156 research outputs found
Evidence of Particle Acceleration in the Superbubble 30 Doradus C with NuSTAR
We present evidence of diffuse, non-thermal X-ray emission from the
superbubble 30 Doradus C (30 Dor C) using hard X-ray images and spectra from
NuSTAR observations. For this analysis, we utilize data from a 200 ks targeted
observation of 30 Dor C as well as 2.8 Ms of serendipitous off-axis
observations from the monitoring of nearby SN 1987A. The complete shell of 30
Dor C is detected up to 20 keV, and the young supernova remnant MCSNR
J0536-6913 in the southeast of 30 Dor C is not detected above 8 keV.
Additionally, six point sources identified in previous Chandra and XMM-Newton
investigations have hard X-ray emission coincident with their locations. Joint
spectral fits to the NuSTAR and XMM-Newton spectra across the 30 Dor C shell
confirm the non-thermal nature of the diffuse emission. Given the best-fit
rolloff frequencies of the X-ray spectra, we find maximum electron energies of
70-110 TeV (assuming a B-field strength of 4G), suggesting 30 Dor C is
accelerating particles. Particles are either accelerated via diffusive shock
acceleration at locations where the shocks have not stalled behind the
H shell, or cosmic-rays are accelerated through repeated acceleration
of low-energy particles via turbulence and magnetohydrodynamic waves in the
bubble's interior.Comment: 14 pages, 8 figures, ApJ, in pres
Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes
To characterize lightning processes that produce terrestrial gamma ray flashes (TGFs), we have analyzed broadband (<1 Hz to 30 kHz) lightning magnetic fields for TGFs detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite in 2004-2009. The majority (96%) of 56 TGF-associated lightning signals contain single or multiple VLF impulses superposed on a slow pulse that reflects a process raising considerable negative charge within 2-6 ms. Some TGF lightning emissions also contain VLF signals that precede any appreciable slow pulse and that we term precursor sferics. The analyses of 9 TGFs related to lightning discharges with location uncertainty <100 km consistently indicate that TGFs are temporally linked to the early portion of the slow process and associated VLF impulses, and not to precursor sferics. The nearly universal presence of a slow pulse suggests that the slow process plays an important role in gamma ray production. In all cases the slow process raises negative charge with a typical mean current moment of +30 kA km. The resulting charge moment change ranges from small values below +10 C km to a maximum of +200 C km, with an average of +64 C km. The current moment waveform extracted from TGF sferics with single or multiple VLF impulses also shows that the slow process initiates shortly before the major TGF-associated fast discharge. These features are generally consistent with the TGF-lightning sequence reported by Lu et al. (2010), suggesting that the majority of RHESSI TGFs are produced during the upward negative leader progression prevalent in normal polarity intracloud flashes
New Star Observations with NuSTAR: Flares from Young Stellar Objects in the Ï Ophiuchi Cloud Complex in Hard X-Rays
We study the structure and dynamics of extreme flaring events on young stellar objects (YSOs) observed in hard X-rays by the Nuclear Spectroscopic Telescope Array (NuSTAR). During 2015 and 2016, NuSTAR made three observations of the star-forming region Ï Ophiuchi, each with an exposure ~50 ks. NuSTAR offers unprecedented sensitivity above ~7 keV, making this data set the first of its kind. Through improved coverage of hard X-rays, it is finally possible to directly measure the high-energy thermal continuum for hot plasmas and to sensitively search for evidence of nonthermal emission from YSO flares. During these observations, multiple flares were observed, and spectral and timing analyses were performed on three of the brightest flares. By fitting an optically thin thermal plasma model to each of these events, we found flare plasma heated to high temperatures (~40â80 MK) and determined that these events are ~1000 times brighter than the brightest flares observed on the Sun. Two of the studied flares showed excess emission at 6.4 keV, and this excess may be attributable to iron fluorescence in the circumstellar disk. No clear evidence for a nonthermal component was observed, but upper limits on nonthermal emission allow for enough nonthermal energy to account for the estimated thermal energy in the flare on protostar IRS 43, which is consistent with the standard model for solar and stellar flares
Observational Artifacts of NuSTAR: Ghost Rays and Stray Light
The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012,
flies two conical approximation Wolter-I mirrors at the end of a 10.15m mast.
The optics are coated with multilayers of Pt/C and W/Si that operate from 3--80
keV. Since the optical path is not shrouded, aperture stops are used to limit
the field of view from background and sources outside the field of view.
However, there is still a sliver of sky (~1.0--4.0 degrees) where photons may
bypass the optics altogether and fall directly on the detector array. We term
these photons Stray-light. Additionally, there are also photons that do not
undergo the focused double reflections in the optics and we term these Ghost
Rays. We present detailed analysis and characterization of these two components
and discuss how they impact observations. Finally, we discuss how they could
have been prevented and should be in future observatories.Comment: Published in Journal of Astronomical Telescopes, Instruments, and
Systems. Open Access. http://dx.doi.org/10.1117/1.JATIS.3.4.04400
NuSTAR detection of X-ray heating events in the quiet Sun
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2â4.1 MK and emission measures of (0.6â15) Ă 1044 cmâ3 describe their spectra well, resulting in thermal energies in the range (2â6) Ă 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares
NuSTAR hard X-ray observation of a sub-A class solar flare
We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04.
Although it was too faint to be observed by the GOES X-ray Sensor, we estimate
the event to be an A0.1 class flare in brightness. This microflare, with only 5
counts per second per detector observed by RHESSI, is fainter than any hard
X-ray (HXR) flare in the existing literature. The microflare occurred during a
solar pointing by the highly sensitive NuSTAR astrophysical observatory, which
used its direct focusing optics to produce detailed HXR microflare spectra and
images. The microflare exhibits HXR properties commonly observed in larger
flares, including a fast rise and more gradual decay, earlier peak time with
higher energy, spatial dimensions similar to the RHESSI microflares, and a
high-energy excess beyond an isothermal spectral component during the impulsive
phase. The microflare is small in emission measure, temperature, and energy,
though not in physical size; observations are consistent with an origin via the
interaction of at least two magnetic loops. We estimate the increase in thermal
energy at the time of the microflare to be 2.4x10^27 ergs. The observation
suggests that flares do indeed scale down to extremely small energies and
retain what we customarily think of as "flarelike" properties.Comment: Status: Accepted by the Astrophysical Journal, 2017 July 1
X-ray polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)
This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASAâs 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3â50 keV (requirement; goal: 2.5â70 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 âŒ1% linear polarization fractions for X-ray sources with fluxes down to âŒ5 mCrab. This paper describes the PolSTAR design as well as the science drivers and the potential science return
Pushing the limits of the NuSTAR detectors
NuSTAR (the Nuclear Spectroscopic Telescope ARray) is a NASA Small Explorer (SMEX) mission launched in June of 2012. Since its launch, NuSTAR has been the preeminent instrument for spectroscopic analysis of the hard X-ray sky over the 3-80 keV bandpass. The low energy side of the bandpass is limited by the absorption along the photon path as well as by the ability of the pixels to trigger on incident photons. The on-board calibration source does not have a low-energy line that we can use to calibrate this part of the response, so instead we use the "nearest-neighbor" readout in the NuSTAR detector architecture to calibrate the individual pixel thresholds for all 8 flight detectors on both focal plane modules (FPMs). These threshold measurements feed back into the quantum efficiency of the detectors at low (<5 keV) energies and, once well-calibrated, may allow the use of NuSTAR data below the current 3 keV limit
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