Context. A short duration burst reminiscent of a soft gamma-ray repeater/anomalous X-ray pulsar behaviour was detected in the direction of LS I +61 303 by the Swift satellite. While the association with this well known gamma-ray binary is likely, a different origin cannot be excluded. Aims. We explore the error box of this unexpected flaring event and establish the radio, near-infrared and X-ray sources in our search for any peculiar alternative counterpart. Methods. We carried out a combined analysis of archive Very Large Array radio data of LS I +61 303 sensitive to both compact and extended emission. We also reanalysed previous near infrared observations with the 3.5 m telescope of the Centro Astronomico Hispano Aleman and X-ray observations with the Chandra satellite. Results. Our deep radio maps of the LS I +61 303 environment represent a significant advancement on previous work and 16 compact radio sources in the LS I +61 303 vicinity are detected. For some detections, we also identify near infrared and X-ray counterparts. Extended emission features in the field are also detected and confirmed. The possible connection of some of these sources with the observed flaring event is considered. Based on these data, we are unable to claim a clear association between the Swift-BAT flare and any of the sources reported here. However, this study represents the most sophisticated attempt to determine possible alternative counterparts other than LS I +61 303.
Deep Dive into Candidate counterparts to the soft gamma-ray flare in the direction of LS I +61303.
Context. A short duration burst reminiscent of a soft gamma-ray repeater/anomalous X-ray pulsar behaviour was detected in the direction of LS I +61 303 by the Swift satellite. While the association with this well known gamma-ray binary is likely, a different origin cannot be excluded. Aims. We explore the error box of this unexpected flaring event and establish the radio, near-infrared and X-ray sources in our search for any peculiar alternative counterpart. Methods. We carried out a combined analysis of archive Very Large Array radio data of LS I +61 303 sensitive to both compact and extended emission. We also reanalysed previous near infrared observations with the 3.5 m telescope of the Centro Astronomico Hispano Aleman and X-ray observations with the Chandra satellite. Results. Our deep radio maps of the LS I +61 303 environment represent a significant advancement on previous work and 16 compact radio sources in the LS I +61 303 vicinity are detected. For some detections, we a
LS I +61 303 (V615 Cas) is a gamma-ray binary originally discovered in 1977 in the radio during a survey for variable sources in the Galactic plane (Gregory & Taylor 1978;Gregory et al. 1979). The orbital period is about 26.5 d (Hutchings & Crampton 1981) and it has been detected in data of frequencies between radio (Taylor & Gregory 1982, 1984) and high energy gamma-rays (Albert et al. 2008). The physical interpretation of the LS I +61 303 emission across the complete electromagnetic spectrum still remains a matter of debate (Romero et al. 2007). Two different models have been proposed to explain the full spectral energy distribution: (i) a microquasar X-ray binary (Bosch-Ramon et al. 2006), and (ii) a nonaccreting pulsar interacting with the envelope of the rapidly rotating Be star (Dubus 2006).
On 2008 September 10, the Swift Burst Alert Telescope (BAT) detected a burst in the direction of LS I +61 303
Send offprint requests to: A. J. Muñoz-Arjonilla within its 15-150 keV energy range (De Pasquale et al. 2008). The calculated location of this soft gamma-ray short-duration burst-event was found to have an error of 2. ′ 2 and the position of LS I +61 303 was found to be clearly consistent with this event (Barthelmy et al. 2008). Given this coincidence, this burst was associated with magnetar-like activity linked to a young highly magnetized pulsar in the binary system (Dubus & Giebels 2008). Unusual X-ray activity (hard high flux and QPOs) had also been reported by Ray et al. (2008) just a few weeks before based on PCA data from the RXTE satellite. If the QPOs originate in LS I +61 303, an accretion disk would be necessary to explain the nature of this source. However, the PCA field of view of ∼ 1 degree includes many additional sources that could be responsible for the QPO behaviour.
Despite these facts, one cannot exclude the possibility of another unrelated source being responsible for the observed gamma-ray flare. A population of faint X-ray sources in the vicinity of LS I +61 303 were reported by Rea & Torres (2008), who suggested that one of these sources 2008) also reported additional X-ray and radio sources coincident with the Swift-BAT error circle and some of them with stellar-like counterparts. We present extensive radio, X-ray, and near infrared observations of the Swift-BAT error circle at the location of this 2008 September 10 event. Both archival data and observations conducted by the authors were used in this paper as described in the log in Table 1. Different populations of sources were detected and their main observational properties are reported. A few interesting objects in the direction of the magnetar-like flare are highlighted and the possibility of them being alternative candidate counterparts is assessed. The census of radio/X-ray sources reported here represents the most complete study to date of alternative counterpart candidates to the Swift-BAT event.
The magnetar-like event that renewed interest in studying LS I +61 303 is probably related to a compact object formed as a result of a supernova event. Therefore, both compact and extended radio features could play a role in our understanding of this phenomenon. The environment of LS I +61 303 in the radio was studied by Martí et al. (1998) (hereafter M98) at 6 cm wavelength using the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) with the array in its CnD configuration providing appropriate sensitivity to both compact and extended sources. We developed the M98 approach improving their sensitivity by combining additional 6 cm VLA archive data acquired by the same compact array configuration (see Table 1) to a total of 25.5 h of on-source time.
The AIPS package of NRAO was used for the standard interferometer data processing and self-calibration. We removed the variable LS I +61 303 core to avoid artifacts and replaced it for cosmetic reasons with a constant point source component with the observed average flux density. We also removed a nearby bright radio source, whose presence affected significantly the dynamic range of the maps. The final result is presented in Fig. 1, where extended emission is enhanced with a slight taper. Compact sources are more clearly evident in the non-tapered radio map of Fig. 2 with a rms noise of 13 µJy beam -1 , significantly lower than that of M98. The observational properties of these compact radio sources are listed in Table 2, where J2000.0 positions are given. -3, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 20, 25, 50, 100, 200, 500, 1000, 2000 and 3000 times the rms noise.
The field of LS I +61 303 was observed at near infrared wavelengths with the 3.5 m telescope at the Centro Astronómico Hispano Alemán (CAHA) in Almería (Spain), one year before the Swift event. The OMEGA2000 camera was used and the images were taken through the J, H and K s filters. CAHA observations were processed following the standard procedures for sky background subtraction, flat-fielding
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