxies Warm gas Cold gas Galaxies Warm gas Missing HaloSat 3

4 Scientific Motivation: Where are the missing Baryons Halos of Galaxies Intergalactic Web vs Milky Way and Magellanic galaxies embedded in a hot halo of gas (from Gupta et al. 2012) Simulation of the warm-hot intergalactic medium (WHIM) HaloSat 4

5 Mission Goal and Science Requirements Goal: measure the mass of the Milky Way s halo Determine the geometry of the halo is it extended or disk-like? Measure how much radiation is made by the halo set by the gas mass Requirement: measure hot gas at ~10 6 K Detect X-rays from oxygen atoms O VII at 561 ev, O VIII at 653 ev Sensitive near 600 ev with 100 ev energy resolution Requirement: determine geometry of halo Observe whole sky Requirement: obtain sufficient X-ray counts Long duration mission View large part of sky (10º 10º fields) Allows use of small detectors (25 mm 2 ) HaloSat 5

6 Impediments Other sources of oxygen line emission: plasma in the solar system (heliosphere) and near Earth (magnetosphere) The HaloSat team includes experts on heliospheric and magnetospheric emission who will develop models we will use to: Plan optimal observation times to minimize foreground emission Model and subtract off the remaining foreground emission Accuracy of current measurements of the halo emission are limited by foregrounds, their minimization is a major plus for HaloSat Koutroumpa's model of Heliospheric O VII solar wind charge exchange (SWCX) emissivity from interstellar neutral H (left) and He (right) flowing through the solar system. 6/8/2016 HaloSat 6

7 Mission Level 1 Requirements ID MR1-001 MR1-002 Requirement X-ray detector shall be sensitive in a band from 400 ev to 2,000 ev with <= 100 ev energy resolution at 600 ev The observatory shall observe at least 75% of the sky with an angular resolution of 10 with goal to observe the whole sky MR1-003 MR1-004 The observatory shall obtain sufficient X-ray counts to measure the total emission in the sum of O VII and O VIII with a statistical accuracy of 0.5 LU for fields with a brightness of 5 LU (LU = line unit = photons/cm2/s/ster). Science observations will be planned to minimize magnetospheric, heliospheric, and particle backgrounds HaloSat 7

8 6U CubeSat 6U CubeSat form factor Build Launch in 2018 Operate for 213 days (required) to 365 days (goal) Use commercial CubeSat bus HaloSat 8

9 X-Ray Detector X-ray detectors from Amptek, Inc., with active area of 25 mm 2 behind Si 3 N 4 window SDD is inside sealed can and cooled by a thermoelectric cooler (TEC) Power for cooling is large fraction of power budget Same detectors as NICER Lab testing with Ti-L shows ΔE ~80 ev FWHM at 451 ev Amptek SDD with TEC in TO-8 can (0.55 diameter) HaloSat Instrument Peer Review 9

10 Sensor Assembly SDD views sky through a 13.3 mm diameter hole that is 135 mm away ( ) Aspect control ±1.0 << field of view APD Scintillator Scintillator To veto charged particle background, SDD is enclosed in a scintillator readout with avalanche photodiodes (APDs) Three identical detectors SDD Mounting plate = heat path HaloSat 10

11 Functional Diagram Independent electronics for each detector assembly 11

12 Mechanical Design All three detectors in one instrument enclosure Sensor assembly X-ray aperture Co-aligned at Iowa and delivered as a single sub-assembly HaloSat 12

13 Mechanical Design Mechanical interface is on bottom plate Three pairs of PCBs mounted on top, not enclosed Electrical interface is via three independent connectors HaloSat 13

14 Operations Observations on night side, two ~1000 s exposures per orbit Accumulate 10,000 detector s for each of ~400 targets Scheduled to minimize helio/magnetospheric background Science after crossing dusk terminator Charging Mode Science Mode Charging after crossing dawn terminator HaloSat 14

15 Simulated Spectrum Simulation of 10,000 detectorsecond exposure of typical field gives statistical accuracy of 8% in total oxygen line flux sets required observation time Red - oxygen emission lines at O VII (561 ev) and O VIII (653 ev) Green - absorbed hot plasma with oxygen emission removed for the distant (halo) emission Orange - unabsorbed cooler plasma with oxygen emission removed for local emission Blue - absorbed power-law for extragalactic emission HaloSat 15