42 research outputs found
Black hole masses of tidal disruption event host galaxies
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
22 mag and =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
310 MM210 M.
The TDE host galaxy sample is dominated by low mass black holes (10
M), as expected from theoretical predictions. The blackbody peak
luminosity of TDEs with M10 M is consistent
with the Eddington limit of the SMBH, whereas the two TDEs with M10 M have peak luminosities below their SMBH
Eddington luminosity, in line with the theoretical expectation that the
fallback rate for M10 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 10
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 disk as the direct origin of
the blackbody radiation at peak brightness.Comment: 16 pages, 9 figures. Submitted to MNRAS; including minor revisions
suggested by the refere
Gaia transient detection efficiency: hunting for nuclear transients
We present a study of the detectability of transient events associated with
galaxies for the Gaia European Space Agency astrometric mission. We simulated
the on-board detections, and on-ground processing for a mock galaxy catalogue
to establish the properties required for the discovery of transient events by
Gaia, specifically tidal disruption events (TDEs) and supernovae (SNe).
Transients may either be discovered by the on-board detection of a new source
or by the brightening of a previously known source. We show that Gaia
transients can be identified as new detections on-board for offsets from the
host galaxy nucleus of 0.1--0.5,arcsec, depending on magnitude and scanning
angle. The Gaia detection system shows no significant loss of SNe at close
radial distances to the nucleus. We used the detection efficiencies to predict
the number of transients events discovered by Gaia. For a limiting magnitude of
19, we expect around 1300 SNe per year: 65% SN Ia, 28% SN II and 7% SN Ibc, and
~20 TDEs per year.Comment: 17 pages, 10 figures, accepted by MNRA
The SED Machine: a robotic spectrograph for fast transient classification
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 on line. At the present time, 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 (R100) 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 proved lived up to its promise. The success of the SEDM
demonstrates the value of spectrographs optimized to spectral classification.
Introduction of similar spectrographs on existing telescopes will help
alleviate the follow-up drought and thereby accelerate the rate of discoveries.Comment: 21 pages, 20 figure
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
The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor
Luminous Red Novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system's common envelope (CE) shortly before its merger. Here we present the results of our photometric and spectroscopic follow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, 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 ± 5days. During this phase, it showed a rather stable photospheric temperature of ~3300K and a luminosity of ~10⁴⁰ erg s⁻¹. The photosphere of AT2018bwo at early times appeared larger and cooler than other similar LRNe, likely due to an extended mass-loss episode before the merger. Towards the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. 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 years before the transient event suggest a progenitor star with T_(prog) ∼ 6500K, R_(prog) ∼ 100 R_⊙ and L_(prog) ∼ 2 × 10⁴ 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 AT2018bwo, we infer a primary mass of 12-16 M_⊙, which is 9-45% larger than the ~11M⊙ 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 0.15-0.5M⊙ with a velocity of ~500 km s⁻¹
A New Class of Changing-Look LINERs
We report the discovery of six active galactic nuclei (AGN) caught "turning
on" during the first nine months of the Zwicky Transient Facility (ZTF) survey.
The host galaxies were classified as 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 AGN, reminiscent of the changing-look LINER
iPTF 16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and
ground-based optical spectra revealed the transformation into a narrow-line
Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II 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.Comment: Submitted to ApJ, 31 pages, 17 Figures (excluding Appendix due to
file size constraints but will be available in electronic version
Measuring nickel masses in Type Ia supernovae using cobalt emission in nebular phase spectra
The light curves of Type Ia supernovae (SNe Ia) are powered by the
radioactive decay of Ni to Co at early times, and the decay of
Co to Fe from ~60 days after explosion. We examine the evolution
of the [Co III] 5892 A emission complex during the nebular phase for SNe Ia
with multiple nebular spectra and show that the line flux follows the square of
the mass of Co as a function of time. This result indicates both
efficient local energy deposition from positrons produced in Co decay,
and long-term stability of the ionization state of the nebula. We compile 77
nebular spectra of 25 SN Ia from the literature and present 17 new nebular
spectra of 7 SNe Ia, including SN2014J. From these we measure the flux in the
[Co III] 5892 A line and remove its well-behaved time dependence to infer the
initial mass of Ni () produced in the explosion. We then examine
Ni yields for different SN Ia ejected masses ( - calculated
using the relation between light curve width and ejected mass) and find the
Ni masses of SNe Ia fall into two regimes: for narrow light curves (low
stretch s~0.7-0.9), is clustered near ~ 0.4 and
shows a shallow increase as increases from ~1-1.4; at high
stretch, clusters at the Chandrasekhar mass (1.4) while
spans a broad range from 0.6-1.2. This could constitute
evidence for two distinct SN Ia explosion mechanisms.Comment: 16 pages, 12 figures (main text), plus data tables in appendix.
Spectra released on WISeREP. Submitted to MNRAS, comments welcom