LOFAR Image Plane Transients: Bell #1

Jess Broderick

Transcript

1. LOFAR image plane transients: Bell #1 Jess Broderick (University of Southampton) Martin Bell, John Swinbank, Adam Stewart, Rob Fender, Tom Hassall, Teo Muñoz-Darias, Gosia Pietka and the LOFAR TKP
2. We have repeatedly observed a field centred on PSR B0329+54: ~20 deg2 in HBA. Bright (~200 mJy) source varying by factor ≥ 10 Not present in previous radio surveys (nor in some other radio observations we've made) Broderick et al. Zenith field transient: ILT J0320.3+5512 casapy averaged image, σ ~3 mJy/beam, resolution ~ a few arcmin
3. Observations and data reduction * 22 observations from 2010 April to 2012 May * HBA_ZERO or HBA_DUAL (avg. frequency ~140-160 MHz) * Compress in time and frequency * Remove baselines > 6 km * Make 24 'band' measurement sets (10 sub-bands at a time) * Calibrate using a VLSS/WENSS/NVSS sky model (gsm.py) * Combine the band measurement sets into 3 larger ones (i.e. 8 bands at a time) * Image the 3 bands with AWimager using baselines < 4 km * Average images after convolving to a common (worst) resolution (~80-150 arcsec)
4. Stewart 2010 rms ~25-40 mJy/beam 2012 rms ~4 mJy/beam
5. Bell #1 light curve
6. What is this object? Large network of multiwavelength (radio, IR, optical, X-ray, γ-ray, GW) partner facilities. Multiple optical observations made, also X-ray upper limits from RXTE ASM and MAXI No optical counterpart to (stacked) m ~23.5. No bright X-ray counterpart. These limits rule out: accreting binary (CV, X-ray binary), AGN What is left? Some flavour of neutron star? Most extreme combination of coherent emission and lack of strong thermal emission How many? Rate naively implies 1000s such objects around the sky that should be easily detected by LOFAR
7. What is this object? Large network of multiwavelength (radio, IR, optical, X-ray, γ-ray, GW) partner facilities. Multiple optical observations made, also X-ray upper limits from RXTE ASM and MAXI No optical counterpart to (stacked) m ~22.5. No bright X-ray counterpart. These limits rule out: accreting binary (CV, X-ray binary), AGN What is left? Some flavour of neutron star? Most extreme combination of coherent emission and lack of strong thermal emission How many? Rate naively implies 1000s such objects around the sky that should be easily detected by LOFAR Liverpool Telescope r'-band
8. What is this object? Large network of multiwavelength (radio, IR, optical, X-ray, γ-ray, GW) partner facilities. Multiple optical observations made, also X-ray upper limits from RXTE ASM and MAXI No optical counterpart to (stacked) m ~22.5. No bright X-ray counterpart. These limits rule out: accreting binary (CV, X-ray binary), AGN What is left? Some flavour of neutron star? Most extreme combination of coherent emission and lack of strong thermal emission (checking beamformed data ASAP) How many? Rate naively implies 1000s such objects around the sky that should be easily detected by LOFAR Muñoz-Darias & Stewart, also see talk by Pietka tomorrow
9. Beamformed data Hassall, Pietka Limits from the LOFAR beamformed observations
10. Hassall, Pietka Limits from the Lovell beamformed observations
11. August 2010 – March 2011 June 2011 – April 2012 Averaged images
12. August 2010 – March 2011 June 2011 – April 2012
16. 1.96 deg separation 3C 86 (z not known) exactly at the midpoint!
17. N S S (WENSS) = 51 mJy, S (NVSS) = 11.4 mJy, S (LOFAR predicted) ~ 120 mJy
18. Bell #1 light curve
19. J0321+5512 light curve (8.5 arcmin E of Bell #1) Calibration problem?
20. Light curves of some sources near Bell #1 55200 55300 55400 55500 55600 55700 55800 55900 56000 56100 56200 0 500 1000 1500 2000 2500 3000 MJD Flux (mJy) PSR 0329+54
21. • Imaging the data with the AWimager * 3 x 80 sub-band strategy works well (originally was 6 x 40) * New, much faster version available! • Refining the radio position * Some very limited success so far when longer baselines (> 4 km) are included in the reduction. * No optical counterpart at the position of the higher-resolution radio peak. • Calculating a spectral index * S/N not good enough; average ~ 0.9 +/- 3.5 • Intra-run variability * No evidence for this, but S/N issues.
22. Peeling 3C 86 seems to also remove Bell #1 (but possibly not its 'friend'; more analysis to be done).
23. rms ~ 3-4 mJy/beam
24. rms ~ 4 mJy/beam * All VLSS sources detected
25. New sources! Bell #1 'off' avg. map, May 2012, NVSS * No detections in VLSS, WENSS and NVSS * S (LOFAR) ~40 mJy * ~15 sources like this with S (LOFAR) ~40-60 mJy (steep spectrum sources? transients?)
26. Non-detections * No detection with LOFAR * S (WENSS) 35 mJy, S (NVSS) 77.1 mJy * ~25 sources like this (variability? spectral turnover at low frequency?)
27. * MSSS HBA mosaic, 9 fields, 2 snapshots (400 s / snapshot), 8 bands * Resolution ~140 arcsec, rms ~10-15 mJy/beam * ~120 deg2 MSSS HBA Stewart, Broderick
28. Summary and future work * Is Bell #1 actually real after all? * Bell #1 paper in preparation (Broderick et al.) * Use new, faster AWimager * Investigate peeling further * Run the TraP on the images * Will get more observations of this field in Cycle 0