We present the discovery of a likely new redback millisecond pulsar (MSP) binary associated with the Fermi γ-ray source 4FGL J2333.1-5527. Using optical photometric and spectroscopic observations ...from the Southern Astrophysical Research telescope, we identify a low-mass, main-sequence-like companion in a 6.9 hr, highly inclined orbit around a suspected massive neutron star primary. Archival XMM-Newton X-ray observations show this system has a hard power-law spectrum Γ = 1.6 0.3 and LX ∼ 5 × 1031 erg s−1, consistent with redback MSP binaries. Our data suggest that for secondary masses typical of redbacks, the mass of the neutron star is likely well in excess of ∼1.4 M , but future timing of the radio pulsar is necessary to bolster this tentative conclusion. This work shows that a bevy of nearby compact binaries still await discovery, and that unusually massive neutron stars continue to be common in redbacks.
Shocks and dust formation in nova V809 Cep Babul, Aliya-Nur; Sokoloski, Jennifer L; Chomiuk, Laura ...
Monthly notices of the Royal Astronomical Society,
08/2022, Letnik:
515, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
The discovery that many classical novae produce detectable GeV γ-ray emission has raised the question of the role of shocks in nova eruptions. Here, we use radio observations of nova V809 ...Cep (nova Cep 2013) with the Jansky Very Large Array to show that it produced non-thermal emission indicative of particle acceleration in strong shocks for more than a month starting about 6 weeks into the eruption, quasi-simultaneous with the production of dust. Broadly speaking, the radio emission at late times – more than 6 months or so into the eruption – is consistent with thermal emission from $10^{-4}\, {\rm M}_\odot$ of freely expanding, 104 K ejecta. At 4.6 and 7.4 GHz, however, the radio light curves display an initial early-time peak 76 d after the discovery of the eruption in the optical (t0). The brightness temperature at 4.6 GHz on day 76 was greater than 105 K, an order of magnitude above what is expected for thermal emission. We argue that the brightness temperature is the result of synchrotron emission due to internal shocks within the ejecta. The evolution of the radio spectrum was consistent with synchrotron emission that peaked at high frequencies before low frequencies, suggesting that the synchrotron from the shock was initially subject to free–free absorption by optically thick ionized material in front of the shock. Dust formation began around day 37, and we suggest that internal shocks in the ejecta were established prior to dust formation and caused the nucleation of dust.
There is a significant discrepancy between recent predictions of the Milky Way classical nova rate of ∼ 60 per year and the annual discovery rate of ∼ 10. Why the recovery fraction of these events is ...∼ 15% even with large advancements to observational time domain astronomy remains largely unexplained. Because of the location of Earth within the Galaxy, discovering Galactic transients requires a large field of view. For the past few decades, observations from a network of amateur astronomers were largely responsible for discovering classical novae, so the sky-coverage as a function of position and depth was difficult to model. Fortunately, in the past decade, many time domain surveys with fields of view that cover large areas of the sky have been commissioned, making the sky-coverage more well defined. To date, there are no Galactic nova rate predictions made using data from a time domain survey that is capable of observing the entire sky. In this thesis, the first estimate of the Galactic nova rate using observations from two all-sky surveys is made.The All-Sky Automated Survey for Supernovae (ASAS-SN) is the first survey to systematically observe the entire sky every night, providing unprecedented cadence of the sky for transients and variable stars. The space-based survey Gaia has a broad observing filter and a fine (∼ 0.1 arcsecond) pixel scale, so it is capable of detecting Galactic plane transients in crowded fields that are heavily affected by extinction. These are the only two all-sky surveys to report classical nova candidates, and they have contributed to marginally increasing the discovery rate of Galactic novae to 13 per year on average since 2017. In addition to the increase in discoveries, this thesis exploits the systematic observing patterns to estimate what fraction of the Galaxy’s novae these surveys detect. To make this estimate, I have constructed a statistical model of Galactic classical novae by utilizing the recently published models of stellar density and extinction of the Milky Way. Using ASAS-SN photometry, I measure the outburst amplitude of novae to be normally distributed with mean and standard deviation μ = 11.43 ± 0.25 mag and σ = 2.57 ± 0.20 mag, respectively. By using recently available all-sky 3D dust maps, I estimate that ∼ 50% of Galactic novae are hidden by extinction from being detected by ASAS-SN. Finally, I estimate that the recovery fraction of the global population of Galactic novae for ASAS-SN ≈ 33%, Gaia ≈ 42%, and a joint effort of the two surveys ≈ 54%, predicting that the Galactic nova rate is 26 ± 5 yr−1, significantly lower than recent estimates.
Abstract
We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV
γ
-ray source 4FGL J0540.0–7552. The source shows the persistent presence of an optical ...accretion disk and exhibits extreme X-ray and optical variability. It also has an X-ray spectrum well-fit by a hard power law with Γ = 1.8 and a high ratio of X-ray to
γ
-ray flux. Together, these properties are consistent with the classification of the binary as a transitional millisecond pulsar (tMSP) in the subluminous disk state. Uniquely among the candidate tMSPs, 4FGL J0540.0–7552 shows consistent optical, X-ray, and
γ
-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV
γ
-rays sometime in mid-2013. In its current subluminous disk state, and like one other candidate tMSP in the Galactic field, 4FGL J0540.0–7552 appears to always be in an X-ray “flare mode,” indicating that this could be common phenomenology for tMSPs.
Abstract
We have discovered a new X-ray-emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV
γ
-ray source 4FGL J1120.0–2204, the second brightest Fermi source ...that still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm (
T
eff
∼ 8500 K) companion in a 15.1 hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only ∼820 pc. Unlike the typical “spider” or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the ∼0.17
M
⊙
companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf. Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in ∼2 Gyr, the properties of the binary will match those of several millisecond pulsar–white dwarf binaries with very short (<1 day) orbital periods. This makes 4FGL J1120.0–2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process.
ABSTRACT
Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The nova’s moderate proximity (2.7 kpc, from Gaia) provided an opportunity to explore its ...multiwavelength properties in great detail. Here, we report on YZ Ret as part of a long-term project to identify the physical mechanisms responsible for high-energy emission in classical novae. We use simultaneous Fermi/LAT and NuSTAR observations complemented by XMM–Newton X-ray grating spectroscopy to probe the physical parameters of the shocked ejecta and the nova-hosting white dwarf. The XMM–Newton observations revealed a supersoft X-ray emission which is dominated by emission lines of C v, C vi, N vi, N vii, and O viii rather than a blackbody-like continuum, suggesting CO-composition of the white dwarf in a high-inclination binary system. Fermi/LAT-detected YZ Ret for 15 d with the γ-ray spectrum best described by a power law with an exponential cut-off at 1.9 ± 0.6 GeV. In stark contrast with theoretical predictions and in keeping with previous NuSTAR observations of Fermi-detected classical novae (V5855 Sgr and V906 Car), the 3.5–78-keV X-ray emission is found to be two orders of magnitude fainter than the GeV emission. The X-ray emission observed by NuSTAR is consistent with a single-temperature thermal plasma model. We do not detect a non-thermal tail of the GeV emission expected to extend down to the NuSTAR band. NuSTAR observations continue to challenge theories of high-energy emission from shocks in novae.
Abstract
We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher ...Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star–black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg
2
for the 90th percentile best localization), covering a total of 51 deg
2
and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an
r
-band decline rate of 0.68 mag day
−1
, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most −17.8 mag (50% confidence). Our data are not constraining for “red” kilonovae and rule out “blue” kilonovae with
M
> 0.5
M
⊙
(30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles <17° assuming an initial jet opening angle of ∼5.°2 and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.
Classical Novae at Radio Wavelengths Chomiuk, Laura; Linford, Justin D.; Aydi, Elias ...
The Astrophysical journal. Supplement series,
12/2021, Letnik:
257, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
We present radio observations (1–40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets ...display a striking diversity in their optical parameters (e.g., spanning optical fading timescales,
t
2
= 1–263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with nonthermal emission observed at earlier times. We identify high brightness temperature emission (
T
B
> 5 × 10
4
K) as an indication of synchrotron emission in at least nine (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves in a machine-readable table and encourage the use of these data by the broader community in multiwavelength studies and modeling efforts.
We have discovered a new X-ray emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV \(\gamma\)-ray source 4FGL J1120.0-2204, the second brightest Fermi source that ...still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm (\(T_{\textrm{eff}}\sim8500\) K) companion in a 15.1-hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only \(\sim 820\) pc. Unlike the typical "spider" or white dwarf secondaries in short-period millisecond pulsar binaries, our observations suggest the \(\sim 0.17\,M_{\odot}\) companion is in an intermediate stage, contracting on the way to becoming an extremely low-mass helium white dwarf (a "pre-ELM" white dwarf). Although the companion is apparently unique among confirmed or candidate millisecond pulsar binaries, we use binary evolution models to show that in \(\sim 2\) Gyr, the properties of the binary will match those of several millisecond pulsar-white dwarf binaries with very short (\(< 1\) d) orbital periods. This makes 4FGL J1120.0-2204 the first system discovered in the penultimate phase of the millisecond pulsar recycling process.
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