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X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line-of-sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line-of-sight, and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of a SWCX emission model against the emission observed at low ecliptic latitudes, where the solar wind data are monitored, and at high ecliptic latitudes, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane were consistent with the line intensities predicted by the model, but were underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high temperature Galactic Halo emission components are reported for both spectral sets.

Previous detections of an X-ray emission line near 3.5 keV in galaxy clusters and other dark-matter-dominated objects have been interpreted as observational evidence for the decay of sterile neutrino dark matter. Motivated by this, we report on a search for a 3.5 keV emission line from the Milky Way's galactic dark matter halo with HaloSat. As a single pixel, collimated instrument, HaloSat observations are impervious to potential systematic effects due to grazing incidence reflection and CCD pixelization, and thus may offer a check on possible instrumental systematic errors in previous analyses. We report nondetections of a ~3.5 keV emission line in four HaloSat observations near the Galactic center. In the context of the sterile neutrino decay interpretation of the putative line feature, we provide 90% confidence level upper limits on the 3.5 keV line flux for a field centered 18.6 degrees from the Galactic center and the corresponding 7.1 keV sterile neutrino mixing angle: F ≤ 0.077 ph/((sq. cm)(s)(sr)) and sin^2(2θ) ≤ 4.25 x 10^(-11). The HaloSat mixing angle upper limit was calculated using a modern parameterization of the Milky Way's dark matter distribution, and in order to compare with previous limits, we also report the limit calculated using a common historical model. The HaloSat mixing angle upper limit places constraints on a number of previous mixing angle estimates derived from observations of the Milky Way's dark matter halo and galaxy clusters, and excludes several previous detections of the line. The upper limits cannot, however, entirely rule out the sterile neutrino decay interpretation of the 3.5 keV line feature.

We constrain the LMC’s global parameters by modeling the first soft X-ray (0.4–6.8 keV) observations of the entire Large Magellanic Cloud (LMC) produced in a single pointing with moderate spectral resolution by the HaloSat CubeSat. These data are best fit with the sum of two thermal plasma components in collisional ionization equilibrium and a power-law. We find cool (0.210±0.014keV) and hot (0.89±0.14keV) components. The LMC’s total X-ray luminosity is(1.08±0.14)×1039erg s−1.X-ray binaries make up a large fraction of the emission with a luminosity of(6.0±0.8)×1038ergs−1, followed by cool gas from superbubbles, supernovae, and diffuse emission with a luminosity of(3.0±0.3)×1038erg s−1. The hot gas from star formation contributes the smallest fraction, with a luminosity of(1.9±0.5)×1038erg s−1. We estimate the total volume of the cool gas to be between(0.2–1.2)×1010pc3and the hot gas to be between(1.0–5)×107pc3for filling factors off= 1and0.2. These volumes result in a total thermal energy for the cool gas between (1.4–3)×1054ergs for electron densities of 0.017–0.04 cm−3, and a thermal energy for the hot gas between (1.7–4)×1053ergs for electron densities of 0.12–0.3 cm−3. This yields cooling timescales for the cool and hot gas of(1.5–3)×108years and(1.8–6)×107years, respectively

HaloSat is a CubeSat-class microsatellite sensitive in the0.4−7.0keV energy band and designed to survey the entire sky in search of soft X-ray emission from highly ionized oxygen residing in the halo of the Milky Way galaxy. Those observations will help constrain the mass and spatial distribution of the Milky Way halo and help us understand if hot galactic halos constitute a significant contribution to the overall cosmological baryon budget. In this paper we describe the science instrument calibration products, including channel-to-energy transformation, instrument energy resolution and instrument response, and the on-ground efforts that led to their creation. We also describe the alignment process used to obtain the field of view information for the HaloSat science instrument.

The Cygnus Superbubble(CSB)is a region of soft X-ray emission approximately 13 degrees wide in the direction of the local spiral arm. Such a large region might be the result of strong stellar winds and supernovae from nearby stellar nurseries, or it could be the result of a single event—a hypernova. HaloSat observed four nonoverlapping10 degree diameter fields in the CSB region over the 0.4-7 keV band. The CSB absorption and temperature was found to be consistent over all four fields, with a weighted average of 6.1×1021cm−2and 0.190 keV, respectively. These observations suggest that the CSB is a cohesive object with a singular origin. The total thermal energy for the CSB is estimated at 4×1052erg, based upon a shell-like physical model of the CSB. Absorption and distance estimates to Cyg OB associations are examined. The CSB absorption is found to be most consistent with the absorption seen in Cyg OB1, implying that the CSB lies at a similar distance of 1.1–1.4 kpc.

We present HaloSat X-ray observations of the entirety of the bright X-ray emitting feature known14as the North Polar Spur (NPS). The large field of view of HaloSat enabled coverage of the entire15bright NPS in only fourteen fields, which were each observed for≈30,000 s. We find that the NPS16fields are distinct in both brightness and spectral shape from the surrounding halo fields. We fit the17NPS as two thermal components in ionization equilibrium with temperatures kTcool≈0.087 keV18and kThot≈0.28 keV. We note a temperature gradient in the NPS hot component with an inner19arc temperature warmer than the outer arc. The emission measures we find for the cool component20of the NPS is a factor of 3-5 greater than that of the hot component, which suggests that the bulk of21the NPS material is in the≈0.1 keV component. We evaluate distance estimates of 0.4 kpc and 8.022kpc for the NPS. Our findings suggest a preference for a distant NPS with an energy of≈6×105423erg, an age of≈10 Myr, and pressures consistent with a 10μG magnetic field associated with the24Fermi bubbles. The electron density≈10×10−3cm−3is consistent with estimates for the shock25region surrounding a galactic scale event.

The Milky Way galaxy is surrounded by a circumgalactic medium (CGM) that may play a key role in galaxy evolution as the source of gas for star formation and a repository of metals and energy produced by star formation and nuclear activity. The CGM may also be a repository for baryons seen in the early universe, but undetected locally. The CGM has an ionized component at temperatures near 2 x 10^6 K studied primarily in the soft X-ray band. Here we report a survey of the southern Galactic sky with a soft X-ray spectrometer optimized to study diffuse soft X-ray emission. The X-ray emission is best fit with a disc-like model based on the radial profile of the surface density of molecular hydrogen, a tracer of star formation, suggesting that the X-ray emission is predominantly from hot plasma produced via stellar feedback. Strong variations in the X-ray emission on angular scales of ~10 degrees indicate that the CGM is clumpy. Addition of an extended, and possibly massive, halo component is needed to match the halo density inferred from other observations.

The Vela and Puppis A supernova remnants(SNRs)comprise a large emission region of∼8°diameter in the soft X-ray sky. The HaloSat CubeSat mission provides the first soft X-ray (0.4–7 keV) observation of the entire Vela SNR and Puppis A SNR region with a single pointing and moderate spectral resolution. HaloSat observations of the Vela SNR are best fit with a two-temperature thermal plasma model consisting of a cooler component with=-+kT0.1910.010.01keV in collisional ionization equilibrium and a hotter component with=-+kT1.0620.270.45 keV in nonequilibrium ionization. Observations of the Puppis A SNR are best fit with a single-component plane-parallel shocked plasma model with=-+kT0.860.050.06 keV in nonequilibrium ionization. For the first time, we find the total X-ray luminosities of both components of the Vela SNR spectrum in the 0.5–7 keV energy band to be= ́-+L4.410X1.41.434 erg s−1 for the cooler component and= ́-+L4.110X1.51.834 erg s−1for the hotter component. We find the total X-ray luminosities of the Vela and Puppis A SNRs to be= ́L8.4 10X34 erg s−1 and= ́-+L6.710X0.91.136 erg s−1

HaloSat is the first mission funded by NASA’s Astrophysics Division to use the CubeSat platform. Using three co-aligned silicon drift detectors, the HaloSat observatory measures soft (0.4 to 7 keV) x-ray emission from sources of diffuse emission such as the hot, gaseous halo of the Milky Way. We describe the design and construction of the science payload on HaloSat and the reasoning behind many of the choices. As a direct result of the design choices and adherence to best practices during construction, the HaloSat science payload continues to perform well after more than one year on-orbit.

This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASA's 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3-50 keV (requirement; goal: 2.5-70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. The PolSTAR design is based on the technology developed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission launched in June 2012. In particular, it uses the same X-ray optics, extendable telescope boom, optical bench, and CdZnTe detectors as NuSTAR. The mission has the sensitivity to measure approx. 1% linear polarization fractions for X-ray sources with fluxes down to approx. 5 mCrab. This paper describes the PolSTAR design as well as the science drivers and the potential science return.