Detection of short high-energy transients in the local universe with SVOM/ECLAIRs

Abstract

The coincidental detection of the gravitational wave event GW 170817 and the gamma-ray burst GRB 170817A marked the advent of multi-messenger astronomy and represented a milestone in the study of GRBs. Significant progress in this field is expected in the coming years with the increased sensitivity of gravitational waves detectors and the launch of new facilities for the high-energy survey of the sky. In this context, the launch of SVOM in mid-2022, with its two wide-field high-energy instruments ECLAIRs and GRM, will foster the possibilities of coincidental transient detection with gravitational waves and gamma-rays events. The purpose of this paper is to assess the ability of SVOM/ECLAIRs to detect and quickly characterize high-energy transients in the local Universe (z ≤ 0.3), and to discuss the contribution of this instrument to multi-messenger astronomy and to gamma-ray burst (GRB) astrophysics in the 2020’s. A list of local HE transients, along with their main characteristics, is constructed through an extensive literature survey. This list includes 41 transients: 24 long GRBs, 10 short GRBs and 7 SGR Giant Flares. The detectability of these transients with ECLAIRs is assessed with detailed simulations using tools developed for the SVOM mission, including a GEANT4 simulation of the energy response and a simulated trigger algorithm representative of the onboard trigger algorithm. SVOM/ECLAIRs would have been able to detect 88% of the short high-energy transients in our list: 22 out of 24 long GRBs, 8 out of 10 short GRBs and 6 out of 7 SGR Giant Flares. The SNR for almost all detections will be sufficiently high to allow the on-board ECLAIRs trigger algorithm to derive the localisation of the transient, transmitting it to the SVOM satellite and ground-based instruments. Coupled with the anti-solar pointing strategy of SVOM, this will enable an optimal follow-up of the events, allowing the observation of their afterglows, supernovae/kilonovae counterparts, and host galaxies. We conclude the paper with a discussion of the unique contribution expected from SVOM and of the possibility of simultaneous GW detection for each type of transient in our sample.

Appendices

Appendix A: Long GRBs

This section discusses the properties of the 24 long GRBs in our sample, starting with some comments about peculiar GRBs.

• GRB 150518A has only been detected by MAXI/GSC located on the ISS (Sakamoto et al. 2015) and partially by Konus-Wind (Golenetskii et al. 2015a). Its energy has not been measured by Konus-Wind because of a data gap during the main emitting episode. The light curve has been partially recorded by MAXI/GSC, but Konus-Wind observations seem to indicate that this burst was already emitting $\sim 250\ \mathrm {s}$ before MAXI/GSC observations, implying that the light curve and the GRB properties recovered by MAXI/GSC are only a fraction of an ultra-long GRB. For this reason, the simulations that have been made on this burst are incomplete and only account for the part visible by MAXI/GSC.

• GRB 190829A has two distinct peaks separated by ${\sim} 45\ \ \mathrm {s}$, with completely different spectra. The case is similar for GRB 180728A, with a precursor and then the main emission of the burst. Thus, the emission properties of those two GRBs have been divided into two parts.

Tables 1, Table 2 and Table 3 summarize the features detected for the long GRBs.

The population observed is diverse in terms of observed characteristics:

• Concerning the fluence, the minimum fluence observed is $1.0\times 10^{-8}\ \mathrm {erg\,cm^{-2}}$ for GRB 150818A while the maximum one is $1.63\times 10^{-4}\ \mathrm {erg\,cm^{-2}}$ for GRB 030329, with the median equal to $3.55\times 10^{-6}\ \mathrm {erg\,cm^{-2}}$.

• For the 1s peak flux, the minimum is $0.1^{+0.06}_{-0.03}\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GRB 100316D and the maximum is $451^{+25}_{-25}\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GRB 030329, with the median equal to $2.65 \pm 0.63\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GRB 060505.

• For the peak energy of the fluence spectrum, 13 out of the 30 long GRBs do not have any peak energy information. This is mainly because they have been observed with instruments such as Swift/BAT whose energy band is often too narrow to measure properly the spectrum peak energy. GRB 020903, GRB 120422A and GRB 040701 only have a lower estimation. For the remaining 14 long GRBs with peak energy, the minimum $E_{\mathrm{peak}}$ for long GRBs is equal to $4.9 \pm 0.4\ \mbox{keV}$ for GRB 060218 and the maximum equal to $302^{+214}_{-85}\ \mbox{keV}$ for the first part of GRB 060614, with a median of $68\ \mbox{keV}$ from GRB 030329.

• For $T_{\mathrm{90}}\$-values, the minimum value is $4.0\ \mathrm {s}$ for GRB 060505, the maximum $2100\pm 100\ \mathrm {s}$ for GRB 060218 and the median is $34\ \mathrm {s}$.

There are also some differences in the intrinsic properties:

• The closest long GRB that has been detected is GRB 980425, at a redshift of 0.0085 ($\approx 38\ \mbox{Mpc}$). The furthest one in our local GRB sample is at a redshift of $z = 0.297$ ($\approx 1590\ \mbox{Mpc}$), GRB 111225A. the median redshift of the sample is $z = 0.135$ ($\approx 660\ \mbox{Mpc}$).

• The minimum isotropic energy detected in the long GRB sample is $E_{\mathrm{iso}} = 5\times 10^{47}\ \mbox{erg}$ for GRB 111005A (Tanga et al. 2018), while the maximum isotropic energy is equal to $1.86\times 10^{52}\ \mbox{erg}$ for GRB 030329. The median value for the long GRBs is $E_{\mathrm{iso}} = 1.65\times 10^{50}\ \mbox{erg}$, significantly lower than the median $E_{\mathrm{iso}}$ of classical long GRBs observed at larger distances.

Based on the observations on Fig. 3, the 1s peak flux in the energy range 15–150 keV for the 2 closest long GRBs (GRB 980425 and GRB 111005A) is of the order of $1\ \mathrm {ph\,cm^{-2}\,s^{-1}}$, which is slightly lower than for classical GRBs (the 1s peak flux median in the Swift/BAT catalog is $\sim 1.4\ \mathrm {ph\,cm^{-2}\,s^{-1}}$, see Lien et al. 2016), while these low-energy GRBs are one order of magnitude closer. This suggests that these two GRBs are the tip of the iceberg of a much larger population (Soderberg et al. 2004b). Two long GRBs of the sample, GRB 040701 and GRB 150518A, have no peak flux mentioned in the literature and therefore do not appear in Fig. 3.

Appendix B: Short GRBs

Tables 4, 5 and 6 summarize the information regarding the 10 short GRBs in our sample.

Table 4 Detected features for Short GRBs

Table 5 Spectral properties of Short GRBs

Table 6 Intrinsic properties of Short GRBs

The detection of the kilonova AT2017gfo associated with GRB 170817A (Tanvir et al. 2017) was the first and only detection of a kilonova in real-time. However, by looking into the archive data, some short GRBs have been tentatively associated with a kilonova.

• GRB 050709 has an optical and infrared signal at $\mathrm{t} > 2.5$ days after the trigger, which could be dominated by a kilonova (Jin et al. 2016).

• For GRB 070809, Jin et al. (2020) has found a possible optical kilonova at $\mathrm{t}\sim 0.47$ d after the trigger.

• The work of Lamb et al. (2019) has enabled to find for GRB 160821B the best-sampled kilonova light curve, without having a gravitational wave trigger.

• Troja et al. (2018) associate GRB 150101B to an off-axis short GRB with a kilonova, similar to the well-known GRB 170817A.

The short GRBs in our sample have diverse prompt emission properties:

• Concerning the fluence, the minimum fluence observed is $9\times 10^{-9}\ \mathrm {erg\,cm^{-2}}$ for GRB 050509B while the maximum one is $5.33\times 10^{-6}\ \mathrm {erg\,cm^{-2}}$ for GRB 061201, with the median equal to $3.6\times 10^{-7}\ \mathrm {erg\,cm^{-2}}$.

• For the 64ms peak flux, the minimum is $3.21\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GRB 070809 and the maximum is $92.1\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GRB 050709, with the median equal to $7.74\ \mathrm {ph\,cm^{-2}\,s^{-1}}$.

• For the peak energy of the fluence spectrum, 6 out of the 10 short GRBs do not have any peak energy information. GRB 050709 only has a peak energy for the first part of the GRB, the second part of the burst being best fitted by a power-law (Villasenor et al. 2005). For the remaining 4 short GRBs with a peak energy, the minimum $E_{\mathrm{peak}}$ is equal to $84 \pm 19\ \mbox{keV}$ for GRB 160821B and the maximum equal to $302^{+214}_{-85}\ \mbox{keV}$ for the first part of GRB 060614, with a median of $68\ \mbox{keV}$ from GRB 030329.

• For $T_{\mathrm{90}}\$-values, the minimum value is $0.024 \ \mathrm {s}$ for GRB 050509B, the maximum $160 \ \mathrm {s}$ for GRB 050709 and the median is $0.90\ \mathrm {s}$.

There are also some differences in the intrinsic properties:

• The closest short GRB that has been detected is GRB 170817A, at a redshift of 0.0093 ($\approx 42\ \mbox{Mpc}$). The furthest one in our local GRB sample is at a redshift of $z = 0.287$ ($\approx 1530\ \mbox{Mpc}$), GRB 050724. the median redshift of the sample is $z = 0.160$ ($\approx 790\ \mbox{Mpc}$).

• The minimum isotropic energy detected in the short GRB sample is $E_{\mathrm{iso}} = 4\pm 1\times 10^{46}\ \mbox{erg}$ for GRB 170817A (Abbott et al. 2017a), while the maximum isotropic energy is equal to $7.65\times 10^{50}\ \mbox{erg}$ for GRB 050724. The median value for the short GRBs is $E_{\mathrm{iso}} = 8.6\times 10^{49}\ \mbox{erg}$: there are two orders of magnitude between GRB 170817A and the second dimmest short GRB, GRB 050509B.

Appendix C: SGR giant-flares

Tables 7, 8 and 9 summarize the information detected for the SGR giant flares.

• Concerning the fluence, the minimum fluence observed is $5.6\times 10^{-9}\ \mathrm {erg\,cm^{-2}}$ for GF 050906 while the maximum one is $0.61\ \mathrm {erg\,cm^{-2}}$ for GF 041227, with the median equal to $2.7\times 10^{-5}\ \mathrm {erg\,cm^{-2}}$.

• For the 64ms peak flux, the minimum is $1.95\,\mathrm {ph\,cm^{-2}\,s^{-1}}$ for GF 050906 and the maximum is $11.9{\times} 10^{6}\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GF 041227, with the median equal to $990\ \mathrm {ph\,cm^{-2}\,s^{-1}}$ for GF 070201.

• For the peak energy of the fluence spectrum, 6 out of the 7 SGR giant flares have spectra showing a peak energy. This is the consequence of the extremely high-luminosity of such events, in addition to the large diversity of instruments that have observed some of them (see Table 7). GF 050906 has a fluence model that is best fitted by a power law, mainly because it was a faint burst ($5.6 \times 10^{-9}\ \mathrm {erg\,cm^{-2}}$) only detected by Swift/BAT. The minimum $E_{\mathrm{peak}}$-value is equal to $240\ \mbox{keV}$ for GF 980827 with an OTTB spectrum, the maximum equal to $2080\ \mbox{keV}$ for GF 051103 with a Band spectrum, with a median of $548\ \mbox{keV}$.

  Table 7 Detected features for SGR Giant Flares

  Table 8 Spectral properties of SGR Giant Flare

• For $T_{\mathrm{90}}$-values, the minimum value is $0.1 \ \mathrm {s}$ for GF 051103, the maximum $0.310 \ \mathrm {s}$ for GF 980827 and the median is $0.2 \ \mathrm {s}$ for GF 200415A.

Table 9 Intrinsic properties of SGR Giant Flares

There are also some differences in the intrinsic properties:

• The closest Giant Flare detected is GF 041227. At an assumed distance of $8.7\ \mathrm{kpc}$, it comes from one of the two galactic SGRs that have emitted a giant flare (the other being GF 980827). GF 050906 is the furthest one at a distance of $130\ \mbox{Mpc}$. The median distance of the sample is $0.780\ \mbox{Mpc}$.

• The minimum isotropic energy detected for the giant flares is $E_{\mathrm{iso}} = 4.3\pm 1\times 10^{44}\ \mbox{erg}$ for GF 980827, while the maximum isotropic energy is equal to $8.0\times 10^{46}\ \mbox{erg}$ for GF 050906. The median value is $E_{\mathrm{iso}} = 1.2\times 10^{46}\ \mbox{erg}$ for GF 041227.

As it can be seen in Table 9, GF 050906 is an outlier among the giant flares, being detected at a distance that is forty times larger than the second most distant Giant Flare GF 051103. This will be discussed further in Sect. C.

Appendix D: Summary of SVOM/ECLAIRs characteristics

Table 10 Main characteristics and expected performance of SVOM/ECLAIRs

Appendix E: Signal-to-Noise Ratio for local high-energy transients

Table 11 SVOM/ECLAIRs Signal To Noise Ratio for Long GRBs

Table 12 SVOM/ECLAIRs Signal To Noise Ratio for Short GRBs

Table 13 SVOM/ECLAIRs Signal To Noise Ratio for SGR Giant Flares

Appendix F: Light curves used in the simulations

Fig. 11

light curves used to simulate the photon counts from GRBs in the movegrb-simulator program. The x-axis is in $\ \mathrm {s}$ while the y-axis is the expected $\mathrm{cnt}/\ \mathrm {s}$ per bin