553 research outputs found
The Afterglow of GRB 130427A from 1 to 10^(16) GHz
We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst (GRB) of the past 29 yr. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 days after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant late-time flaring or rebrightening activity. The entire data set from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments
PTF 11kx: A Type Ia Supernova with a Symbiotic Nova Progenitor
There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi
A Case Study of On-the-Fly Wide-Field Radio Imaging Applied to the Gravitational-wave Event GW 151226
We apply a newly-developed On-the-Fly mosaicing technique on the NSF's Karl
G. Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive
search for an afterglow from the Advanced LIGO binary black hole merger event
GW 151226. In three epochs between 1.5 and 6 months post-merger we observed a
100 sq. deg region, with more than 80% of the survey region having a RMS
sensitivity of better than 150 uJy/beam, in the northern hemisphere having a
merger containment probability of 10%. The data were processed in
near-real-time, and analyzed to search for transients and variables. No
transients were found but we have demonstrated the ability to conduct blind
searches in a time-frequency phase space where the predicted afterglow signals
are strongest. If the gravitational wave event is contained within our survey
region, the upper limit on any late-time radio afterglow from the merger event
at an assumed mean distance of 440 Mpc is about 1e29 erg/s/Hz. Approximately
1.5% of the radio sources in the field showed variability at a level of 30%,
and can be attributed to normal activity from active galactic nuclei. The low
rate of false positives in the radio sky suggests that wide-field imaging
searches at a few Gigahertz can be an efficient and competitive search
strategy. We discuss our search method in the context of the recent afterglow
detection from GW 170817 and radio follow-up in future gravitational wave
observing runs.Comment: 11 pages. 6 figures. 1 table. Accepted for publication in ApJ Letter
Are Delayed Radio Flares Common in Tidal Disruption Events? The Case of the TDE iPTF 16fnl
Radio emission from tidal disruption events (TDEs) originates from an
interaction of an outflow with the super-massive black hole (SMBH) circum
nuclear material (CNM). In turn, this radio emission can be used to probe
properties of both the outflow launched at the event and the CNM. Until
recently, radio emission was detected only for a relatively small number of
events. While the observed radio emission pointed to either relativistic or
sub-relativistic outflows of different nature, it also indicated that the
outflow has been launched shortly after the stellar disruption. Recently,
however, delayed radio flares, several months and years after stellar
disruption, were reported in the case of the TDE ASASSN-15oi. These delayed
flares suggest a delay in the launching of outflows and thus may provide new
insights into SMBH accretion physics. Here, we present a new radio dataset of
another TDE, iPTF16fnl, and discuss the possibility that a delayed radio flare
has been observed also in this case, ~ 5 months after optical discovery,
suggesting that this phenomenon may be common in TDEs. Unlike ASASSN-15oi, the
data for iPTF16fnl is sparse and the delayed radio flare can be explained by
several alternative models: among them are a complex varying CNM density
structure and a delayed outflow ejection
Oxygen and helium in stripped-envelope supernovae
We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe IIb), 45 Type Ib SNe (SNe Ib), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe (SNe Ib/c). We have compared the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the He I λλ5876, 7065 and O I λ7774 absorption lines. Consistent with previous work, we find that SNe Ic show higher pEWs and velocities in O I λ7774 compared to SNe IIb and Ib. The pEWs of the He I λλ5876, 7065 lines are similar in SNe Ib and IIb after maximum light. The He I λλ5876, 7065 velocities at maximum light are higher in SNe Ib compared to SNe IIb. We identify an anticorrelation between the He I λ7065 pEW and O I λ7774 velocity among SNe IIb and Ib. This can be interpreted as a continuum in the amount of He present at the time of explosion. It has been suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing could be responsible for hiding He in SNe Ic. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNe Ic (V_m = √2E_k/M_(ej) ≈ 15 000 km s^(−1)) and SNe Ib (V_m ≈ 9000 km s^(−1)) would imply an average He mass difference of ∼1.4 M⊙, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNe Ic do not hide He but lose He due to envelope stripping
Sensitive Search for Radio Variables and Transients in the Extended Chandra Deep Field South
We report on an analysis of the Extended Chandra Deep Field South (E-CDFS) region using archival data from the Very Large Array, with the goal of studying radio variability and transients at the sub-milliJansky level. The 49 epochs of E-CDFS observations at 1.4 GHz sample timescales from 1 day to 3 months. We find that only a fraction (1%) of unresolved radio sources above 40 μJy are variable at the 4σ level. There is no evidence that the fractional variability changes along with the known transition of radio-source populations below 1 mJy. Optical identifications of the sources show that the variable radio emission is associated with the central regions of an active galactic nucleus or a star-forming galaxy. After a detailed comparison of the efficacy of various source-finding algorithms, we use the best to carry out a transient search. No transients were found. This implies that the areal density of transients with peak flux density greater than 0.21 mJy is less than 0.37 deg^(–2) (at a confidence level of 95%). This result is approximately an order of magnitude below the transient rate measured at 5 GHz by Bower et al. but it is consistent with more recent upper limits from Frail et al. Our findings suggest that the radio sky at 1.4 GHz is relatively quiet. For multi-wavelength transient searches, such as the electromagnetic counterparts to gravitational waves, this frequency may be optimal for reducing the high background of false positives
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