Current time domain facilities are finding several hundreds of transient astronomical events a year. The discovery rate is expected to increase in the future as soon as new surveys such as the Zwicky ...Transient Facility (ZTF) and the Large Synoptic Sky Survey (LSST) come online. Presently, the rate at which transients are classified is approximately one order or magnitude lower than the discovery rate, leading to an increasing "follow-up drought". Existing telescopes with moderate aperture can help address this deficit when equipped with spectrographs optimized for spectral classification. Here, we provide an overview of the design, operations and first results of the Spectral Energy Distribution Machine (SEDM), operating on the Palomar 60-inch telescope (P60). The instrument is optimized for classification and high observing efficiency. It combines a low-resolution (R ∼ 100) integral field unit (IFU) spectrograph with "Rainbow Camera" (RC), a multi-band field acquisition camera which also serves as multi-band (ugri) photometer. The SEDM was commissioned during the operation of the intermediate Palomar Transient Factory (iPTF) and has already lived up to its promise. The success of the SEDM demonstrates the value of spectrographs optimized for spectral classification.
Abstract
We present a systematic study of the most luminous (
M
IR
Vega magnitudes brighter than −14) infrared (IR) transients discovered by the
SPitzer
InfraRed Intensive Transients Survey (SPIRITS) ...between 2014 and 2018 in nearby galaxies (
D
< 35 Mpc). The sample consists of nine events that span peak IR luminosities of
M
4.5,peak
between −14 and −18.2, show IR colors between 0.2 < (3.6–4.5) < 3.0, and fade on timescales between 55 days <
t
fade
< 480 days. The two reddest events (
A
V
> 12) show multiple, luminous IR outbursts over several years and have directly detected, massive progenitors in archival imaging. With analyses of extensive, multiwavelength follow-up, we suggest the following possible classifications: five obscured core-collapse supernovae (CCSNe), two erupting massive stars, one luminous red nova, and one intermediate-luminosity red transient. We define a control sample of all optically discovered transients recovered in SPIRITS galaxies and satisfying the same selection criteria. The control sample consists of eight CCSNe and one Type Iax SN. We find that 7 of the 13 CCSNe in the SPIRITS sample have lower bounds on their extinction of 2 <
A
V
< 8. We estimate a nominal fraction of CCSNe in nearby galaxies that are missed by optical surveys as high as
(90% confidence). This study suggests that a significant fraction of CCSNe may be heavily obscured by dust and therefore undercounted in the census of nearby CCSNe from optical searches.
Abstract
Luminous red novae (LRNe) are transients characterized by low luminosities and expansion velocities, and they are associated with mergers or common-envelope ejections in stellar binaries. ...Intermediate-luminosity red transients (ILRTs) are an observationally similar class with unknown origins, but they are generally believed to be either electron-capture supernovae in super-asymptotic giant branch stars or outbursts in dusty luminous blue variables (LBVs). In this paper, we present a systematic sample of eight LRNe and eight ILRTs detected as part of the Census of the Local Universe (CLU) experiment on the Zwicky Transient Facility (ZTF). The CLU experiment spectroscopically classifies ZTF transients associated with nearby (<150 Mpc) galaxies, achieving 80% completeness for
m
r
< 20 mag. Using the ZTF-CLU sample, we derive the first systematic LRNe volumetric rate of
7.8
−
3.7
+
6.5
×
10
−
5
Mpc
−3
yr
−1
in the luminosity range −16 ≤
M
r
≤ −11 mag. We find that, in this luminosity range, the LRN rate scales as
dN
/
dL
∝
L
−
2.5
±
0.3
—significantly steeper than the previously derived scaling of
L
−1.4±0.3
for lower-luminosity LRNe (
M
V
≥ −10 mag). The steeper power law for LRNe at high luminosities is consistent with the massive merger rates predicted by binary population synthesis models. We find that the rates of the brightest LRNe (
M
r
≤ −13 mag) are consistent with a significant fraction of them being progenitors of double compact objects that merge within a Hubble time. For ILRTs, we derive a volumetric rate of
2.6
−
1.4
+
1.8
×
10
−
6
Mpc
−3
yr
−1
for
M
r
≤ −13.5 mag, which scales as
dN
/
dL
∝
L
−
2.5
±
0.5
. This rate is ∼1%–5% of the local core-collapse supernova rate and is consistent with theoretical ECSN rate estimates.
Abstract
The mass of the central black hole in a galaxy that hosted a tidal disruption event (TDE) is an important parameter in understanding its energetics and dynamics. We present the first ...homogeneously measured black hole masses of a complete sample of 12 optically/UV-selected TDE host galaxies (down to g
host ≤ 22 mag and z = 0.37) in the Northern sky. The mass estimates are based on velocity dispersion measurements, performed on late time optical spectroscopic observations. We find black hole masses in the range of 3 × 105 M⊙ ≤ M
BH ≤ 2 × 107 M⊙. The TDE host galaxy sample is dominated by low-mass black holes (∼ 106 M⊙), as expected from theoretical predictions. The blackbody peak luminosity of TDEs with M
BH ≤ 107.1 M⊙ is consistent with the Eddington limit of the supermassive black hole (SMBH), whereas the two TDEs with M
BH ≥ 107.1 M⊙ have peak luminosities below their SMBH Eddington luminosity, in line with the theoretical expectation that the fallback rate for M
BH ≥ 107.1 M⊙ is sub-Eddington. In addition, our observations suggest that TDEs around lower mass black holes evolve faster. These findings corroborate the standard TDE picture in 106 M⊙ black holes. Our results imply an increased tension between observational and theoretical TDE rates. By comparing the blackbody emission radius with theoretical predictions, we conclude that the optical/UV emission is produced in a region consistent with the stream self-intersection radius of shallow encounters, ruling out a compact accretion disc as the direct origin of the blackbody radiation at peak brightness.
Searching for the next Galactic Luminous red nova Addison, Harry; Blagorodnova, Nadejda; Groot, Paul J ...
Monthly Notices of the Royal Astronomical Society,
10/2022, Letnik:
517, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Luminous red novae (LRNe) are astrophysical transients believed to be caused by the partial ejection of a binary star’s common envelope (CE) and the merger of its components. The formation ...of the CE is likely to occur during unstable mass transfer, initiated by a primary star which is evolving off the main sequence (a Hertzsprung gap star) and a lower mass companion. In agreement with observations, theoretical studies have shown that outflows from the pre-CE phase produce a detectable brightening of the progenitor system a few years before the ejection event. Based on these assumptions, we present a method to identify Galactic LRNe precursors, the resulting precursor candidates, and our follow-up analysis to uncover their nature. We begin by constructing a sample of progenitor systems, i.e. Hertzsprung gap stars, by statistically modelling the density of a colour magnitude diagram formed from ‘well behaved’ Gaia DR2 sources. Their time-domain evolution from the Zwicky Transient Facility (ZTF) survey is used to search for slowly brightening events, as pre-CE precursor candidates. The nature of the resulting candidates is further investigated using archival data and our own spectroscopic follow-up. Overall, we constructed a sample of ∼5.4 × 104 progenitor sources, from which 21 were identified as candidate LRNe precursors. Further analysis revealed 16 of our candidates to be Hα emitters, with their spectra often suggesting hotter (albeit moderately extincted) A-type or B-type stars. Because of their long-term variability in optical and mid-infrared wavelengths, we propose that many of our candidates are mass-transferring binaries with compact companions surrounded by dusty circumstellar discs or alternatively magnetically active stellar merger remnants.
The Zwicky Transient Facility (ZTF) is a new robotic time-domain survey currently in progress using the Palomar 48-inch Schmidt Telescope. ZTF uses a 47 square degree field with a 600 megapixel ...camera to scan the entire northern visible sky at rates of ∼3760 square degrees/hour to median depths of g ∼ 20.8 and r ∼ 20.6 mag (AB, 5 in 30 sec). We describe the Science Data System that is housed at IPAC, Caltech. This comprises the data-processing pipelines, alert production system, data archive, and user interfaces for accessing and analyzing the products. The real-time pipeline employs a novel image-differencing algorithm, optimized for the detection of point-source transient events. These events are vetted for reliability using a machine-learned classifier and combined with contextual information to generate data-rich alert packets. The packets become available for distribution typically within 13 minutes (95th percentile) of observation. Detected events are also linked to generate candidate moving-object tracks using a novel algorithm. Objects that move fast enough to streak in the individual exposures are also extracted and vetted. We present some preliminary results of the calibration performance delivered by the real-time pipeline. The reconstructed astrometric accuracy per science image with respect to Gaia DR1 is typically 45 to 85 milliarcsec. This is the RMS per-axis on the sky for sources extracted with photometric S/N ≥ 10 and hence corresponds to the typical astrometric uncertainty down to this limit. The derived photometric precision (repeatability) at bright unsaturated fluxes varies between 8 and 25 millimag. The high end of these ranges corresponds to an airmass approaching ∼2-the limit of the public survey. Photometric calibration accuracy with respect to Pan-STARRS1 is generally better than 2%. The products support a broad range of scientific applications: fast and young supernovae; rare flux transients; variable stars; eclipsing binaries; variability from active galactic nuclei; counterparts to gravitational wave sources; a more complete census of Type Ia supernovae; and solar-system objects.
While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have ...hindered progress toward this promise. Here we present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light-curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, lower optical luminosities, and higher disruption rates compared to the rest of the sample. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities, and the lowest rates. A high detection rate of Bowen lines in TDEs with small photometric radii could be explained by the high density that is required for this fluorescence mechanism. The stellar debris can provide a source for this dense material. Diffusion of photons through this debris may explain why the rise and fade timescale of the TDEs in our sample are not correlated. We also report, for the first time, the detection of soft X-ray flares from a TDE on ∼day timescales. Based on the fact that the X-ray flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow.
Luminous red novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system’s common envelope shortly before its merger. Here we present the results of our ...photometric and spectroscopic follow-up campaign of AT 2018bwo (DLT 18x), a LRN discovered in NGC 45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of
M
r
= −10.97 ± 0.11 and maintained this brightness during its optical plateau of
t
p
= 41 ± 5 days. During this phase, it showed a rather stable photospheric temperature of ∼3300 K and a luminosity of ∼10
40
erg s
−1
. Although the luminosity and duration of AT 2018bwo is comparable to the LRNe V838 Mon and M31-2015LRN, its photosphere at early times appears larger and cooler, likely due to an extended mass-loss episode before the merger. Toward the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The IR spectrum at +103 days after discovery was comparable to that of a M8.5 II type star, analogous to an extended AGB star. The reprocessed emission by the cooling dust was also detected in the mid-infrared bands ∼1.5 years after the outburst. Archival
Spitzer
and
Hubble
Space Telescope data taken 10−14 yrs before the transient event suggest a progenitor star with
T
prog
∼ 6500 K,
R
prog
∼ 100
R
⊙
, and
L
prog
= 2 × 10
4
L
⊙
, and an upper limit for optically thin warm (1000 K) dust mass of
M
d
< 10
−6
M
⊙
. Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT 2018bwo, we infer a primary mass of 12–16
M
⊙
, which is 9–45% larger than the ∼11
M
⊙
obtained using single-star evolution models. The system, consistent with a yellow-supergiant primary, was likely in a stable mass-transfer regime with −2.4 ≤ log(
Ṁ
/
M
⊙
yr
−1
) ≤ −1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15–0.5
M
⊙
with a velocity of ∼500 km s
−1
.
A New Class of Changing-look LINERs Frederick, Sara; Gezari, Suvi; Graham, Matthew J. ...
Astrophysical journal/The Astrophysical journal,
09/2019, Letnik:
883, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We report the discovery of six active galactic nuclei (AGNs) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as ...low-ionization nuclear emission-line region galaxies (LINERs) by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGNs, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up Hubble Space Telescope ultraviolet and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong Fe vii, x, xiv and He ii λ 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ∼60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare, implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.
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