GRB 240606B

James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC)  report:

Swift/BAT did not localize GRB 240606B onboard (T0: 2024-06-06T13:33:06.5 UTC, GECAM trigger 392).

The GECAM notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). 

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 4.096 s analysis time bin starting at T0 - 2.048 s with a sqrt(TS) of 28.9.
An arcminute localization is found with DeltaLLHOut of 29.3 and a DeltaLLHPeak of 17.9.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 40.234, -60.637 deg which is
   RA(J2000)  = 02h 40m 56.16s
   Dec(J2000) = -60d 38′ 13.2″
with an estimated uncertainty of 4 arcmin radius.

XRT and UVOT follow-up has been requested.
Results of follow-up observations will be reported in future circulars.

GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the GECAM GRB 240606B. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021695

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the GECAM event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

This circular is an official product of the Swift-XRT team.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), D.N.
Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), A.P. Beardmore (U.
Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Swift/BAT-GUANO-detected burst GRB 240606B, collecting 4.0 ks of Photon
Counting (PC) mode data between T0+22.9 ks and T0+35.1 ks. 

Four uncatalogued X-ray sources are detected consistent with being
within 493 arcsec of the BAT-GUANO position, of which one ("Source 1")
is fading with 2.1 sigma significance and is undetected by eRosita to
deeper upper limits. It is likely the afterglow.

Using 3507 s of PC mode data and 3 UVOT images, we find an enhanced XRT
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 40.29507, -60.61201 which is
equivalent to:

RA (J2000): 02h 41m 10.82s
Dec(J2000): -60d 36' 43.2"

with an uncertainty of 2.4 arcsec (radius, 90% confidence). This
position is 53 arcsec from the BAT-GUANO position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.2 (+0.9, -1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+0.4, -0.3). The
best-fitting absorption column is  6 (+11, -4) x 10^20 cm^-2,
consistent with the Galactic value of 2.4 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.1 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6 (+11, -4) x 10^20 cm^-2
Galactic foreground: 2.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.7 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x
10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021695.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021695.

This circular is an official product of the Swift-XRT team.

Yue Wang, Shao-Lin Xiong, Chen-Wei Wang report on behalf of the GECAM team:

GECAM-B was triggered both in-flight and on-ground by a long burst, GRB 240606B, at 2024-06-06T13:33:06.050 UTC (T0), which initialized Swift/BAT-GUANO observation (GCN # 36628).

According to the realtime alert data, the GECAM-B light curve shows a long pulse with a duration about ~10 sec (15-1020 keV). 

The time-averaged spectrum of GECAM-B realtime data from T0-2 to T0+2 s could be
adequately fit by a cut-off power-law with a flux about 6.67E-7 erg/cm^2/s in 20-1000 keV.

The GECAM light curve could be found here: 
https://twikinew.ihep.ac.cn/pubgecam/Sandbox/GRB/GRB240606B_lightcurve.png

The GECAM-B localization is consistent with the Swift/BAT localization within the error.
We note that these results are based on realtime alert data and thus very preliminary. Refined analysis will be reported later.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM) mission originally consists of two microsatellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

S. Bala (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 13:33:06.09 UT on 06 June 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240606B (trigger 739373591/240606565).
which was also detected by Swift/BAT-GUANO (DeLaunay et al. 2024, GCN 36628
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 ).
The Fermi GBM on-ground location is consistent with the Swift-BAT/GUANO position.

The angle from the Fermi LAT boresight is 141 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 11 s (50-300 keV). The time-averaged spectrum
from T0-6.9 to T0+8.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.2 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 310 +/- 60 keV.
A Band function fits the spectrum equally well
with Epeak= 238 +/- 75 keV, alpha = -1.11 +/- 0.16 and beta = -2.28 +/- 0.41.

The event fluence (10-1000 keV) in this time interval is
(6.4 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.51 s in the 10-1000 keV band is 7.9 +/- 0.7 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 240606B
(Swift/BAT-GUANO arcminute localization: DeLaunay et al., GCN 36628
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 ;
GECAM-B detection: Wang et al., GCN 36633
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 ;
Fermi-GBM detection: Bala & Meegan, GCN 36640
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 )
triggered Konus-Wind at T0=48787.583 s UT (13:33:07.583).

The burst light curve shows a multipeaked structure
which starts at ~T0-53.0 s and has a total duration of ~58.4 s.
The emission is seen up to ~4 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240606_T48787/

As observed by Konus-Wind, the burst
had a fluence of 5.07(-0.10,+1.54)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+0.318 s,
of 1.68(-0.34,+0.61)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

Since the bulk of the burst emission was detected before
the trigger, the spectral analysis was performed using
the KW 3-channel light curve data.

Modelling the KW 3-channel time-integrated spectrum
(measured from T0-53.014 s to T0+5.360 s)
by a power law with exponential cutoff (CPL) model,
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep),
yields alpha = -0.55(-0.83,+3.21) and Ep = 116(-21,+62) keV.

A CPL fit to the spectrum near the peak count rate,
measured from T0-0.400 s to T0+0.816 s, gives
alpha -1.20(-0.14,+0.14), and Ep = 475(-128,+239) keV.

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7-Dimensional Telescope collaboration

We searched for the optical counterpart of the GRB, GRB 240606B (DeLaunay et al., GCN #36628
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 ) using the 7-Dimensional Telescopes (7DT). Approximately 20.2 hours following the initial detection (2024-06-06T13:33:06.5 UTC), we targeted the localization center provided by the Swift XRT at RA, Dec = 40.29507 deg, -60.61201 deg with an uncertainty of 2.4 arcsecs (Osborne et al., GCN #36631
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 ). Observations were made with eleven 7DT units in eight medium-band filters, denoted as m450, m500, m550, m650, m700, m750, m800, and m850, in which the numeric values indicate their central wavelengths in nanometers. Each medium-band filter has a bandwidth of 25nm. 

No significant transient event was identified in the preliminary result. Photometric flux calibration was performed using synthetic photometries derived from the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. The 5-sigma upper limits (AB) range from 17.8 to 21.0 mag in the medium-band filters. To improve the depth for detection, we combined all images taken with the medium-band filters from m400 to m875. The combined image was treated as an r-band equivalent, and photometric measurements were performed. This approach yielded a 5-sigma upper limit of 20.9 AB magnitudes. Despite the increased depth, no significant transient detection was identified. Observations were conducted under suboptimal conditions, potentially limiting our search sensitivity.

Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
m450 2024-06-07T09:47:55 1200 20.7
m500 2024-06-07T09:47:23 1200 21.0
m550 2024-06-07T09:47:22 1200 20.1
m650 2024-06-07T09:46:09 1200 20.1
m700 2024-06-07T09:47:20 1200 19.6
m750 2024-06-07T09:47:16 1200 19.2
m800 2024-06-07T09:47:21 1200 18.0
m850 2024-06-07T09:47:18 1200 17.8

The 7-Dimensional Telescope (7DT), located in Chile and comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters, aims to detect optical counterparts of GW sources and conduct the 7-Dimensional Sky Survey (7DS) of the Southern Hemisphere. Further information about the 7DT is available at http://gwuniverse.snu.ac.kr/.