James C. French

Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

Magnetars are young neutron stars with very strong magnetic fields of the order of 1014–1015 G. They are detected in our Galaxy either as soft γ-ray repeaters or anomalous X-ray pulsars. Soft γ-ray repeaters are a rare type of γ-ray transient sources that are occasionally detected as bursters in the high-energy sky. No optical counterpart to the γ-ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of ∼3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the ‘persistent’ soft γ-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.A. J. Castro-Tirado, A. de Ugarte Postigo, J. Gorosabel, M. Jeĺınek, T. A. Fatkhullin, V. V. Sokolov, P. Ferrero, D. A. Kann, S. Klose, D. Sluse, M. Bremer, J. M. Winters, D. Nuernberger, D. Pérez-Ramı́rez, M. A. Guerrero, J. French, G. Melady, L. Hanlon, B. McBreen, F. J. Aceituno, R. Cunniffe, P. Kubánek, S. Vitek, S. Schulze, A. C. Wilson, R. Hudec, J. M. González-Pérez, T. Shahbaz, S. Guziy, S. B. Pandey L. Pavlenko, E. Sonbas, S. A. Trushkin, N. N. Bursov, N. A. Nizhelskij and L. Sabau-Graziati

A photometric redshift of z = 1.8+0.4 -0.3 for the AGILE GRB 080514B

The AGILE gamma-ray burst GRB 080514B is the first detected to have emission above 30 MeV and an optical afterglow. However, no spectroscopic redshift for this burst is known. We report on our ground-based optical/NIR and millimeter follow-up observations of this event at several observatories, including the multi-channel imager GROND on La Silla, supplemented by Swift UVOT and Swift XRT data. The spectral energy distribution (SED) of the optical/NIR afterglow is found to decline sharply bluewards to the UV bands, which can be utilized in estimating the redshift. Fitting the SED from the Swift UVOT uvw2 band to the H band, we estimate a photometric redshift of z = 1.8 +0.4-0.3, which is consistent with the reported pseudo-redshift based on gamma-ray data. We find that the afterglow properties of GRB 080514B do not differ from those exhibited by the global sample of long bursts. Compared with the long burst sample, we conclude that this burst was special because of its high-energy emission properties, even though both its afterglow and host galaxy are not remarkable in any way. Obviously, high-energy emission in the gamma-ray band does not automatically correlate with the occurrence of special features in the corresponding afterglow light.

The Photometry Pipeline of the Watcher Robotic Telescope

The Watcher robotic telescope was developed primarily to perform rapid optical follow-up observations of Gamma-Ray Bursts (GRBs). Secondary scientific goals include blazar monitoring and variable star studies. An automated photometry pipeline to rapidly analyse data from Watcher has been implemented. Details of the procedures to get image zero-point, source instrumental measurement, and limiting magnitude are presented. Sources of uncertainty are assessed and the performance of the pipeline is tested by comparison with a number of catalogue sources.

Watcher: A Telescope for Rapid Gamma-Ray Burst Follow-Up Observations

The Watcher telescope is planned to begin operation in Spring 2004 in South Africa.The system has been designed to respond primarily to very precise (arcminute) gamma‐ray burst locations distributed via the internet by the GCN. Watcher will be fully automatic and the planned response time for GRBs is ∼ 30 seconds or better. In addition, the telescope will be used for blazar monitoring and the photometric detection of extra‐solar planets when GRBs are not being observed.

SWIFT J195509+261406: Dramatic Flaring Activity from a New Galactic Magnetar

Most of the transient sources that are detected in the gamma-ray sky are produced by extragalactic gamma-ray bursts (GRBs). However, it is known that there are some other astronomical objects that can produce high-energy bursts within the Milky Way. SWIFT J195509+261406, just one degree off the Galactic plane, is one of them. It was discovered on the 10th July 2007 by the Swift satellite and was since then observable for a period of a fortnight. During this time SWIFT J195509+261406 experimented dramatic flaring activity that could be observed in near infrared, optical and X-rays. We gathered multi-wavelength observations of SWIFT J195509+261406 including optical, near infrared, millimeter and radio observations. Our dataset covers the time from 1 min after the burst onset to more than 4 months later. Following the initial burst in the gamma-ray band, we recorded more than 40 flaring episodes in the optical bands (reaching up to I c ∼ 15) over a time span of 3 days, plus a faint infrared flare that was observed at late times. After this time, the source slowly faded away until it became undetectable. Using the observations compiled in this work we propose that this source is part of the magnetar family, linking soft gamma-ray repeaters and anomalous X-ray pulsars to dim isolated neutron stars.

Assessment of a pixel-to-pixel metrological approach to the measurement of astronomical magnitudes

This work provides a thorough insight into the performance of a new approach to the measurement of astronomical magnitudes. This new photometric procedure only considers as part of the source those pixels with a value higher than the sky background value within a specified confidence interval. The resultant procedure is robust, simple and fast, and it provides the instrumental magnitude and corresponding uncertainty for each celestial object within a CCD image. A numerical and observational comparison with the more traditional aperture photometry method is also presented.

GRB 070610: Flares from a peculiar Galactic source

The majority of bright gamma‐ray transients that we observe in the sky, are produced by extragalactic gamma‐ray bursts (GRBs). However, we know that there are other kinds of sources that can produce intense bursts of high‐energy photons within our own Galaxy. GRB 070610/Swift J195509+261406, being just 1 degree away from the Galactic plane, is one of these sources and presents a peculiar behavior, as it shows strong flaring activity that has been observed in the optical and near infrared.We have gathered multi‐wavelength observations of GRB 070610 including optical, near infrared, millimeter and radio observations. Our dataset covers the time from 1 minute after the burst onset to more than 4 months later. Following the burst in the gamma‐ray band, the source displayed more than 40 flaring episodes in the optical bands (reaching up to Ic∼15) over a time span of three days, plus a faint infrared flare that was observed at late times. After this time, the source slowly faded away until it became undetectable.

Similarities in the Temporal Properties of Gamma‐Ray Bursts and Soft Gamma‐Ray Repeaters

Magnetars are modelled as sources that derive their output from magnetic energy that substantially exceeds their rotational energy. An implication of the recent polarization measurement of GRB 021206 is that the emission mechanism may be dominated by a magnetic field that originates in the central engine. Similarities in the temporal properties of SGRs and GRBs are considered in light of the fact that the central engine in GRBs may be magnetically dominated. The results show that 1) the time intervals between outbursts in SRG 1806 − 20 and pulses in GRBs are consistent with lognormal distributions and 2) the cumulative outputs of SGRs and GRBs increase linearly with time. This behaviour can be successfully modelled by a relaxation system that maintains a steady state situation.