Research Article

Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

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Science  13 Jul 2018:
Vol. 361, Issue 6398, eaat1378
DOI: 10.1126/science.aat1378
  • Multimessenger observations of blazar TXS 0506+056.

    The 50% and 90% containment regions for the neutrino IceCube-170922A (dashed red and solid gray contours, respectively), overlain on a V-band optical image of the sky. Gamma-ray sources in this region previously detected with the Fermi spacecraft are shown as blue circles, with sizes representing their 95% positional uncertainty and labeled with the source names. The IceCube neutrino is coincident with the blazar TXS 0506+056, whose optical position is shown by the pink square. The yellow circle shows the 95% positional uncertainty of very-high-energy γ-rays detected by the MAGIC telescopes during the follow-up campaign. The inset shows a magnified view of the region around TXS 0506+056 on an R-band optical image of the sky.

    IMAGES: PHOASAS-SN FOR THE V-BAND OPTICAL; KANATA FOR THE R-BAND IN MAGNIFIED VIEW
  • Fig. 1 Event display for neutrino event IceCube-170922A.

    The time at which a DOM observed a signal is reflected in the color of the hit, with dark blues for earliest hits and yellow for latest. Times shown are relative to the first DOM hit according to the track reconstruction, and earlier and later times are shown with the same colors as the first and last times, respectively. The total time the event took to cross the detector is ~3000 ns. The size of a colored sphere is proportional to the logarithm of the amount of light observed at the DOM, with larger spheres corresponding to larger signals. The total charge recorded is ~5800 photoelectrons. Inset is an overhead perspective view of the event. The best-fitting track direction is shown as an arrow, consistent with a zenith angle Embedded Image degrees below the horizon.

  • Fig. 2 Fermi-LAT and MAGIC observations of IceCube-170922A’s location.

    Sky position of IceCube-170922A in J2000 equatorial coordinates overlaying the γ-ray counts from Fermi-LAT above 1 GeV (A) and the signal significance as observed by MAGIC (B) in this region. The tan square indicates the position reported in the initial alert, and the green square indicates the final best-fitting position from follow-up reconstructions (18). Gray and red curves show the 50% and 90% neutrino containment regions, respectively, including statistical and systematic errors. Fermi-LAT data are shown as a photon counts map in 9.5 years of data in units of counts per pixel, using detected photons with energy of 1 to 300 GeV in a 2° by 2° region around TXS0506+056. The map has a pixel size of 0.02° and was smoothed with a 0.02°-wide Gaussian kernel. MAGIC data are shown as signal significance for γ-rays above 90 GeV. Also shown are the locations of a γ-ray source observed by Fermi-LAT as given in the Fermi-LAT Third Source Catalog (3FGL) (23) and the Third Catalog of Hard Fermi-LAT Sources (3FHL) (24) source catalogs, including the identified positionally coincident 3FGL object TXS 0506+056. For Fermi-LAT catalog objects, marker sizes indicate the 95% CL positional uncertainty of the source.

  • Fig. 3 Time-dependent multiwavelength observations of TXS 0506+056 before and after IceCube-170922A.

    Significant variability of the electromagnetic emission can be observed in all displayed energy bands, with the source being in a high-emission state around the time of the neutrino alert. From top to bottom: (A) VHE γ-ray observations by MAGIC, H.E.S.S., and VERITAS; (B) high-energy γ-ray observations by Fermi-LAT and AGILE; (C and D) x-ray observations by Swift XRT; (E) optical light curves from ASAS-SN, Kiso/KWFC, and Kanata/HONIR; and (F) radio observations by OVRO and VLA. The red dashed line marks the detection time of the neutrino IceCube-170922A. The left set of panels shows measurements between MJD 54700 (22 August 2008) and MJD 58002 (6 September 2017). The set of panels on the right shows an expanded scale for time range MJD 58002 to MJD 58050 (24 October 2017). The Fermi-LAT light curve is binned in 28-day bins on the left panel, while finer 7-day bins are used on the expanded panel. A VERITAS limit from MJD 58019.40 (23 September 2017) of Embedded Image Embedded Image Embedded Image is off the scale of the plot and not shown.

  • Fig. 4 Broadband spectral energy distribution for the blazar TXS 0506+056.

    The SED is based on observations obtained within 14 days of the detection of the IceCube-170922A event. The Embedded Image vertical axis is equivalent to a Embedded Image scale. Contributions are provided by the following instruments: VLA (38), OVRO (39), Kanata Hiroshima Optical and Near-InfraRed camera (HONIR) (52), Kiso, and the Kiso Wide Field Camera (KWFC) (43), Southeastern Association for Research in Astronomy Observatory (SARA/UA) (53), ASAS-SN (54), Swift Ultraviolet and Optical Telescope (UVOT) and XRT (55), NuSTAR (56), INTEGRAL (57), AGILE (58), Fermi-LAT (16), MAGIC (35), VERITAS (59), H.E.S.S. (60), and HAWC (61). Specific observation dates and times are provided in (25). Differential flux upper limits (shown as colored bands and indicated as “UL” in the legend) are quoted at the 95% CL, while markers indicate significant detections. Archival observations are shown in gray to illustrate the historical flux level of the blazar in the radio-to-keV range as retrieved from the ASDC SED Builder (62), and in the γ-ray band as listed in the Fermi-LAT 3FGL catalog (23) and from an analysis of 2.5 years of HAWC data. The γ-ray observations have not been corrected for absorption owing to the EBL. SARA/UA, ASAS-SN, and Kiso/KWFC observations have not been corrected for Galactic attenuation. The electromagnetic SED displays a double-bump structure, one peaking in the optical-ultraviolet range and the second one in the GeV range, which is characteristic of the nonthermal emission from blazars. Even within this 14-day period, there is variability observed in several of the energy bands shown (see Fig. 3), and the data are not all obtained simultaneously. Representative Embedded Image neutrino flux upper limits that produce on average one detection like IceCube-170922A over a period of 0.5 (solid black line) and 7.5 years (dashed black line) are shown, assuming a spectrum of Embedded Image at the most probable neutrino energy (311 TeV).

Supplementary Materials

  • Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

    The IceCube Collaboration, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S, INTEGRAL, Kanata, Kiso, Kapteyn, Liverpool Telescope, Subaru, Swift/NuSTAR, VERITAS, and VLA/17B-403 teams

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