26 research outputs found
Progenitor, Precursor and Evolution of the Dusty Remnant of the Stellar Merger M31-LRN-2015
M31-2015-LRN is a likely stellar merger discovered in the Andromeda Galaxy in
2015. We present new optical to mid-infrared photometry and optical
spectroscopy for this event. Archival data shows that the source started to
brighten 2 years before the nova event. During this precursor phase, the
source brightened by 3 mag. The lightcurve at 6 and 1.5 months before the
main outburst may show periodicity, with periods of 160.3 and 28.11.4
days respectively. This complex emission may be explained by runaway mass loss
from the system after the binary undergoes Roche-lobe overflow, leading the
system to coalesce in tens of orbital periods. While the progenitor spectral
energy distribution shows no evidence of pre-existing warm dust in system, the
remnant forms an optically thick dust shell at 4 months after the
outburst peak. The optical depth of the shell increases dramatically after 1.5
years, suggesting the existence of shocks that enhance the dust formation
process. We propose that the merger remnant is likely an inflated giant
obscured by a cooling shell of gas with mass M ejected at
the onset of the common envelope phase.Comment: 16 pages, 10 figures. Accepted for publication in MNRA
Finding Anomalous Periodic Time Series: An Application to Catalogs of Periodic Variable Stars
Catalogs of periodic variable stars contain large numbers of periodic
light-curves (photometric time series data from the astrophysics domain).
Separating anomalous objects from well-known classes is an important step
towards the discovery of new classes of astronomical objects. Most anomaly
detection methods for time series data assume either a single continuous time
series or a set of time series whose periods are aligned. Light-curve data
precludes the use of these methods as the periods of any given pair of
light-curves may be out of sync. One may use an existing anomaly detection
method if, prior to similarity calculation, one performs the costly act of
aligning two light-curves, an operation that scales poorly to massive data
sets. This paper presents PCAD, an unsupervised anomaly detection method for
large sets of unsynchronized periodic time-series data, that outputs a ranked
list of both global and local anomalies. It calculates its anomaly score for
each light-curve in relation to a set of centroids produced by a modified
k-means clustering algorithm. Our method is able to scale to large data sets
through the use of sampling. We validate our method on both light-curve data
and other time series data sets. We demonstrate its effectiveness at finding
known anomalies, and discuss the effect of sample size and number of centroids
on our results. We compare our method to naive solutions and existing time
series anomaly detection methods for unphased data, and show that PCAD's
reported anomalies are comparable to or better than all other methods. Finally,
astrophysicists on our team have verified that PCAD finds true anomalies that
might be indicative of novel astrophysical phenomena
Strong Ultraviolet Pulse From a Newborn Type Ia Supernova
Type Ia supernovae are destructive explosions of carbon oxygen white dwarfs.
Although they are used empirically to measure cosmological distances, the
nature of their progenitors remains mysterious, One of the leading progenitor
models, called the single degenerate channel, hypothesizes that a white dwarf
accretes matter from a companion star and the resulting increase in its central
pressure and temperature ignites thermonuclear explosion. Here we report
observations of strong but declining ultraviolet emission from a Type Ia
supernova within four days of its explosion. This emission is consistent with
theoretical expectations of collision between material ejected by the supernova
and a companion star, and therefore provides evidence that some Type Ia
supernovae arise from the single degenerate channel.Comment: Accepted for publication on the 21 May 2015 issue of Natur
VAST: An ASKAP Survey for Variables and Slow Transients
The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an
unprecedented opportunity to investigate the transient sky at radio
wavelengths. In this paper we present VAST, an ASKAP survey for Variables and
Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP
to enable the discovery and investigation of variable and transient phenomena
from the local to the cosmological, including flare stars, intermittent
pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar
scintillation, radio supernovae and orphan afterglows of gamma ray bursts. In
addition, it will allow us to probe unexplored regions of parameter space where
new classes of transient sources may be detected. In this paper we review the
known radio transient and variable populations and the current results from
blind radio surveys. We outline a comprehensive program based on a multi-tiered
survey strategy to characterise the radio transient sky through detection and
monitoring of transient and variable sources on the ASKAP imaging timescales of
five seconds and greater. We also present an analysis of the expected source
populations that we will be able to detect with VAST.Comment: 29 pages, 8 figures. Submitted for publication in Pub. Astron. Soc.
Australi
iPTF 16hgs: A Double-peaked Ca-rich Gap Transient in a Metal-poor, Star-forming Dwarf Galaxy
Calcium-rich gap transients represent an intriguing new class of faint and fast-evolving supernovae that exhibit strong [Ca ii] emission in their nebular phase spectra. In this paper, we present the discovery and follow-up observations of a faint and fast-evolving transient, iPTF 16hgs, that exhibited a double-peaked light curve. Exhibiting a Type Ib spectrum in the photospheric phase and an early transition to a [Ca ii] dominated nebular phase, iPTF 16hgs shows properties consistent with the class of Ca-rich gap transients, with two important exceptions. First, while the second peak of the light curve is similar to other Ca-rich gap transients, the first blue and fast-fading peak (declining over ≈2 days) is unique to this source. Second, we find that iPTF 16hgs occurred in the outskirts (projected offset of ≈6 kpc ≈ 1.9 R_(eff)) of a low-metallicity (≈0.4 Z_⊙), star-forming, dwarf spiral galaxy. Deep limits from late-time radio observations suggest a low-density environment for the source. If iPTF 16hgs shares explosion physics with the class of Ca-rich gap transients, the first peak can be explained by the presence of 0.01 M_⊙ of ^(56)Ni in the outer layers the ejecta, reminiscent of some models of He-shell detonations on WDs. However, if iPTF 16hgs is physically unrelated to the class, the first peak is consistent with shock cooling emission (of an envelope with a mass of ≈0.08 M_⊙ and radius of ≈13 R_⊙) in a core-collapse explosion of a highly stripped massive star in a close binary system
Interacting supernovae and supernova impostors. LSQ13zm: an outburst heralds the death of a massive star
We report photometric and spectroscopic observations of the optical transient LSQ13zm. Historical data reveal the presence of an eruptive episode (that we label as ‘2013a’) followed by a much brighter outburst (‘2013b’) three weeks later, that we argue to be the genuine supernova explosion. This sequence of events closely resemble those observed for SN 2010mc and (in 2012) SN 2009ip. The absolute magnitude reached by LSQ13zm during 2013a (MR = ?14.87 ± 0.25?mag) is comparable with those of supernova impostors, while that of the 2013b event (MR = ?18.46 ± 0.21?mag) is consistent with those of interacting supernovae. Our spectra reveal the presence of a dense and structured circumstellar medium, probably produced through numerous pre-supernova mass-loss events. In addition, we find evidence for high-velocity ejecta, with a fraction of gas expelled at more than 20 000?km s?1. The spectra of LSQ13zm show remarkable similarity with those of well-studied core-collapse supernovae. From the analysis of the available photometric and spectroscopic data, we conclude that we first observed the last event of an eruptive sequence from a massive star, likely a Luminous Blue Variable, which a short time later exploded as a core-collapse supernova. The detailed analysis of archival images suggest that the host galaxy is a star-forming Blue Dwarf Compact Galaxy
Detection of Broad H alpha Emission Lines in the Late-Time Spectra of a Hydrogen-Poor Superluminous Supernova
iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z = 0.3434, with a slow-evolving light curve and spectral features similar to SN2007bi. It rises in 83–148 days to reach a peak bolometric luminosity of ~1.3 × 1044 erg s−1, then decays slowly at 0.015 mag day−1. The measured ejecta velocity is ~ 13,000 km s−1. The inferred explosion characteristics, such as the ejecta mass (70–220 M⊙), and the total radiative and kinetic energy (Erad ~ 1051 erg, Ekin ~ 2 × 1053 erg), are typical of slow-evolving H-poor SLSN events. However, the late-time spectrum taken at +251 days (rest, post-peak) reveals a Balmer Hα emission feature with broad and narrow components, which has never been detected before among other H-poor SLSNe. The broad component has a velocity width of ~4500 km s−1 and a ~300 km s−1 blueward shift relative to the narrow component. We interpret this broad Hα emission with a luminosity of ~2 × 1041 erg s−1 as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of ~4 × 1016 cm from the explosion site. This interaction causes the rest-frame r-band LC to brighten at late times. The fact that the late-time spectra are not completely absorbed by the shock-ionized H-shell implies that its Thomson scattering optical depth is likely ≤1, thus setting upper limits on the shell mass ≤30 M⊙. Of the existing models, a Pulsational Pair Instability supernova model can naturally explain the observed 30 M⊙ H-shell, ejected from a progenitor star with an initial mass of (95–150) M⊙ about 40 years ago. We estimate that at least ~15% of all SLSNe-I may have late-time Balmer emission lines
ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf
The detonation of a helium shell on a white dwarf (WD) has been proposed as a possible explosion triggering mechanism for SNe Ia. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of ≈18 days from explosion, the transient reached a peak absolute magnitude of M_R ≈ −18.2 mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like SNe Ia, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color (g − r ≈ 2 mag) arising from nearly complete line blanketing of flux blueward of 5000 Å. This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after ≈30 days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive (≈0.15 M⊙) helium shell on a sub-Chandrasekhar mass (≈0.75 M⊙) WD after including mixing of ≈0.2 M⊙ of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells