273 research outputs found
PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy
[Abridged] We present observations of PS16dtm, a luminous transient that
occurred at the nucleus of a known Narrow-line Seyfert 1 galaxy hosting a
10 M black hole. The transient was previously claimed to be a Type
IIn SLSN due to its luminosity and hydrogen emission lines. The light curve
shows that PS16dtm brightened by about two magnitudes in ~50 days relative to
the archival host brightness and then exhibited a plateau phase for about 100
days followed by the onset of fading in the UV. During the plateau PS16dtm
showed no color evolution, maintained a blackbody temperature of 1.7 x 10
K, and radiated at approximately of the SMBH. The spectra exhibit
multi-component hydrogen emission lines and strong FeII emission, show little
evolution with time, and closely resemble the spectra of NLS1s while being
distinct from those of Type IIn SNe. Moreover, PS16dtm is undetected in the
X-rays to a limit an order of magnitude below an archival X-ray detection of
its host galaxy. These observations strongly link PS16dtm to activity
associated with the SMBH and are difficult to reconcile with a SN origin or any
known form of AGN variability, and therefore we argue that it is a TDE in which
the accretion of the stellar debris powers the rise in the continuum and
excitation of the pre-existing broad line region, while providing material that
obscures the X-ray emitting region of the pre-existing AGN accretion disk. A
detailed TDE model fit to the light curve indicates that PS16dtm will remain
bright for several years; we further predict that the X-ray emission will
reappear on a similar timescale as the accretion rate declines. Finally, we
place PS16dtm in the context of other TDEs and find that TDEs in AGN galaxies
are an order of magnitude more efficient and reach Eddington luminosities,
likely due to interaction of the stellar debris with the pre-existing accretion
disk.Comment: 19 pages, 17 figures, Submitted to Ap
The fine line between total and partial tidal disruption events
Flares from tidal disruption events are unique tracers of quiescent black
holes at the centre of galaxies. The appearance of these flares is very
sensitive to whether the star is totally or partially disrupted, and in this
paper we seek to identify the critical distance of the star from the black hole
(r_d) that enables us to distinguish between these two outcomes. We perform
here Mesh-free Finite Mass, traditional, and modern Smoothed Particle
Hydrodynamical simulations of star-black hole close encounters, with the aim of
checking if the value of r_d depends on the simulation technique. We find that
the critical distance (or the so-called critical disruption parameter beta_d)
depends only weakly on the adopted simulation method, being beta_d=0.92\pm 0.02
for a gamma=5/3 polytrope and beta_d=2.01\pm 0.01 for a gamma=4/3 polytrope.Comment: 8 pages, 8 figures, 2 tables; accepted to A&
Optical/UV-to-X-Ray Echoes from the Tidal Disruption Flare ASASSN-14li
We carried out the first multi-wavelength (optical/UV and X-ray) photometric
reverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We find
that its X-ray variations are correlated with and lag the optical/UV
fluctuations by 324 days. Based on the direction and the magnitude of the
X-ray time lag, we rule out X-ray reprocessing and direct emission from a
standard circular thin disk as the dominant source of its optical/UV emission.
The lag magnitude also rules out an AGN disk-driven instability as the origin
of ASASSN-14li and thus strongly supports the tidal disruption picture for this
event and similar objects. We suggest that the majority of the optical/UV
emission likely originates from debris stream self-interactions. Perturbations
at the self-interaction sites produce optical/UV variability and travel down to
the black hole where they modulate the X-rays. The time lag between the
optical/UV and the X-rays variations thus correspond to the time taken by these
fluctuations to travel from the self-interaction site to close to the black
hole. We further discuss these time lags within the context of the three
variants of the self-interaction model. High-cadence monitoring observations of
future TDFs will be sensitive enough to detect these echoes and would allow us
to establish the origin of optical/UV emission in TDFs in general.Comment: Publish in ApJ Letter
iPTF15eqv: Multi-wavelength Expos\'e of a Peculiar Calcium-rich Transient
The progenitor systems of the class of "Ca-rich transients" is a key open
issue in time domain astrophysics. These intriguing objects exhibit unusually
strong calcium line emissions months after explosion, fall within an
intermediate luminosity range, are often found at large projected distances
from their host galaxies, and may play a vital role in enriching galaxies and
the intergalactic medium. Here we present multi-wavelength observations of
iPTF15eqv in NGC 3430, which exhibits a unique combination of properties that
bridge those observed in Ca-rich transients and Type Ib/c supernovae. iPTF15eqv
has among the highest [Ca II]/[O I] emission line ratios observed to date, yet
is more luminous and decays more slowly than other Ca-rich transients. Optical
and near-infrared photometry and spectroscopy reveal signatures consistent with
the supernova explosion of a < 10 solar mass star that was stripped of its
H-rich envelope via binary interaction. Distinct chemical abundances and ejecta
kinematics suggest that the core collapse occurred through electron capture
processes. Deep limits on possible radio emission made with the Jansky Very
Large Array imply a clean environment ( 0.1 cm) within a radius of
cm. Chandra X-ray Observatory observations rule out alternative
scenarios involving tidal disruption of a white dwarf by a black hole, for
masses > 100 solar masses). Our results challenge the notion that
spectroscopically classified Ca-rich transients only originate from white dwarf
progenitor systems, complicate the view that they are all associated with large
ejection velocities, and indicate that their chemical abundances may vary
widely between events.Comment: 24 pages, 16 figures. Closely matches version published in The
Astrophysical Journa
Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817
We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the
binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using
the final observation as a template, we uncover a source at the position of
GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and
23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from
the image subtraction); no obvious source is detected at 3.6 micron to a
3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer
by a factor of about 2-3 times compared to our previously published kilonova
model, which is based on UV, optical, and near-IR data at <30 days. However,
the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent
with our model. We suggest that the discrepancy is likely due to a transition
to the nebular phase, or a reduced thermalization efficiency at such late time.
Using the Spitzer data as a guide, we briefly discuss the prospects of
observing future binary neutron star mergers with Spitzer (in LIGO/Virgo
Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing
Run 4 and beyond).Comment: 6 pages, 2 figures, submitted to ApJ
Planetary Phase Variations of the 55 Cancri System
Characterization of the composition, surface properties, and atmospheric
conditions of exoplanets is a rapidly progressing field as the data to study
such aspects become more accessible. Bright targets, such as the multi-planet
55 Cancri system, allow an opportunity to achieve high signal-to-noise for the
detection of photometric phase variations to constrain the planetary albedos.
The recent discovery that that inner-most planet, 55 Cancri e, transits the
host star introduces new prospects for studying this system. Here we calculate
photometric phase curves at optical wavelengths for the system with varying
assumptions for the surface and atmospheric properties of 55 Cancri e. We show
that the large differences in geometric albedo allows one to distinguish
between various surface models, that the scattering phase function cannot be
constrained with foreseeable data, and that planet b will contribute
significantly to the phase variation depending upon the surface of planet e. We
discuss detection limits and how these models may be used with future
instrumentation to further characterize these planets and distinguish between
various assumptions regarding surface conditions.Comment: 7 pages, 3 figures, accepted for publication in Ap
A Luminous, Fast Rising UV-Transient Discovered by ROTSE: a Tidal Disruption Event?
We present follow-up observations of an optical transient (OT) discovered by
ROTSE on Jan. 21, 2009. Photometric monitoring was carried out with ROTSE-IIIb
in the optical and Swift in the UV up to +70 days after discovery. The light
curve showed a fast rise time of ~10 days followed by a steep decline over the
next 60 days, which was much faster than that implied by 56Ni - 56Co
radioactive decay. The SDSS DR10 database contains a faint, red object at the
position of the OT, which appears slightly extended. This and other lines of
evidence suggest that the OT is of extragalactic origin, and this faint object
is likely the host galaxy. A sequence of optical spectra obtained with the
9.2-m Hobby-Eberly Telescope (HET) between +8 and +45 days after discovery
revealed a hot, blue continuum with no visible spectral features. A few weak
features that appeared after +30 days probably originated from the underlying
host. Fitting synthetic templates to the observed spectrum of the host galaxy
revealed a redshift of z = 0.19. At this redshift the peak magnitude of the OT
is close to -22.5, similar to the brightest super-luminous supernovae; however,
the lack of identifiable spectral features makes the massive stellar death
hypothesis less likely. A more plausible explanation appears to be the tidal
disruption of a sun-like star by the central super-massive black hole. We argue
that this transient likely belongs to a class of super-Eddington tidal
disruption events.Comment: 13 pages, accepted for publication in ApJ; some references adde
The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars and Underluminous Supernovae
We study a complete, colour-selected sample of double-degenerate binary
systems containing extremely low mass (ELM) <0.25 Msol white dwarfs (WDs). We
show, for the first time, that Milky Way disk ELM WDs have a merger rate of
approximately 4 x 10^(-5)/yr due to gravitational wave radiation. The merger
end-product depends on the mass ratio of the binary. The ELM WD systems that
undergo stable mass transfer can account for >3% of AM CVn stars. More
importantly, the ELM WD systems that may detonate merge at a rate comparable to
the estimated rate of underluminous SNe, rare explosions estimated to produce
only ~0.2 Msol worth of ejecta. At least 25% of our ELM WD sample belong to the
old thick disk and halo components of the Milky Way. Thus, if merging ELM WD
systems are the progenitors of underluminous SNe, transient surveys must find
them in both elliptical and spiral galaxies.Comment: MNRAS Letters, in pres
The Discovery of Binary White Dwarfs that will Merge within 500 Myr
We present radial velocity observations of four extremely low-mass (0.2 Msol)
white dwarfs. All four stars show peak-to-peak radial velocity variations of
540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out
main-sequence companions. In addition, no milli-second pulsar companions are
detected in radio observations. Thus the invisible companions are most likely
white dwarfs. Two of the systems are the shortest period binary white dwarfs
yet discovered. Due to the loss of angular momentum through gravitational
radiation, three of the systems will merge within 500 Myr. The remaining system
will merge within a Hubble time. The mass functions for three of the systems
imply companions more massive than 0.46 Msol; thus those are carbon/oxygen core
white dwarfs. The unknown inclination angles prohibit a definitive conclusion
about the future of these systems. However, the chance of a supernova Ia event
is only 1% to 5%. These systems are likely to form single R Coronae Borealis
stars, providing evidence for a white dwarf + white dwarf merger mechanism for
these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70%
chance of having a low-mass white dwarf companion. This system will probably
form a single helium-enriched subdwarf O star. All four white dwarf systems
have unusal mass ratios of < 0.2-0.8 that may also lead to the formation of AM
CVn systems.Comment: ApJ, accepted versio
Stellar GADGET: A smooth particle hydrodynamics code for stellar astrophysics and its application to Type Ia supernovae from white dwarf mergers
Mergers of two carbon-oxygen white dwarfs have long been suspected to be
progenitors of Type Ia Supernovae. Here we present our modifications to the
cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar
physics including but not limited to mergers of white dwarfs. We demonstrate a
new method to map a one-dimensional profile of an object in hydrostatic
equilibrium to a stable particle distribution. We use the code to study the
effect of initial conditions and resolution on the properties of the merger of
two white dwarfs. We compare mergers with approximate and exact binary initial
conditions and find that exact binary initial conditions lead to a much more
stable binary system but there is no difference in the properties of the actual
merger. In contrast, we find that resolution is a critical issue for
simulations of white dwarf mergers. Carbon burning hotspots which may lead to a
detonation in the so-called violent merger scenario emerge only in simulations
with sufficient resolution but independent of the type of binary initial
conditions. We conclude that simulations of white dwarf mergers which attempt
to investigate their potential for Type Ia supernovae should be carried out
with at least 10^6 particles.Comment: 11 pages, 6 figures, accepted for publication in MNRA
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