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
   

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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
   

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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)
   

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4. Stewart
   2010
   rms ~25-40 mJy/beam
   2012
   rms ~4 mJy/beam
   

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5. Bell #1 light curve
   

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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
   

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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
   

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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
   

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9. Beamformed data
   Hassall, Pietka
   Limits from
   the LOFAR
   beamformed
   observations
   

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10. Hassall, Pietka
    Limits from
    the Lovell
    beamformed
    observations
    

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11. August 2010 – March 2011 June 2011 – April 2012
    Averaged images
    

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12. August 2010 – March 2011 June 2011 – April 2012
    

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16. 1.96 deg separation 3C 86 (z not known)
    exactly at the midpoint!
    

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17. N
    S
    S (WENSS) = 51 mJy, S (NVSS) = 11.4 mJy, S (LOFAR predicted) ~ 120 mJy
    

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18. Bell #1 light curve
    

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19. J0321+5512 light curve (8.5 arcmin E of Bell #1)
    Calibration problem?
    

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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
    

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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.
    

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22. Peeling 3C 86 seems to also remove Bell #1 (but
    possibly not its 'friend'; more analysis to be done).
    

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23. rms ~ 3-4 mJy/beam
    

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24. rms ~ 4 mJy/beam
    * All VLSS sources detected
    

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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?)
    

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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?)
    

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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
    

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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
    

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29. View Slide