Orphan Afterglows Research Articles

Exploring the Optical Transient Sky with the Palomar Transient Factory

ABSTRACTThe Palomar Transient Factory (PTF) is a wide-field experiment designed to investigate the optical transient and variable sky on time scales from minutes to years. PTF uses the CFH12k mosaic camera, with a field of view of 7.9 deg2 and a plate scale of 1″ pixel-1, mounted on the Palomar Observatory 48 inch Samuel Oschin Telescope. The PTF operation strategy is devised to probe the existing gaps in the transient phase space and to search for theoretically predicted, but not yet detected, phenomena, such as fallback supernovae, macronovae,. Ia supernovae, and the orphan afterglows of gamma-ray bursts. PTF will also discover many new members of known source classes, from cataclysmic variables in their various avatars to supernovae and active galactic nuclei, and will provide important insights into understanding galactic dynamics (through RR Lyrae stars) and the solar system (asteroids and near-Earth objects). The lessons that can be learned from PTF will be essential for the preparation of future large synoptic sky surveys like the Large Synoptic Survey Telescope. In this article we present the scientific motivation for PTF and describe in detail the goals and expectations for this experiment.

Constraining the rate of GRB visible afterglows with the CFHTLS very wide survey

Aims.We analyze images of the CFHTLS Very Wide Survey to search for visible orphan afterglows from gamma-ray bursts (GRBs).

Estimation of the detectability of optical orphan afterglows

Aims. By neglecting sideways expansion of gamma-ray burst (GRB) jets and assuming their half-opening angle distribution, we estimate the detectability of orphan optical afterglows. Methods. This estimation is carried out by calculating the durations of off-axis optical afterglows whose flux density exceeds a certain observational limit. Results. We show that the former assumption leads to more detectable orphans, while the latter suppresses the detectability strongly compared with the model with half-opening angle θ j = 0.1. We also considered the effects of other parameters, and find that the effects of the ejecta energy E j and post-jet-break temporal index -α 2 are important but that the effects of the electron-energy distribution index p, electron energy equipartition factor ∈ e , and environment density n are insignificant. If E j and α 2 are determined by other methods, one can constrain the half-opening angle distribution of jets by observing orphan afterglows. Adopting a set of standard parameters, the detectable rate of orphan afterglows is about 1.3 x 10 -2 deg -2 yr -1 , it the observed limiting magnitude is 20 in R-band.

Constraining the GRB collimation with a survey for orphan afterglows

GRB afterglows without detected prompt‐emission counterparts are powerful tools to estimate the collimation of GRB jets. Here we report on the analysis of a dedicated survey for the search of these orphan afterglows. The survey was obtained the Wide Field Imager at the 2.2 m MPI/ESO telescope at La Silla, Chile and includes monitoring of ∼12 deg2 with a limiting magnitude of R=23 in up to 25 nights typically spaced by one to two nights. Four new optical transients were discovered, three of these associated with a flare star, a cataclysmic variable and a dwarf nova. The fourth source shows indications for an extra‐galactic origin. We discuss the results in the context of the GRB collimation.

Early afterglows in wind environments revisited

When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyse the reverse-forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g. an optical flash may emerge. The reverse shock behaves differently in two approximations: the relativistic and Newtonian cases, which depend on the parameters, e.g. the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than 114E 1/4 53 � −1/4 0,12 A −1/4 ∗,−1 , the early reverse shock is Newtonian. This may take place for the wider of a two-component jet, an orphan afterglow caused by a low initial Lorentz factor and so on. The synchrotron self-absorption effect is significant especially for the Newtonian reverse shock case, as the absorption frequency ν a is larger than the cooling frequency ν c and the minimum synchrotron frequency ν m for typical parameters. For the optical to X-ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities and compare them with those in the interstellar medium model. Ke yw ords: shock waves ‐ stars: winds, outflows ‐ gamma-rays: bursts.

Beaming Effects in GRBs and Orphan Afterglows

The overall dynamical evolution and radiation mechanism of $\gamma$-ray burst jets are briefly introduced. Various interesting topics concerning beaming in $\gamma$-ray bursts are discussed, including jet structures, orphan afterglows and cylindrical jets. The possible connection between $\gamma$-ray bursts and neutron star kicks is also addressed.

Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients

Orphan afterglows are transient events that are produced by cosmological fireballs and resemble gamma ray burst (GRB) afterglows, yet are not accompanied by gamma rays. Such transients may be produced by jetlike GRBs observed off-axis, and therefore hold great promise as a test of gamma ray burst collimation. However, orphans may also be produced by fireballs, i.e., cosmological fireballs whose ejecta carry too many baryons to produce a GRB. A well designed orphan afterglow search can distinguish between on-axis dirty fireballs and off-axis orphans in at least two ways. First, by combining real-time triggers from a wide area, multicolor search with deeper followup observations, the light curve can be tracked for a time exceeding 2*t_1, where t_1 is the age of the event at first observation. Such a light curve allows simultaneous fits to t_1 and the time decay slope alpha with sufficient accuracy to distinguish on- and off-axis orphans. Second, radio followup of orphan afterglows will show whether the radio flux is falling in time (as expected for an off-axis orphan) or not (as expected for on-axis events). Additional tests involving multi-band monitoring of the cooling, self-absorption, and f_nu peak frequencies are also possible, although much more observationally demanding. A further complication in orphan searches is that dirty fireballs are likely to also be collimated, and that collimated dirty fireballs viewed off-axis will individually be practically indistinguishable from off-axis GRB afterglows. To recognize their presence, orphan afterglow surveys must be sufficiently extensive to catch at least some dirty fireballs on-axis.

Beaming Effects in Gamma-Ray Burst Afterglows

Based on a refined dynamical model, afterglows from jetted γ-ray burst (GRB) remnants are investigated numerically. Measuring of GRB beaming by using orphan afterglow surveys is addressed. The possible existence of a kind of cylindrical jets is also discussed.

The Detectability of Orphan Afterglows

The realization that gamma-ray bursts (GRBs) release a constant amount of energy implies that post-jet-break afterglow evolution is largely universal. For a given redshift, all afterglows should be detected up to a fixed observer angle. We estimate the observed magnitude and the implied detectability of orphan afterglows. We show that for reasonable limiting magnitudes (mlim = 25), orphan afterglows will typically be detected from small (~10°) angles away from the GRB jet axis. A detected orphan afterglow generally corresponds to a "near miss" of a GRB whose jet is pointing just slightly away from us. With our most optimistic parameters, we expect that 15 orphan afterglows will be recorded in the Sloan Digital Sky Survey, and 35 transients will be recorded in a dedicated 2 m class telescope operating full time for a year in an orphan afterglow search. The rate is smaller by a factor of 15 for our "canonical" parameters. We show that for a given facility, an optimal survey should be shallower, covering a larger area, rather than deeper. The limiting magnitude should not be, however, lower than ~23, as in this case, more transients from on-axis GRBs will be discovered than orphan afterglows. About 15% of the transients could be discovered with a second exposure of the same area provided that it follows after 3, 4, and 8 days for mlim = 23, 25, and 27, respectively.

On-axis orphan afterglows

In many GRB inner engine models the highly relativistic GRB jets are engulfed by slower moving matter. This could result in different beaming for the prompt γ-ray emission and for the lower energy afterglow. In this case we should expect that some observer will see on-axis orphan afterglows: X-ray, optical and radio afterglows within the initial relativistic ejecta with no preceding GRB; the prompt γ-ray emission is pointing elsewhere. We show that the observations of the WFC on BeppoSAX constrain with high certainty the prompt X-ray beaming factor to be less than twice the prompt γ-ray beaming. The results of Ariel 5 are consistent with this interpretation. The RASS from ROSAT and HEAO-1 constrain the X-ray beaming factor at 400 and 20 min after the burst, respectively, to be comparable and certainly not much larger than the γ-ray beaming factor. There is no direct limit on the optical beaming. However, we show that observations of several months with existing hardware could result in a useful limit on the optical beaming factor of GRB afterglows.

Orphan Afterglows of Collimated Gamma‐Ray Bursts: Rate Predictions and Prospects for Detection

We make a quantitative prediction for the detection rate of orphan gamma-ray burst (GRB) afterglows as a function of flux sensitivity in X-ray, optical, and radio wave bands, based on a recent model of collimated GRB afterglows. We find that the orphan afterglow rate strongly depends on the opening angle of the jet (roughly ?), as expected from simple geometrical consideration, if the total jet energy is kept constant as suggested by recent studies. The relative beaming factor brel, i.e., the ratio of all afterglow rates including orphans to those associated with observable prompt GRBs, could be as high as brel 100 for searches deeper than R ~ 24, depending on afterglow parameters. To make the most plausible predictions, we average the model emission for 10 sets of afterglow parameters obtained through fits to 10 well-observed, collimated GRB jets, weighted by the sky coverage of each jet. Our model expectations are consistent with the results (or constraints) obtained by all past searches. We estimate the number of orphan afterglows in the first 1500 deg2 field of the Sloan Digital Sky Survey (SDSS) to be about 0.2. The relative beaming factor brel is rapidly increasing with the search sensitivity: brel ~ 3 for the SDSS sensitivity to transient objects in the northern sky (R ~ 19), ~14 for the past high-z supernova searches (R ~ 23), and ~50 for the sensitivity of the Subaru Suprime-Cam (R ~ 26). Predictions are made for the current facilities and future projects in X-ray, optical, and radio bands. Among them, the southern sky observation of the SDSS (sensitive to transients down to R ~ 23) could detect ~40 orphan afterglows during the 5 yr operation. The Allen Telescope Array would find about 200 afterglows in a radio band at ~0.1-1 mJy with brel ~ 15.

An Untriggered Search for Optical Bursts

We present an untriggered search for optical bursts with the ROTSE-I telephoto array. Observations were taken which monitor an effective 256 square degree field continuously over 125 hours to m_{ROTSE}=15.7. The uniquely large field, moderate limiting magnitude and fast cadence of $\sim$10 minutes permits transient searches in a new region of sensitivity. Our search reveals no candidate events. To quantify this result, we simulate potential optical bursts with peak magnitude, m_{p}, at t=10 s, which fade as f=(\frac{t}{t_{0}}) ^{\alpha_{t}}, where \alpha_t < 0. Simple estimates based on observational evidence indicate that a search of this sensitivity begins to probe the possible region occupied by GRB orphan afterglows. Our observing protocol and image sensitivity result in a broad region of high detection efficiency for light curves to the bright and slowly varying side of a boundary running from [\alpha_{t},m_{p}]=[-2.0,6.0] to [-0.3,13.2]. Within this region, the integrated rate of brief optical bursts is less than 1.1\times 10^{-8} {\rm s}^{-1} {\rm deg}^{-2}. At $\sim$22 times the observed GRB rate from BATSE, this suggests a limit on \frac{\theta_{opt}}{\theta_{\gamma}}\lesssim 5 where \theta_{opt} and \theta_{\gamma} are the optical and gamma-ray collimation angles, respectively. Several effects might explain the absence of optical bursts, and a search of the kind described here but more sensitive by about 4 magnitudes should offer a more definitive probe.

SDSS J124602.54+011318.8: A Highly Variable Active Galactic Nucleus, Not an Orphan Gamma‐Ray Burst Afterglow

ABSTRACTThe optically variable source SDSS J124602.54+011318.8 first appears in Sloan Digital Sky Survey (SDSS) data as a bright point source with nonstellar colors. Subsequent SDSS imaging and spectroscopy showed that the point source declined or disappeared, revealing an underlying host galaxy at redshift 0.385. Based on these properties, the source was suggested to be a candidate “orphan afterglow”: a moderately beamed optical transient associated with a gamma‐ray burst (GRB) whose highly beamed radiation cone does not include our line of sight. We present new imaging and spectroscopic observations of this source. When combined with a careful reanalysis of archival optical and radio data, the observations prove that SDSS J124602.54+011318.8 is in fact an unusual radio‐loud active galactic nucleus (AGN), probably in the BL Lac class. The object displays strong photometric variability on timescales of weeks to years, including several bright flares similar to the one reported initially. The SDSS observations are therefore almost certainly not related to a GRB. The optical spectrum of this object changes dramatically in correlation with its optical brightness. At the bright phase, weak, narrow oxygen emission lines and probably a broader Hα line are superposed on a blue continuum. As the flux decreases, the spectrum becomes dominated by the host galaxy light, with emerging stellar absorption lines, while both the narrow and broad emission lines have larger equivalent widths. We discuss briefly the implications of this discovery on the study of AGNs and other optically variable or transient phenomena.

Off-Axis Afterglow Emission from Jetted Gamma-Ray Bursts

We calculate gamma-ray burst (GRB) afterglow light curves from a relativistic jet as seen by observers at various viewing angles, θobs, relative to the jet axis. We describe three increasingly more realistic models and compare the resulting light curves. An observer at θobs < θ0, where θ0 is the initial jet opening angle, should see a light curve very similar to that for an on-axis observer. An observer at θobs > θ0 sees a rising light curve at early times, peaking when the jet Lorentz factor is ~1/θobs, and approaching that seen by an on-axis observer, at later times. A strong linear polarization (≲40%) may occur near the peak in the light curve and slowly decay with time. We show that, if GRB jets have a universal energy, then orphan afterglows are detectable up to a maximum offset angle that is independent of the jet initial aperture and thus at a rate proportional to the true GRB rate. We also discuss the implications of the proposed connection between SN 1998bw and GRB 980425.

Failed gamma-ray bursts and orphan afterglows

It is believed that orphan afterglow searches can help to measure the beaming angle in gamma-ray bursts (GRBs). Great expectations have been put on this method. We point out that the method is in fact not as simple as we originally expected. As a result of the baryon-rich environment that is common to almost all popular progenitor models, there should be many failed gamma-ray bursts, i.e. fireballs with Lorentz factor much less than 100-1000, but still much larger than unity. In fact, the number of failed gamma-ray bursts may even be much larger than that of successful bursts. Owing to the existence of these failed gamma-ray bursts, there should be many orphan afterglows even if GRBs are due to isotropic fireballs, then the simple discovery of orphan afterglows never means that GRBs are collimated. Unfortunately, to distinguish between a failed-GRB orphan and a jetted but off-axis GRB orphan is not an easy task. The major problem is that the trigger time is unknown. Some possible solutions to the problem are suggested.

The Difficulty in Using Orphan Afterglows to Measure Gamma‐Ray Burst Beaming

If gamma-ray burst (GRB) emission is strongly collimated, then GRBs occur throughout the universe at a rate much higher than is detected. Since the emission from the optical afterglow is thought to be more isotropic than the gamma-ray emission, it has been hypothesized that a search for orphan afterglows (those without the triggering GRB) would allow strong constraints to be placed on the degree of GRB collimation. In this paper we establish that, within the context of leading models of GRB jet evolution, measurement of the GRB beaming angle θjet using optical orphan searches is extremely difficult, perhaps impossible in practice. We show that this is because, in the leading model of GRB jets, the effective afterglow beaming angle scales with the jet angle, Ωopt ∝ Ωjet for small angles, and so the ratio of detected orphan afterglows to GRBs is independent of the jet opening angle. Thus, the number of expected afterglow detections is the same for moderate jet angles (~20°) as for arbitrarily small jet angles (≪01). For nearly isotropic GRB geometry, or for radio afterglow searches in which the jet has become nonrelativistic, the ratio of afterglows to GRBs may give information on collimation. However, using a simple model we estimate the expected number of orphan detections in current supernova surveys and find this number to be less than 1 for all jet opening angles. Even for future supernova surveys, the small detection rate and lack of dependence on collimation angle appear to ruin the prospects of determining GRB beaming by this method. Radio searches may provide the best hope to find the missing orphans.

Snapshot Identification of Gamma‐Ray Burst Optical Afterglows

Gamma ray burst afterglows can be identified in single epoch observations using three or more optical filters. This method relies on color measurements to distinguish the power law spectrum of an afterglow from the curved spectra of stars. Observations in a fourth filter will further distinguish between afterglows and most galaxies up to redshifts z ~ 1. Many afterglows can also be identified with fewer filters using ultraviolet excess, infrared excess, or Lyman break techniques. By allowing faster identification of gamma ray burst afterglows, these color methods will increase the fraction of bursts for which optical spectroscopy and other narrow-field observations can be obtained. Because quasar colors can match those of afterglows, the maximum error box size where an unambiguous identification can be expected is set by the flux limit of the afterglow search and the quasar number-flux relation. For currently typical error boxes (10 -- 100 square arcminutes), little contamination is expected at magnitudes R < 21.5 +- 0.5. Archival data demonstrates that the afterglow of GRB 000301C could have been identified using this method. In addition to finding gamma ray burst counterparts, this method will have applications in ``orphan afterglow'' searches used to constrain gamma ray burst collimation.