HaloSat Overview

Philip Kaaret (philip-kaaret@uiowa.edu)

August 17, 2016

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

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.

HaloSat 3

Image from WMAP

Cold gas

Galaxies

Warm gas

Missing

Baryons Today

Cold gas Galaxies Warm gas Missing

Scientific Motivation: Where are the missing Baryons

HaloSat 4

Halos of Galaxies

Milky Way and Magellanic galaxies embedded in a hot halo of gas (from

Gupta et al. 2012)

Intergalactic Web

Simulation of the warm-hot intergalactic medium (WHIM)

vs

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 ~106 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 mm2)

HaloSat 5

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 magnetosphericemission 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

6/8/2016 HaloSat 6

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.

Mission Level 1 Requirements

HaloSat 7

ID Requirement

MR1-001X-ray detector shall be sensitive in a band from 400 eV to 2,000 eV with <= 100 eV energy resolution at 600 eV

MR1-002The observatory shall observe at least 75% of the sky with an angular resolution of 10° with goal to observe the whole sky

MR1-003

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).

MR1-004Science observations will be planned to minimize magnetospheric, heliospheric, and particle backgrounds

6U CubeSat

6U CubeSat form factor

Build 2016-2017

Launch in 2018

Operate for 213 days (required) to 365 days (goal)

Use commercial CubeSat bus

HaloSat 8

X-Ray Detector

• X-ray detectors from Amptek, Inc., with active area of 25 mm2

behind Si3N4 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

HaloSat – Instrument Peer Review 9

Amptek SDD with TEC in

TO-8 can (0.55” diameter)

Sensor Assembly

• SDD views sky through a 13.3 mm diameter hole that is 135 mm away (9.2° - 13.4° )

• Aspect control ±1.0°<< field of view

• To veto charged particle background, SDD is enclosed in a scintillatorreadout with avalanche photodiodes (APDs)

• Three identical detectors

HaloSat 10

SDD

Scintillator

Mounting plate = heat path

Scintillator

APD

Independent electronics for each detector assembly

11

Functional Diagram

Mechanical Design

Sensor assembly

HaloSat 12

X-ray aperture

All three detectors in one instrument enclosure

Co-aligned at Iowa and delivered as a single sub-assembly

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

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

HaloSat 14

Charging Mode

Science Mode

Science after crossing dusk terminator

Charging after crossing dawn terminator

Simulated Spectrum

• Simulation of 10,000 detector-second 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