In the last entry to this blog I talked about the birth of gravitational wave multi-messenger astrophysics and how the first joint detection of gravitational waves and electromagnetic waves in August 2017 opened a totally new window to the universe. But the summer of 2017 produced even more surprises!
On September 19, 2017: at 20:54:30.43 Coordinated Universal Time (UTC), a high-energy neutrino of about 290TeV (IceCube-170922A) was detected in an automated analysis that is part of IceCube’s real-time alert system. An alert was distributed to observers 43 seconds later. The direction of the neutrino (reconstructed to an area of about 1 square degree and consistent with the location of a known gamma-ray blazar TXS 0506+056) became visible at the location of the H.E.S.S. observatory in Namibia around 4 hours later. The event perfectly fulfilled the criteria I had outlined in the follow-up program of high-energy neutrinos (cf. Multimessenger searches). We could thus start H.E.S.S. observations and acquired an initial dataset of 1.3h during that night. After a first look at this data and without detecting significant gamma-ray emission I announced this in ATEL #10787. Additional observations were obtained on subsequent nights but still no gamma-ray emission could be detected from the region. Upper limits at 7.5*10^-12 erg / cm^2 /s (95% C.L.) on the gamma-ray flux level were subsequently derived. Too bad for us, but the story fortunately does not stop there...
The IceCube observatory in the deep ice below the South Pole. The event IC170922 is illustrated by the colorful markers indicating the arrival time of the signals. Credit: IceCube/NSF
Observations of lower energy gamma-rays obtained with the LAT instrument onboard the Fermi satellite showed the blazar TXS 0506+056 to be in a flaring state since April 2017. Strong flux variations by almost an order of magnitude with respect to the long-term average had been observed over several weeks. This first observation of a neutrino in spatial coincidence with the gamma-ray emitting blazar during an active phase suggests sparked the interest of the wider astronomical community and triggered an extensive multi-wavelength campaign with observations ranging from radio frequencies to high-energy gamma-rays. During this campaign high-energy gamma rays with energies up to 400 GeV were detected by the MAGIC instrument located at the Roque de los Muchachos Observatory on the Canary Island of La Palma. The emission in X-rays showed clear evidence for spectral variability, the flux in the optical V band was the highest ever observed in recent years and polarization has been detected at the level of 7% in the R band. A summary of these observations can be found in a joint publication in Science 361. Finally, the redshift of TXS 0506+056 was recently determined to be z = 0.3365 +- 0.0010 using the largest single mirror optical telescope, the Gran Telescopio Canarias (S. Paiano et al. ApJL 854), providing crucial input into our understanding on the level of attenuation of the TeV flux following its propagation through extragalactic space.
Another global multi-messenger effort allowed to detect the correlation between a high-energy neutrino (IC170922) and a flaring blazar (TXS 0506+056). Credit: N.Fuller/NSF/IceCube
Is this the first source of high-energy neutrinos?
If true, this would mean that blazars may indeed be a source of high-energy cosmic rays and thus provide a crucial step towards resolving a century old puzzle. Whoa... But I believe it is important to keep a cool head, and note that there remains a roughly 0.1% probability that the coincidence of the neutrino event with the flare of TXS 0506+056 is purely a random chance coincidence. Think of it like rolling a dice 4 times continuously getting the exact same number: not totally out of the ordinary, right? There is also an ongoing discussion about an additional 44% probability that the neutrino was induced by a CR hitting the Earths atmosphere and would thus not point back to any astrophysical origin in the first place.
One thing is clear: this correlation is extremely interesting and promises a significant breakthrough. Confirmation can only come with further observations. Thanks to our preparations, H.E.S.S. and the upcoming Cherenkov Telescope Array are prime instruments to do just this... Looking forward to the next neutrino alert. It may just be around the corner...
AS-SN optical V-band widefield image of the sky centered on the known position (+) of the TXS 0506+056 blazar. Two known objects from catalogues of gamma-ray sources generated Fermi Large Area Telescope are shown as blue circles, one being TXS 0506+056, with diameter representing the 95% position uncertainty. Also shown are the 50% and 90% containment areas (solid-grey and dashed-red contours, respectively) for the best-fit directional reconstruction of a high-energy neutrino detected on 22 September 2017 that indicate positional coincidence with the blazar TXS 0506+056. Subsequent very high-energy gamma-ray observations from the MAGIC imaging air Cherenkov telescopes also detected this source, with 68% positional uncertainties of those observations shown as the yellow circle. Inset is a zoomed view of the region of interest surrounding TXS 0506+056. From IceCube Collaboration et al., Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A. Science 361