Discovery of a very Lyman-α-luminous quasar at z = 6.62

Distant luminous quasars provide important information on the growth of the first supermassive black holes, their host galaxies and the epoch of reionization. The identification of quasars is usually performed through detection of their Lyman-α line redshifted to 0.9 microns at z > 6.5. Here, we report the discovery of a very Lyman-α luminous quasar, PSO J006.1240 + 39.2219 at redshift z = 6.618, selected based on its red colour and multi-epoch detection of the Lyman-α emission in a single near-infrared band. The Lyman-α line luminosity of PSO J006.1240 + 39.2219 is unusually high and estimated to be 0.8 × 1012 Solar luminosities (about 3% of the total quasar luminosity). The Lyman-α emission of PSO J006.1240 + 39.2219 shows fast variability on timescales of days in the quasar rest frame, which has never been detected in any of the known high-redshift quasars. The high luminosity of the Lyman-α line, its narrow width and fast variability resemble properties of local Narrow-Line Seyfert 1 galaxies which suggests that the quasar is likely at the active phase of the black hole growth accreting close or even beyond the Eddington limit.

From power-law fit F λ ~ λ α to the continuum of PSO J006.1240 + 39.2219 between 9500-9900 and 10000-10150 Å, we estimated a spectral slope of α = − 1.10 ± 0.48. By extrapolating the power law to 1450 × (1 + z) Å, we measured the absolute magnitude of the quasar at rest-frame wavelength 1450 Å to be M 1450 = − 26.1 ± 0.4, where the error includes the uncertainty in the spectral slope and redshift. Applying a bolometric correction factor of 4.4 to the ultraviolet (UV) luminosity 14 , we estimated a total quasar luminosity of 2.8 × 10 13  L (where  L is the solar luminosity). The relation between black hole mass and bolometric luminosity for the known z ~ 6 quasars follows well the expected relation for accretion at the Eddington limit 15 . If PSO J006.1240 + 39.2219 accretes at the Eddington limit, its luminosity implies a black hole mass of 10 8 -10 9  M 7,15 . Given the relation between mass of black holes and square of the width of broad emission lines 16 , the narrow Lyα line of PSO J006.1240 + 39.2219 implies up to an order of a magnitude smaller black hole mass than expected from the quasar luminosity, and the super-Eddington accretion rate 7,15 .
From the spectrum of PSO J006.1240 + 39.2219 with the subtracted continuum we measured its Lyα line luminosity to be 0.8 × 10 12  L , which is about 3% of the total luminosity of the quasar. We compared the luminosity of the Lyα line of PSO J006.1240 + 39.2219 with that of the other quasars discovered at z > 6.5. As shown in Fig. 2, the Lyα luminosity of PSO J006.1240 + 39.2219 is larger than the Lyα luminosity of the z > 6.5 quasars by more than a factor of two. It is also more than ten times larger than the Lyα luminosity of the most luminous Lyman Alpha Emitting galaxies (LAEs) seen during the epoch of reionization 17 . The relative contribution of the Lyα line into the total quasar luminosity is somewhat uncertain as due to the uncertainty in the continuum fit. However, we calculated that for power-law slopes between − 0.5 and − 2.5, the Lyα emission always dominates the UV continuum and contributes 2-4% into the bolometric luminosity of the quasar. In the other z > 6.5 quasars this contribution is only 0.1-0.5% (see Fig. 3). The rest-frame equivalent width (EW) of the Lyα line of PSO J006.1240 + 39.2219 is also large. Similar to the previous studies, we estimated the EW of the quasar by integrating the line flux above the continuum within 1160 < λ rest < 1290 Å, which includes the Lyα and NV lines. The measured EW of PSO J006.1240 + 39.2219 is equal to 182 Å and corresponds to the high end of the equivalent width distribution of the known z > 5.6 quasars with the peak at EW ≈ 35 Å 9 . Many of the z > 5.6 quasars have absolute magnitudes brighter than that of PSO J006.1240 + 39.2219, but only small fraction of them exhibit strong Lyα emission lines (Their composite spectrum is shown in Fig. 1). The typical EW of these quasars is ~140 Å 9 . The EWs of the known z > 6.5 quasars are smaller than 35 Å (except for PSO J338 + 29 with  EW 70Å 4 ), i.e., corresponds to the lower end of the EW distribution of the high-redshift quasars, which can be explained by stronger HI absorption at z > 6.5. The large EW of PSO J006.1240 + 39.2219 compared to these quasars implies weaker nearby intergalactic HI absorption.
The multi-epoch photometry of PSO J006.1240 + 39.2219 shows the change of the quasar brightness. From the PS1 images taken between June 2010 and July 2013, we measured the y PS1 -band brightness of the quasar at different epochs. The resulting quasar light curve is shown in Fig. 4. The brightness of the quasar at the time of our spectroscopic observations is also presented. From the quasar light curve we find that the quasar is variable on rest-frame timescales of days and months, with an amplitude exceeding its multi-epoch mean brightness by more than 2.5σ. The overall peak-to-peak amplitude of the observed variations is ~0.7 mag. Between 2010 and 2011, the quasar became brighter by about 0.24 mag within 50 days in the quasar rest frame. In 2013, PSO J006.1240 + 39.2219 changed its brightness from 20.15 ± 0.09 to 19.66 ± 0.07 mag over a period of ~2 days in the quasar rest frame. These high-amplitude variations are larger than brightness changes of 0.1-0.2 mag expected from the UV/optical structure function 18 and damped random walk model 19 of quasar variability on similar timescales. However, we note, that the Lyα line is ionized by the extreme UV and soft X-ray radiation which can be highly variable. For instance, the soft X-ray flux of NLS1s can change by a factor of ten on timescales of days 20,21  3 22 ) and the size of the Lyα emitting region 23,24 . The y PS1 band measures the total flux from the Lyα line and nearby UV continuum. Therefore, observed variability of PSO J006.1240 + 39.2219 can be caused both by the line and continuum variations. However, the continuum brightness, corresponding to the y PS1 -band multi-epoch mean quasar flux, is > ∼ 21 mag, which is below the detection limit for single exposures in the 3π PS1 survey = . (m 20 12 mag) y lim PS1 25 . The relative flux contribution of the Lyα line into the y PS1 -band total flux of the quasar is more than 70%. Therefore, the observed flux mostly comes from the Lyα line of PSO J006.1240 + 39.2219. The rapid y PS1 -band variations of the quasar provide the evidence of variable Lyα emission which responds fast to the variations of the extreme UV and soft X-ray flux and, therefore, originates close to the central engine 26 . The small size of the Lyα emitting region, as expected from variability of the quasar, suggests a rather small mass of the central black hole 16 .

Discussion
We reported the discovery of the Lyα-luminous narrow-line quasar, PSO J006.1240 + 39.2219, with the first evidence of broad-band quasar variability at high redshift. We find a similarity between the properties of PSO J006.1240 + 39.2219 and the NLS1 galaxies. The NLS1s exhibit rapid UV variability and narrow broad lines, as a result of the smaller black hole masses, an order of a magnitude smaller than the black hole masses of the broad-line quasars of the same luminosities. Similar to the NLS1s, the strong narrow Lyα line of PSO J006.1240 + 39.2219 without a prominent broad-line component and its short-term variability provide the evidence of the smaller black hole mass of this quasar than that expected from the luminosity -black hole mass relation.
The high luminosity of the Lyα line of PSO J006.1240 + 39.2219 implies that the extreme UV and soft X-ray component of the quasar continuum is strong and sustains its Lyα emission at a very high level 27,28 . We estimate the luminosity of this high-energy continuum to be L ion = 1.8 L(Lyα)/ α f esc Ly ≈ 5 × 10 12  L 29 , where we assume that the average energy of ionizing photons is 13.6 eV and escape fraction of the Lyα photons is ≈ . . The adopted escape fraction represents the volume-averaged value that is found to evolve approximately as power law esc Ly 2 57 between redshift 0 and 6 30 . We note, that the volume-averaged escape fraction includes effects of absorption by the IGM that might lead to the smaller values of α f esc Ly at z > 6. In spite of the uncertainty in α f esc Ly , being the most luminous Lyα emitter, PSO J006.1240 + 39.2219 is the powerful source of ionizing radiation which likely has an important contribution into ionization of the IGM surrounding the quasar. From the observed spectrum we measure the size of the quasar ionized HII region scaled to M 1450 = − 27 to be R NZ = 4 ± 1 Mpc, which is slightly larger than a near zone of 2.5-3.5 Mpc expected from the empirical relation between R NZ and redshift 31 (see 'The near-zone size' in the Methods).
The observed y PS1 -band brightness variations of PSO J006.1240 + 39.2219 are likely due to variability of its Lyα emission as it substantially dominates the y PS1 -band flux of the quasar. The size of the Lyα emitting region of PSO J006.1240 + 39.2219 inferred from the timescale of the Lyα rapid variations is about 2 light days. This is similar but slightly smaller than the BLR regions of the local NLS1 galaxies 24,32,33 . From the observations of reverberation time lags between the UV/X-ray continuum and Lyα line (and also between the UV/X-ray continuum and Balmer lines), the typical size of the Lyα emitting region of the NLS1s is estimated to be 3-10 light days. For comparison, the time lags (and correspondingly the BLR sizes) in broad-line quasars are ⩾1 month 34,35 . We caution that if the UV continuum of PSO J006.1240 + 39.2219 is highly variable (e.g., changing by about 1 mag on short timescales) its variability imposed on the variations of the Lyα flux would lead to underestimation of the size of the Lyα emitting region inferred from the observed short-term variations.
From the similarity of PSO J006.1240 + 39.2219 with the NLS1 galaxies we infer that this quasar is young, at the early phase of its black hole and bulge formation. These Lyα-line luminous young quasars seen at early cosmic epochs might be capable of ionizing large volumes of gas and might play a significant role in cosmic reionization.

Methods
Quasar candidate selection. We searched for z ps1 -band dropouts in the first and second internal data releases of the PS1 survey (PV1 and PV2) using the i ps1 -, z ps1 -and y ps1 -band photometric catalogues. First, from the y ps1 -band catalogue we selected point sources assuming that the difference between their point spread function (PSF) and aperture magnitudes is less than 0.3 mag, and the chi-square of the PSF fit is χ < . . From the resulting sample we selected the z ps1 -band dropout quasar candidates using the following criteria: (where σ y PS1 is the y ps1 -band photometric error), i ps1 > 24 and z ps1 > 24 mag. These criteria are similar to those adopted in the previous searches of high-redshift quasars from PS1 4 . Unlike the previous works, we additionally checked for multi-epoch detections of our z ps1 -band dropout candidates in the y ps1band. The PS1 survey conducted repeated scans of the sky and provided multi-epoch photometry for detected sources in all PS1 bands. The z ps1 -dropout candidates detected at least at two different epochs were considered by us as reliable. In this way we excluded short-lived transients and other possible artifacts from our colour-selected sample. The strongest of the multi-epoch candidates had photometric measurements at five different epochs while no detection in the z PS1 band. This candidate is the high-redshift quasar presented in this work. The selected high-redshift quasar candidates were also checked for the counterparts in the Wide-Field Infrared Survey Explorer all-sky source catalogue 36 (AllWISE) within a match radius of 3 arcsec. However, none of them was detected in the WISE bands. Using this result, we place upper limits on their WISE W1 and W2 magnitudes to be W1 > 19.7 and W2 > 19.3 mag (i.e., fainter than the WISE W1 and W2 limiting magnitudes).
Spectroscopic follow-up. We performed simultaneous photometric and spectroscopic observations of twelve z > 6.5 quasar candidates with the Subaru Faint Object Camera And Spectrograph 37 (FOCAS) of the 8.2-m Subaru telescope. The observations were carried out on November 2, 2015. We used FOCAS long-slit mode, VPH900 grating and the SO58 order cut filter, giving us a wavelength coverage of 7500-10450 Å and a dispersion of 0.74 Å pixel −1 . The slit was 0.8 arcsec wide resulting in a spectroscopic resolution of R ~ 1500. The seeing during the observations varied between 0.26-0.48 arcsec. Prior to spectroscopy we took acquisition images of the candidates in the FOCAS Y band. The spectra were taken only for three candidates, two of which were reliably detected during acquisition. Out of these three targets, only one had the blue-end cutoff typical for high-redshift sources. We took five 1000s through-slit exposures of this target which was identified as a quasar based on its spectrum. The stacked spectrum of the quasar and its uncertainty were calculated using median combine of the individual exposures. The quasar spectrum was absolute flux calibrated using the spectrophotometric standard star BD + 28d4211 observed on the same night.

Redshift measurements.
To estimate the redshift of the quasar, we measured the redshifted positions of the NV, OI + SiII and CII emission lines at λ rest = 1239.85, 1305.42 and 1336.60 Å. The redshift was determined by calculating the cross-correlation function between region 9300-10200 Å of the quasar spectrum and the redshifted composite SDSS quasar spectrum 38 . The fitted wavelength range did not include the Lyα line. The best correlation with a correlation coefficient of 0.86 was achieved for a redshift of z = 6.618 ± 0.02.