ABSTRACT
The North Polar Spur (NPS) is one of the largest structures observed in the Milky Way in both the radio and soft X-rays. While several predictions have been made regarding the origin of the ...NPS, modelling the structure is difficult without precise distance constraints. In this paper, we determine accurate distances to the southern terminus of the NPS and towards latitudes ranging up to 55°. First, we fit for the distance and extinction to stars towards the NPS using optical and near-infrared photometry and Gaia Data Release 2 astrometry. We model these per-star distance–extinction estimates as being caused by dust screens at unknown distances, which we fit for using a nested sampling algorithm. We then compare the extinction to the Spur derived from our 3D dust modelling with integrated independent measures from XMM–Newton X-ray absorption and H i column density measures. We find that we can account for nearly 100 per cent of the total column density of the NPS as lying within 140 pc for latitudes >26° and within 700 pc for latitudes <11°. Based on the results, we conclude that the NPS is not associated with the Galactic Centre or the Fermi bubbles. Instead, it is likely associated, especially at higher latitudes, with the Scorpius–Centaurus association.
Abstract
The fate of stars in the zero-age main-sequence (ZAMS) range ≈8–12
M
⊙
is unclear. They could evolve to form white dwarfs or explode as electron-capture supernovae (SNe) or iron ...core-collapse SNe (CCSNe). Even though the initial mass function indicates that this mass range should account for over 40% of all CCSN progenitors, few have been observationally confirmed, likely due to the faintness and rapid evolution of some of these transients. In this paper, we present a sample of nine Ca-rich/O-poor Type IIb SNe detected by the Zwicky Transient Facility with progenitors likely in this mass range. These sources have a Ca
ii
λ
λ
7291, 7324/O
i
λ
λ
6300, 6364 flux ratio of ≳2 in their nebular spectra. Comparing the measured O
i
luminosity (≲10
39
erg s
−1
) and derived oxygen mass (≈0.01
M
⊙
) with theoretical models, we infer that the progenitor ZAMS mass for these explosions is less than 12
M
⊙
. The ejecta properties (
M
ej
≲ 1
M
⊙
and
E
kin
∼ 10
50
erg) are also consistent. The low ejecta mass of these sources indicates a class of strongly-stripped SNe that is a transition between the regular stripped-envelope SNe and ultra-stripped SNe. The progenitor could be stripped by a main-sequence companion and result in the formation of a neutron star−main sequence binary. Such binaries have been suggested to be progenitors of neutron star−white dwarf systems that could merge within a Hubble time and be detectable with LISA.
Abstract
We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum ...light 1 day <
t
1/2
< 12 days, of which 28 have blue (
g
−
r
≲ −0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had predominantly featureless spectra and luminous radio emission: AT2018lug (The Koala) and AT2020xnd (The Camel). Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad H
α
in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (eight) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 24 of the transients. All X-ray and radio observations resulted in nondetections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs and use ZTF’s SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.
Abstract We present the discovery and analysis of SN 2022oqm, a Type Ic supernova (SN) detected <1 day after the explosion. The SN rises to a blue and short-lived (2 days) initial peak. Early-time ...spectral observations of SN 2022oqm show a hot (40,000 K) continuum with high ionization C and O absorption features at velocities of 4000 km s −1 , while its photospheric radius expands at 20,000 km s −1 , indicating a pre-existing distribution of expanding C/O material. After ∼2.5 days, both the spectrum and light curves evolve into those of a typical SN Ic, with line velocities of ∼10,000 km s −1 , in agreement with the evolution of the photospheric radius. The optical light curves reach a second peak at t ≈ 15 days. By t = 60 days, the spectrum of SN 2022oqm becomes nearly nebular, displaying strong Ca ii and Ca ii emission with no detectable O i , marking this event as Ca-rich. The early behavior can be explained by 10 −3 M ⊙ of optically thin circumstellar material (CSM) surrounding either (1) a massive compact progenitor such as a Wolf–Rayet star, (2) a massive stripped progenitor with an extended envelope, or (3) a binary system with a white dwarf. We propose that the early-time light curve is powered by both the interaction of the ejecta with the optically thin CSM and shock cooling (in the massive star scenario). The observations can be explained by CSM that is optically thick to X-ray photons, is optically thick in the lines as seen in the spectra, and is optically thin to visible-light continuum photons that come either from downscattered X-rays or from the shock-heated ejecta. Calculations show that this scenario is self-consistent.
Based on
Das and Dey (2007), this paper designs and implements a periodic controller to achieve, via zero-placement, robustness of a physical Cart–Inverted Pendulum system with respect to ...differential gain variations in the output sensors. Experimental results that verify the superiority of this controller over linear time-invariant (LTI) ones are also presented.
Abstract We present SN 2023zaw—a subluminous ( M r = −16.7 mag) and rapidly evolving supernova ( t 1/2, r = 4.9 days), with the lowest nickel mass (≈0.002 M ⊙ ) measured among all stripped-envelope ...supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s −1 . The late-time spectra show prominent narrow He i emission lines at ∼1000 km s −1 , indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M ☉ and an envelope radius of ≈50 R ⊙ . The extremely low nickel mass and low ejecta mass (≈0.5 M ⊙ ) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M ⊙ ) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M ☉ ) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.
We use the light-curve and spectral synthesis code JEKYLL to calculate a set of macroscopically mixed type IIb supernova (SN) models, which are compared to both previously published and new ...late-phase observations of SN 2020acat. The models differ in the initial mass, in the radial mixing and expansion of the radioactive material, and in the properties of the hydrogen envelope. The best match to the photospheric and nebular spectra and light curves of SN 2020acat is found for a model with an initial mass of 17 M ⊙ , strong radial mixing and expansion of the radioactive material, and a 0.1 M ⊙ hydrogen envelope with a low hydrogen mass fraction of 0.27. The most interesting result is that strong expansion of the clumps containing radioactive material seems to be required to fit the observations of SN 2020acat both in the diffusion phase and in the nebular phase. These Ni bubbles are expected to expand due to heating from radioactive decays, but the degree of expansion is poorly constrained. Without strong expansion, there is a tension between the diffusion phase and the subsequent evolution, and models that fit the nebular phase produce a diffusion peak that is too broad. The diffusion-phase light curve is sensitive to the expansion of the Ni bubbles because the resulting Swiss-cheese-like geometry decreases the effective opacity and therefore the diffusion time. This effect has not been taken into account in previous light-curve modelling of stripped-envelope SNe, which may lead to a systematic underestimate of their ejecta masses. In addition to strong expansion, strong mixing of the radioactive material also seems to be required to fit the diffusion peak. It should be emphasized, however, that JEKYLL is limited to a geometry that is spherically symmetric on average, and large-scale asymmetries may also play a role. The relatively high initial mass found for the progenitor of SN 2020acat places it at the upper end of the mass distribution of type IIb SN progenitors, and a single-star origin cannot be excluded.
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