Finding high-redshift gamma-ray bursts in combined near-infrared and optical surveys S. Campana1 G. Ghirlanda12 R. Salvaterra3 O.A. Gonzalez4 M. Landoni1 G. Pariani1 A.

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Finding high-redshift gamma-ray bursts in combined

near-infrared and optical surveys

S. Campana1, G. Ghirlanda1,2, R. Salvaterra3, O.A. Gonzalez4, M. Landoni1, G. Pariani1, A.

Riva5, M. Riva1, S.J. Smartt6, N.R. Tanvir7, S.D. Vergani8

1INAF - Osservatorio astronomico di Brera, Via E. Bianchi 46, I-23807, Merate (LC), Italia

2INFN – Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italia

3INAF - Istituto di Astroﬁsica Spaziale e Fisica cosmica, via Alfonso Corti 12, I-20133 Milano,

Italia

4STFC UK Astronomy Technology Centre, The Royal Observatory Edinburgh, EH9 3HJ, Edin-

burgh, UK

5INAF - Osservatorio astroﬁsico di Torino, Strada Osservatorio 20, I-10025, Pino Torinese (TO),

Italia

6Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast,

Belfast BT7 1NN, UK

7School of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH,

8GEPI, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen, F-92190, Meudon,

France

The race for the most distant object in the Universe has been played by long-duration gamma-

ray bursts (GRBs), star-forming galaxies and quasars. GRBs took a temporary lead with the dis-

covery of GRB 090423 at a redshift z= 8.21,2, but now the record-holder is the galaxy GN-z11

at z= 11.03,*. However, galaxies and quasars are very faint (GN-z11 has a magnitude H= 26),

hampering the study of the physical properties of the primordial Universe. On the other hand, GRB

afterglows are brighter by a factor of >

∼100, but with the drawback of being transients, and lasting

only for 1–2 days. When bright (e.g., H= 18.6 mag at 7 hr from the explosion as observed for

GRB 0904231,2), GRB spectroscopic afterglow studies could provide independent constraints on

the reionisation history, on the ionising photon escape fraction and on the cosmic history of metal

enrichment. GRBs could be used to select high-zgalaxies independently from their brightness,

thus sampling the galaxy luminosity function below the sensitivity limit of deep surveys. They

also are expected to provide important information about the nature of the stellar population in

the ﬁrst galaxies by, e.g., identifying explosions from the ﬁrst population of metal-free stars (the

so-called PopIII stars), showing the presence of their distinctive metal enrichment signature and

measuring the stellar initial mass function evolution at early epochs.

The major limitation is that, with current instrumentation, high-redshift (z>

∼6) GRBs are

*The recently released James Webb Space Telescope ﬁrst images have revealed a number of candidates in z∼

9−15 (e.g., [4]).

arXiv:2210.09749v1 [astro-ph.HE] 18 Oct 2022

extremely rare, with just 9 events recognised in 17.5 years of Swift observations5. This is mainly

due to the limited sensitivity and ﬁeld of view of current (the Neil Gehrels Swift observatory)

and soon to be launched (Space-based multi-band astronomical Variable Objects Monitor, SVOM)

space missions, and to the relatively faint nature of high-redshift GRBs. It is also likely that some

of these high-redshift GRBs are lost during the, sometimes patchy, follow-up process. After the

GRB alert sometimes the afterglow is followed up only at optical wavelengths or the telescopes

for the near-infrared (nIR) spectroscopic follow-up are not available or weather conditions do not

allow for rapid observations.

Here we describe a novel approach to the discovery of high-redshift (z>

∼6) GRBs, exploit-

ing their nIR emission properties. The afterglows of high-redshift GRBs are naturally absorbed,

like any other source, at optical wavelengths by hydrogen along the line of sight in the intergalactic

medium (Lyman-αabsorption). We propose to take advantage of the deep monitoring of the sky by

the Vera Rubin Observatory6, to simultaneously observe exactly the same ﬁelds with a dedicated

nIR facility. By comparing the two streams of transients, one can pinpoint transients detected in

the nIR band and not in the optical band. Fast transients detected only in the nIR and with an AB

colour index r−H>

∼3.5 are high-redshift GRBs, with a low contamination rate (see Box).

In order to estimate the number of nIR high-redshift GRBs, we carried out simulations using

a population synthesis code7. A population of long GRBs is simulated based on their luminosity

function (i.e., the number of sources as a function of their luminosity or energy) and their formation

rate as a function of redshift (i.e., cosmic rate density). The free parameters of these functions have

been derived by reproducing the observed properties of well selected samples of GRBs collected

in the past 15 years. In particular, the population is calibrated by reproducing the ﬂuence, peak

ﬂux, observer frame peak energy and observer-frame duration distribution of GRBs detected by

Fermi and Swift. In order to minimise the impact of observational biases, which always play a role

in GRB studies, a ﬂux-limited, 97% complete in redshift, sub-sample of bright GRBs detected by

Swift8was used. This sample provides constraints on the GRB rate density. Long-duration GRBs

are associated with the core-collapse of massive stars, thus indicating they might be good tracer of

the star formation. However, theoretical arguments indicate that GRBs require a low-metal content

in the progenitor9, resulting in an increase of GRBs with redshift over the nominal star formation

rate. This has been addressed including a GRB density evolution with redshift7,8.

In addition to the GRB prompt properties, the population code simulates the afterglow emis-

sion based on the model by Ryan et al.10 for a ﬁreball decelerating in a constant density external

medium (see Table 1 for the adopted parameter values). The afterglow luminosity depends on

the kinetic energy of the jet and on the shock efﬁciencies in amplifying the magnetic ﬁeld and

accelerating the emitting particles and it has been demonstrated that there is a close relationship

between the afterglow luminosity and the prompt-emission high-energy luminosity11. This code

has been adopted for rate estimates for the Transient High-Energy Sky and Early Universe Sur-

veyor (THESEUS)12 and the Gamow Explorer13.

Having set the parameters for a MonteCarlo simulation, we run the code asking what is the

rate of GRBs as a function of redshift for a given limiting magnitude and at a given time after the

GRB event. This is shown in Fig. 1 (top panel) for H < 21 (AB magnitude system). The decay of

the GRB rate as a function of redshift for z > 5follows a power-law evolution ∝z−3.4 (solid line

in Fig. 1 - top panel). Owing to the power-law temporal decay of the afterglow light curve, the data

points sampled at different epochs follow a similar z-decay. The dependence of the rate of high-

redshift (z>

∼6) GRBs on the limiting Hmagnitude is shown in Fig. 1 (bottom panel). We estimate

that about 1/3 of high-redshift Swift GRBs are missed during the follow-up process. We also note

that the population sampled by nIR observations is somewhat different from the one sampled by

current high-energy instruments. Fig. 2 shows the contour levels in plane of the prompt emission

isotropic GRB energy Eiso versus redshift for the simulated long GRB population detectable with

H < 21. For comparison the nine long GRBs known with redshift z > 6are shown (star symbols).

The nIR detection samples relatively less energetic events with respect to those currently detected

by, e.g., Swift and in the near future SVOM.

The high-znIR detection rates are indeed very promising. The only problem is identifying

these transients as GRB afterglows. Lyman-αabsorption comes to the rescue, heavily absorbing

radiation at optical wavelengths for high-redshift objects. If we can simultaneously observe in the

optical and in nIR bands, transients detected only at longer wavelengths are natural candidates to

be high-zGRBs. We can make these candidates stronger by increasing the colour index, i.e., the

magnitude difference between optical and nIR, ensuring the optical observations are sufﬁciently

deep.

In the next few years, we will have of the most formidable optical survey machine: the Vera

Rubin Observatory6. The Rubin Observatory will carry out the Legacy Survey of Space and Time

(LSST), reaching a limiting magnitude r∼24.5 in 30 s exposures (AB magnitude system, single

visit, 5σ). Here we propose the construction of a new nIR facility to observe in tandem with

the Rubin LSST to unveil the high-redshift Universe through the identiﬁcation of GRBs. To be

conservative, we consider a telescope and nIR camera able to reach magnitude H= 21 in 30

s, with the same ﬁeld of view as the Rubin telescope (9.6 deg2). This will allow us to look for

transients with r−H>

∼3.5, enough to assure that the transients are at high redshift14,15,1,2. Then,

we assume 10 hr observations per night, 80% of good weather nights, 30 s exposures, and 73%

open shutter time as a fraction of the observing time16. Based on these metrics, we estimate that

we would detect ∼11.0 GRBs per year at z>

∼6. The corresponding rate at z>

∼10 is ∼2.7 GRBs

per year.

A nIR telescope with the required sensitivity and ﬁeld of view does not currently exist. It

should have the same ﬁeld of view of the Rubin telescope (9.6 deg2) and should reach magnitude

H= 21 in 30 s (in two sub-exposures of ∼15 s). The system should be optimised to observe

in the nIR to improve performance and minimise the telescope diameter. In Table 2, we provide

a summary of the main existing telescopes with nIR imaging instruments. It is readily apparent

that, if the system is not nIR optimised, an 8m-class telescope is needed. Using the Advanced

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Findinghigh-redshiftgamma-rayburstsincombinednear-infraredandopticalsurveysS.Campana1,G.Ghirlanda1,2,R.Salvaterra3,O.A.Gonzalez4,M.Landoni1,G.Pariani1,A.Riva5,M.Riva1,S.J.Smartt6,N.R.Tanvir7,S.D.Vergani81INAF-OsservatorioastronomicodiBrera,ViaE.Bianchi46,I-23807,Merate(LC),Italia2INFN–SezionediMilan...

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Finding high-redshift gamma-ray bursts in combined near-infrared and optical surveys S. Campana1 G. Ghirlanda12 R. Salvaterra3 O.A. Gonzalez4 M. Landoni1 G. Pariani1 A.

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