The Vela Pulsar - the most Highly Energetic Clock

November 2023

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Pulsars, progeny of supernova explosions, are among the most fascinating beasts in the cosmic zoo. They are rapidly rotating objects exhibiting magnetic field intensities many orders of magnitude larger than what we can achieve on Earth (e.g., at CERN) and matter densities that defy imagination: a cubic centimetre of matter at the heart of such stars which are essentially made up of neutrons, therefore called neutron stars, weighs several billion tons! Pulsars were discovered as periodic sources of radio pulses back in 1967 by Jocelyn Bell-Burnell & Antony Hewish [1]. The radio pulses are detected at Earth when beams of electromagnetic radiation emitted from the pulsar magnetic poles sweep across the field of view of radio-telescopes: this is the famous light house picture (which is, however, not the sole possible one). The energy reservoir of pulsars is rotational energy as demonstrated by precise temporal measurements revealing they're slowing down. Nowadays there are more than 3000 of radio pulsars known, including dozens in our neighbouring galaxy, the LMC (ATNF).

Pulsars are also sources of high-energy (HE; 100 MeV-100 GeV) gamma rays, which are widely believed to be emitted by electrons and positrons accelerated to TeV energies. They constitute, as such, outstanding particle accelerators injecting matter and antimatter into the Galaxy, being at the same time formidable natural laboratories for fundamental physics. The number of known gamma-ray emitting pulsars exceeds 300 sources (Fermi-LAT third pulsar catalog, 3PC), i.e. roughly 10% of all pulsars known. This can be understood as being due to intrinsic factors, e.g. a pulsar may not be enough powerful to accelerate particles to sufficient energies, the gamma-ray beam might simply not sweep over Earth, etc, or as being due to the limited sensitivity of gamma-ray instruments — the high-energy photons are much more scarce and hence more difficult to catch than radio pulses!

A striking feature of gamma-ray pulsars is the universality of their emission spectra: they all show strong cutoffs or a break above energies of a few GeV. This made the prospects of their detection in the VHE range (~100 GeV-100 TeV) by ground-based telescopes rather unlikely, although some models predicted very early on a TeV component [3-6]. Previous searches of such a component had resulted in non detections [e.g., 10, 11] except the detection of only one pulsar, that associated with the Crab nebula [7-9]. However, although extending up to ~1 TeV, the VHE spectrum of the Crab pulsar could hardly be qualified as an independent component per se, given that it displays a very soft emission (i.e. a very steeply falling spectrum as a function of energy) in continuation of its GeV emission, above a break at few GeV (see the gray points in Fig. 3).

The Vela pulsar (hereafter, Vela) was among our prime candidates to search for VHE pulsed emission. Vela is a young pulsar located in the Southern sky in the constellation Vela (sail of the ship) which was discovered soon after the discovery of the first pulsar [2]. It is rotating about eleven times per second and its characteristic age is about 11 kyr. Although Vela ranks only as the 10th most powerful pulsar (given its spin-down power of ~7x1036 erg/s), it stands out as being by far the brightest pulsar in radio and GeV gamma-rays, thanks to its very nearby distance of 936 light years. In the GeV range, its rotation phase-folded gamma-ray light curve exhibits two peaks, labelled P1 and P2, separated by 0.43 in phase and connected by a bridge emission containing a third peak labelled P3 (Fig. 1, bottom panel). The light curve evolves with energy and in the tens of GeV range, only one of the pulses, P2, is seen (Fig. 1, bottom and middle panel).

Fig. 1: The phasogram of the Vela Pulsar in the light of gamma rays reveals the pulsed emission as a function of the pulsation phase as derived from radio observations (taken from [14]). This image compares three different energy ranges. The top-most diagram reveals the most recently published measurements [14] obtained with the CT1 - CT4 telescope arrays that allows observations at the highest energies. The middle display illustrates observations obtained with CT5 at lower energies [12], while the bottom panel gives the phasogram at even lower energy gamma rays, measured with the Fermi-LAT instrument [15].

Our first observations of Vela were made in 2004-2005 with the 12-m-diameter CT1–4 telescopes in stereo mode. The search for pulsed emission was conducted using the standard cuts yielding a threshold of 250 GeV as well as an optimised set for achieving a low energy threshold of 170 GeV. We did not detect any signal in the 16h of data available then [10]. It was only after the addition of the CT5 telescope in 2012, that we detected Vela for the first time. Thanks to very large collection area of CT5 (28-m equivalent diameter), we were able to achieve a very low energy threshold of 10 GeV in monoscopic mode and to detect the P2 pulse of Vela up to 80 GeV in 40 hours of data (represented as a green area in Fig. 3 and [12]). The timing of the HESS array was consequently validated, thanks to this first detection of a pulsar, which made Vela the second pulsar to be detected from ground. But it remained still a “GeV” pulsar and not a TeV per se.

Still looking for a VHE signal, in 2013 we carried out a reanalysis of the stereo data from 2004-2005 which were supplemented by observations made in 2006-2007. This time we used an advanced and more sensitive analysis method [13] and focused on Vela's P2 pulse. The analysis resulted in a strong hint for a signal that motivated further Vela observations in 2014-2016. Here, the periodicity test called Cosine test (C-test) which was invented by our late colleague Okkie de Jager, was instrumental, as it is the most powerful test known to search for a single pulsation with known position in phase and width [16]. The additional data confirmed the signal, but with an added surprise: we had carried out the search using four predefined energy thresholds of 0.5, 1, 3 and 7 TeV in order to be able to detect the pulsations, which their spectrum underlying either soft or hard. The surprise was that the data showed the highest significance above 7 TeV, thus indicating a very hard spectrum. The surprise was all the greater as the signal remained significant even above 20 TeV which is about 200 times more energetic than the radiation detected previously from this object. The handful of recorded photons have energies 20 times higher than those detected from the Crab pulsar.

Fig. 2: The Vela Pulsar during the phase interval 0-55-06 at energies above 5 TeV. While the total number of photons recorded is small, the phase modulation allows a clear identification of the pulsed emission. Taken from [3].

Whatever the explanation, the Vela pulsar now officially holds the record as the pulsar with the highest-energy gamma rays discovered to date. This latest addition to Vela’s superlatives challenges the models developed for explaining the GeV emission of pulsars. Producing 20 TeV gamma rays requires particles (electrons and positrons) of at least this energy and a radiation mechanism. The traditional view (I) considers an acceleration by the electric field along the magnetic field lines in cavities (i.e. zones free of charge) in the magnetosphere and an emission process known as Curvature Radiation (CR) up to GeV energies. A recent version of this scenario extends the acceleration regions slightly beyond the light cylinder (LC), into the wind region. The LC is a conceptual image to depict the radius (with respect to the pulsar) at which the corotation speed attains the light speed. The alternative scenario (II) hypothesises an acceleration through a process called magnetic reconnection deep in the equatorial plane of the pulsar. There, a so-called current sheet forms at the junction of magnetic field lines of opposite polarity beyond the LC (the wind zone). In this scheme, the Synchrotron Radiation (SR) is invoked for the GeV emission.

In either scenario, TeV photons can only be understood as being produced by the inverse-Compton (IC) process: whatever the assumptions about the acceleration mechanism and regions, the accelerated high energy electrons must boost much lower energy photons, e.g., in the IR to optical range, through IC scatterings. The difficulties arise when seeking to accelerate particles reaching energies up to 20 TeV. In scenario (I), the maximum attainable energy of particles is limited by the continuous losses through CR. One has to postulate acceleration zones well beyond the LC and to recourse to very low values of a parameter called magnetic conversion efficiency (i.e. the ratio of the magnetic filed channelled into the electric filed parallel to the magnetic field lines). The maximum particle energy is also limited in scenario (II), by SR in this case. To reach the highest energies, one needs further assumptions, e.g., the highest energy particles escape the zones where they loose their energy though SR, or that particles and their emission (both in GeV and TeV ranges) are boosted thanks to a bulk movement of the wind in the laboratory frame (i.e. observers’ frame). In any case the TeV photons shall preserve the phase coherence with those in the GeV range, which is a further challenge for the models.

Fig. 3: Spectral energy distribution of the P2 pulse of the Vela Pulsar (taken from [3]). The spectra recorded with Fermi-LAT and HESS, respectively, are combined to reveal the SED of the pulsed emission over nearly 6 orders of magnitude in photon energy and illustrate the emergence of a very high energy component at energies above 600 GeV. For comparison, the SED of the P2 pulse of the Crab pulsar as measured by Fermi-LAT and MAGIC [9] is also shown. Models based on scenarios (I) and (II) are shown as solid curves in orange and blue colors, respectively. They are not sufficient to explain the H.E.S.S. measurements up to 20 TeV. The dashed and dash-dotted curves show possible paths to fit the data (see descriptions in [3]).

Finally, this discovery opens a new observation window for detection of other pulsars in the TeV to the tens of TeV range with current and upcoming more sensitive instruments such as LHAASO or CTA. It also paves the path for a better comprehension of their potential contribution to positrons in the Galaxy and to Ultra-High-Energy cosmic rays. It will possibly have implications on our understanding of other highly magnetised astrophysical objects such as black-hole magnetospheres or jet-accretion disk systems.

Credit for thumbnail: DESY Science Communication Lab.


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