571 research outputs found
Evidence for High-Frequency QPOs with a 3:2 Frequency Ratio from a 5000 Solar Mass Black Hole
Following the discovery of 3:2 resonance quasi-periodic oscillations (QPOs)
in M82X-1 (Pasham et al. 2014), we have constructed power density spectra (PDS)
of all 15 (sufficiently long) {\it XMM-Newton} observations of the
ultraluminous X-ray source NGC1313X-1 ( 210
erg/sec). We detect a strong QPO at a frequency of 0.290.01 Hz in data
obtained on 2012 December 16. Subsequent searching of all the remaining
observations for a 3:2/2:3 frequency pair revealed a feature at 0.460.02
Hz on 2003 Dec 13 (frequency ratio of 1.590.09). The global significance
of the 0.29 Hz feature considering all frequencies between 0.1 and 4 Hz is
3.5 . The significance of the 0.460.02 Hz QPO is 3.5
for a search at 2/3 and 3/2 of 0.29 Hz. We also detect lower frequency QPOs
(32.92.6 and 79.71.2 mHz). All the QPOs are super-imposed on a
continuum consisting of flat-topped, band-limited noise, breaking into a
power-law at a frequency of 163 mHz and white noise at 0.1 Hz.
NGC1313X-1's PDS is analogous to stellar-mass black holes' (StMBHs) PDS in the
so-called steep power-law state, but with the respective frequencies (both QPOs
and break frequencies) scaled down by a factor of 1000. Using the
inverse mass-to-high-frequency QPO scaling of StMBHs, we estimate NGC1313X-1's
black hole mass to be 50001300 , consistent with an inference
from the scaling of the break frequency. However, the implied Eddington ratio,
L 0.030.01, is significantly lower compared to StMBHs in the
steep power-law state (L 0.2).Comment: Published in ApJ Letter
Late-Time Circumstellar Interaction in a Spitzer Selected Sample of Type IIn Supernovae
Type IIn supernovae (SNe IIn) are a rare (< 10%) subclass of core-collapse
SNe that exhibit relatively narrow emission lines from a dense, pre-existing
circumstellar medium (CSM). In 2009, a warm Spitzer survey observed 30 SNe IIn
discovered in 2003 - 2008 and detected 10 SNe at distances out to 175 Mpc with
unreported late-time infrared emission, in some cases more than 5 years
post-discovery. For this single epoch of data, the warm-dust parameters suggest
the presence of a radiative heating source consisting of optical/X-ray emission
continuously generated by ongoing CSM interaction. Here we present
multi-wavelength follow-up observations of this sample of 10 SNe IIn and the
well-studied Type IIn SN 2010jl. A recent epoch of Spitzer observations reveals
ongoing mid-infrared emission from nine of the SNe in this sample. We also
detect three of the SNe in archival WISE data, in addition to SNe 1987A,
2004dj, and 2008iy. For at least five of the SNe in the sample, optical and/or
X-ray emission confirms the presence of radiative emission from ongoing CSM
interaction. The two Spitzer nondetections are consistent with the forward
shock overrunning and destroying the dust shell, a result that places upper
limits on the dust-shell size. The optical and infrared observations confirm
the radiative heating model and constrain a number of model parameters,
including progenitor mass-loss characteristics. All of the SNe in this sample
experienced an outburst on the order of tens to hundreds of years prior to the
SN explosion followed by periods of less intense mass loss. Although all
evidence points to massive progenitors, the variation in the data highlights
the diversity in SN IIn progenitor evolution. While these observations do not
identify a particular progenitor system, they demonstrate that future,
coordinated, multi-wavelength campaigns can constrain theoretical mass-loss
models.Comment: 10 pages, 6 figures, accepted to AJ (with comments
The peculiar Type Ia supernova iPTF14atg: Chandrasekhar-mass explosion or violent merger?
iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN
2002es, is the first SN Ia for which a strong UV flash was observed in the
early-time light curves. This has been interpreted as evidence for a
single-degenerate (SD) progenitor system where such a signal is expected from
interactions between the SN ejecta and the non-degenerate companion star. Here,
we compare synthetic observables of multi-dimensional state-of-the-art
explosion models for different progenitor scenarios to the light curves and
spectra of iPTF14atg. From our models, we have difficulties explaining the
spectral evolution of iPTF14atg within the SD progenitor channel. In contrast,
we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and
0.76 solar masses, respectively, provides an excellent match to the spectral
evolution of iPTF14atg from 10d before to several weeks after maximum light.
Our merger model does not naturally explain the initial UV flash of iPTF14atg.
We discuss several possibilities like interactions of the SN ejecta with the
circum-stellar medium and surface radioactivity from a He ignited merger that
may be able to account for the early UV emission in violent merger models.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Testing the E_p,i - L_p,iso - T_0.45 correlation on a BeppoSAX and Swift sample of gamma-ray bursts
Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we
tested the correlation between the intrinsic peak energy of the time-integrated
spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the
duration of the most intense parts of the GRB computed as T_0.45 ("Firmani
correlation"). For 41 out of 88 GRBs we could estimate all of the three
required properties. Apart from 980425, which appears to be a definite outlier
and notoriously peculiar in many respects, we used 40 GRBs to fit the
correlation with the maximum likelihood method discussed by D'Agostini,
suitable to account for the extrinsic scatter in addition to the intrinsic
uncertainties affecting every single GRB. We confirm the correlation. However,
unlike the results by Firmani et al., we found that the correlation does have a
logarithmic scatter comparable with that of the E_p,i-E_iso ("Amati")
correlation. We also find that the slope of the product L_p,iso T_0.45 is equal
to ~0.5, which is consistent with the hypothesis that the E_p,i-L_p,iso-T_0.45
correlation is equivalent to the E_p,i-E_iso correlation (slope ~0.5). We
conclude that, based on presently available data, there is no clear evidence
that the E_p,i-L_p,iso-T_0.45 correlation is different (both in terms of slope
and dispersion) from the E_p,i-E_iso correlation.Comment: 9 pages, 4 figures, revised version submitted to MNRA
First search for an x-ray–optical reverberation signal in an ultraluminous x-ray source
Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to active galactic nucleus broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1's X-rays are variable on a timescale of a few hundred seconds (rms of 9.0 ± 0.5%), the optical emission does not show any statistically significant variations. We set a 3? upper limit on the rms optical variability of 3.3%. The ratio of the X-ray to the optical variability is an indicator of X-ray reprocessing efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar ratio for NGC 5408 X-1, the expected rms optical variability is ?2%, which is still a factor of roughly two lower than what was possible with the VLT observations in this study. We find marginal evidence (3?) for optical variability on a ~24 hr timescale. Our results demonstrate that such measurements can be made, but photometric conditions, low sky background levels, and longer simultaneous observations will be required to reach optical variability levels similar to those of X-ray binaries
The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution
We present multi-wavelength observations of the tidal disruption event (TDE)
iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF)
survey at redshift . The optical and ultraviolet (UV) light curves
of the transient show a slow decay over five months, in agreement with previous
optically discovered TDEs. It also has a comparable black-body peak luminosity
of erg/s. The inferred temperature
from the optical and UV data shows a value of (35) K. The
transient is not detected in X-rays up to erg/s within
the first five months after discovery. The optical spectra exhibit two distinct
broad emission lines in the He II region, and at later times also H
emission. Additionally, emission from [N III] and [O III] is detected, likely
produced by the Bowen fluorescence effect. UV spectra reveal broad emission and
absorption lines associated with high-ionization states of N V, C IV, Si IV,
and possibly P V. These features, analogous to those of broad absorption line
quasars (BAL QSOs), require an absorber with column densities cm. This optically thick gas would also explain the
non-detection in soft X-rays. The profile of the absorption lines with the
highest column density material at the largest velocity is opposite that of BAL
QSOs. We suggest that radiation pressure generated by the TDE flare at early
times could have provided the initial acceleration mechanism for this gas.
Spectral UV line monitoring of future TDEs could test this proposal.Comment: 20 pages, 12 figures, published in Ap
Sifting for Sapphires: Systematic Selection of Tidal Disruption Events in iPTF
We present results from a systematic selection of tidal disruption events
(TDEs) in a wide-area (4800~deg), band, Intermediate Palomar
Transient Factory (iPTF) experiment. Our selection targets typical
optically-selected TDEs: bright (60\% flux increase) and blue transients
residing in the center of red galaxies. Using photometric selection criteria to
down-select from a total of 493 nuclear transients to a sample of 26 sources,
we then use follow-up UV imaging with the Neil Gehrels Swift Telescope,
ground-based optical spectroscopy, and light curve fitting to classify them as
14 Type Ia supernovae (SNe Ia), 9 highly variable active galactic nuclei
(AGNs), 2 confirmed TDEs, and 1 potential core-collapse supernova. We find it
possible to filter AGNs by employing a more stringent transient color cut ( 0.2 mag); further, UV imaging is the best discriminator for filtering
SNe, since SNe Ia can appear as blue, optically, as TDEs in their early phases.
However, when UV-optical color is unavailable, higher precision astrometry can
also effectively reduce SNe contamination in the optical. Our most stringent
optical photometric selection criteria yields a 4.5:1 contamination rate,
allowing for a manageable number of TDE candidates for complete spectroscopic
follow-up and real-time classification in the ZTF era. We measure a TDE per
galaxy rate of 1.7 10 gal yr (90\%
CL in Poisson statistics). This does not account for TDEs outside our selection
criteria, thus may not reflect the total TDE population, which is yet to be
fully mapped.Comment: 24 pages, 21 figures. Accepted for publication in the Astrophysical
Journal Supplement Serie
Early Rebrightenings of X-ray Afterglows from Ring-Shaped GRB Jets
Early rebrightenings at a post-burst time of 10^2 - 10^4 s have been observed
in the afterglows of some gamma-ray bursts (GRBs). Unlike X-ray flares, these
rebrightenings usually last for a relatively long period. The continuous energy
injection mechanism usually can only produce a plateau in the afterglow light
curve, but not a rebrightening. Also, a sudden energy injection can give birth
to a rebrightening, but the rebrightening is a bit too rapid. Here we argued
that the early rebrightenings can be produced by the ring-shaped jet model. In
this scenario, the GRB outflow is not a full cone, but a centrally hollowed
ring. Assuming that the line of sight is on the central symmetry axis of the
hollow cone, we calculate the overall dynamical evolution of the outflows and
educe the multiband afterglow light curves. It is found that the early
rebrightenings observed in the afterglows of a few GRBs, such as GRBs 051016B,
060109, 070103 and 070208 etc, can be well explained in this framework. It is
suggested that these long-lasted early rebrightenings in GRB afterglows should
be resulted from ring-shaped jets.Comment: 11 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysic
Early Observations of the Type Ia Supernova iPTF 16abc: A Case of Interaction with Nearby, Unbound Material and/or Strong Ejecta Mixing
Early observations of Type Ia supernovae (SNe Ia) provide a unique probe of
their progenitor systems and explosion physics. Here we report the intermediate
Palomar Transient Factory (iPTF) discovery of an extraordinarily young SN Ia,
iPTF 16abc. By fitting a power law to our early light curve, we infer that
first light for the SN, that is when the SN could have first been detected by
our survey, occurred only days before our first
detection. In the 24 hr after discovery, iPTF 16abc rose by 2 mag,
featuring a near-linear rise in flux for 3 days. Early spectra show
strong C II absorption, which disappears after 7 days. Unlike the
extensivelyobserved SN Ia SN 2011fe, the colors of iPTF 16abc are
blue and nearly constant in the days after explosion. We show that our early
observations of iPTF 16abc cannot be explained by either SN shock breakout and
the associated, subsequent cooling or the SN ejecta colliding with a stellar
companion. Instead, we argue that the early characteristics of iPTF 16abc,
including (i) the rapid, near-linear rise, (ii) the nonevolving blue colors,
and (iii) the strong C II absorption, are the result of either ejecta
interaction with nearby, unbound material or vigorous mixing of radioactive
Ni in the SN ejecta, or a combination of the two. In the next few years,
dozens of very young \textit{normal} SNe Ia will be discovered, and
observations similar to those presented here will constrain the white dwarf
explosion mechanism.Comment: 18 pages, 12 figures, accepted by Ap
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