Polarization in the prompt emission of gamma-ray bursts and their afterglows

Synchrotron is considered the dominant emission mechanism in the production of gamma-ray burst photons in the prompt as well as in the afterglow phase. Polarization is a characteristic feature of synchrotron and its study can reveal a wealth of information on the properties of the magnetic field and of the energy distribution in gamma-ray burst jets. In this paper I will review the theory and observations of gamma-ray bursts polarization. While the theory is well established, observations have prove difficult to perform, due to the weakness of the signal. The discriminating power of polarization observations, however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of Physics focus issue on Gamma Ray Burst

On the detection of very high redshift Gamma Ray Bursts with Swift

We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with Swift, assuming that GRBs form preferentially in low-metallicity environments. The model fits well both the observed BATSE and Swift GRB differential peak flux distribution and is consistent with the number of z>2.5 detections in the 2-year Swift data. We find that the probability to observe a burst at z>5 becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent with the number of confirmed detections. The corresponding fraction of z>5 bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity threshold for GRB formation. We propose to use the computed probability as a tool to identify high redshift GRBs. By jointly considering promptly-available information provided by Swift and model results, we can select reliable z>5 candidates in a few hours from the BAT detection. We test the procedure against last year Swift data: only three bursts match all our requirements, two being confirmed at z>5. Other three possible candidates are picked up by slightly relaxing the adopted criteria. No low-z interloper is found among the six candidates.Comment: 5 pages, 2 figures, MNRAS in pres

Time-resolved optical/near-IR polarimetry of V404 Cyg during its 2015 outburst

We present optical and near-IR linear polarimetry of V404 Cyg during its 2015 outburst and in quiescence. We obtained time resolved r'-band polarimetry when the source was in outburst, near-IR polarimetry when the source was near quiescence and multiple wave-band optical polarimetry later in quiescence. The optical to near-IR linear polarization spectrum can be described by interstellar dust and an intrinsic variable component. The intrinsic optical polarization, detected during the rise of one of the brightest flares of the outburst, is variable, peaking at 4.5 per cent and decaying to 3.5 per cent. We present several arguments that favour a synchrotron jet origin to this variable polarization, with the optical emission originating close to the jet base. The polarization flare occurs during the initial rise of a major radio flare event that peaks later, and is consistent with a classically evolving synchrotron flare from an ejection event. We conclude that the optical polarization flare represents a jet launching event; the birth of a major ejection. For this event we measure a rather stable polarization position angle of -9 degrees E of N, implying that the magnetic field near the base of the jet is approximately perpendicular to the jet axis. This may be due to the compression of magnetic field lines in shocks in the accelerated plasma, resulting in a partially ordered transverse field that have now been seen during the 2015 outburst. We also find that this ejection occurred at a similar stage in the repetitive cycles of flares.Comment: 9 pages, 5 figures, accepted for publication by MNRA

A Search for the Optical/Infrared Counterpart of the Anomalous X-ray Pulsar 1E 1841-045

We have carried out a search for the optical and infrared counterpart of the Anomalous X-ray Pulsar 1E 1841-045, which is located at the center of the supernova remnant Kes73. We present the first deep optical and infrared images of the field of 1E 1841-045, as well as optical spectroscopy results that exclude the brightest objects in the error circle as possible counterparts. A few of the more reddened objects in this region can be considered as particularly interesting candidates, in consideration of the distance and absorption expected from the association with Kes73. The strong interstellar absorption in the direction of the source does not allow to completely exclude the presence of main sequence massive companions.Comment: 8 pages, latex, 6 figures, Submitted to Mon. Not. R. Astron. So