GRB 230506B

S. Dichiara (PSU), C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (U
Tampa), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and M. A.
Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory
Team:

At 12:45:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230506B (trigger=1167266).  Swift slewed immediately to the
burst.
The BAT on-board calculated location is
RA, Dec 326.993, +44.540 which is
   RA(J2000) = 21h 47m 58.32s
   Dec(J2000) = +44d 32' 24.00"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  No BAT light-curve is available at this time.

The XRT began observing the field at 12:47:12 UT, 114 seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with an enhanced position: RA, Dec 326.97876,
44.51108 which is equivalent to:
   RA(J2000)  = 21h 47m 54.90s
   Dec(J2000) = +44d 30' 39.9"
with an uncertainty of 6 arcseconds (radius, 90% containment). This
location is 110 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.

UVOT prompt data are not available at this time.  We are waiting for
the full dataset to search for an UVOT counterpart.

Due to a communications issue, no realtime alerts were received for this
GRB. Therefore, results in this GCN are based on rapidly reported results,
and will be refined in a future GCN Circular.

Burst Advocate for this burst is S. Dichiara (sbd5667 AT psu.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

V.Lipunov, P.Balanutsa, A.Kuznetsov, N.Tiurina, O.Gress, E.Gorbovskoy, G.Antipov,K.Zhirkov, D.Vlasenko,
Ya.Kechin, A.Chasovnikov,V.Senik, V.Topolev, Yu.Tselik, Siyu Wu, V.Vladimirov, D.Cheryasov (Lomonosov Moscow State University, SAI,Physics Department),
C.Francile, F. Podesta, C.Lopez, R. Podesta (Observatorio Astronomico Felix Aguilar OAFA),
D.A.H.Buckley (SAAO),
R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.Corella,L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory),
N.M.Budnev, O.Ershova (ISU,API),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational State University),
K.Vetrov, V.Shumkov, T.Pogrosheva, E.Minkina, O.Ershova (Lomonosov MSU)

MASTER-Amur robotic telescope  (Global MASTER-Net http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Blagoveshchensk State Pedagogical University) was pointed to the SWIFT GRB230506.53 (trigger No 1167266,21h 47m 54.90s , +44d 30m 39.9s, R=0.1) errorbox  6276 sec after trigger time at 2023-05-06 14:29:54 UT, so as alert didn't come from GCN socket system.

The observations began at zenith distance = 72 deg. The sun  altitude  is -22.3 deg.The galactic latitude b = -7 deg., longitude l = 92 deg.


Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2214809

We obtained following upper limits.

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

    6367 |         MASTER-Amur |   C |   180 | 16.8 |
    6567 |         MASTER-Amur |   C |   180 | 17.2 |
    6767 |         MASTER-Amur |   C |   180 | 17.3 |
    7037 |         MASTER-Amur |   C |   720 | 18.0 |  Coadd
    6966 |         MASTER-Amur |   C |   180 | 17.3 |
    7166 |         MASTER-Amur |   C |   180 | 17.4 |
    7366 |         MASTER-Amur |   C |   180 | 17.4 |
Filter C is a clear (unfiltred) band.

Observations and analysis will be  continued.
The message may be cited.

S. Dichiara (PSU), C. Gronwall (PSU), J. A. Kennea (PSU),
A. Y. Lien (U Tampa), K. L. Page (U Leicester),
B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 12:45:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230506B (trigger=1167266).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 326.993, +44.540 which is 
   RA(J2000) = 21h 47m 58.32s
   Dec(J2000) = +44d 32' 24.00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  No BAT light-curve is available at this time. 

The XRT began observing the field at 12:47:12 UT, 114 seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with an enhanced position: RA, Dec 326.97876,
44.51108 which is equivalent to:
   RA(J2000)  = 21h 47m 54.90s
   Dec(J2000) = +44d 30' 39.9"
with an uncertainty of 6 arcseconds (radius, 90% containment). This
location is 110 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

UVOT prompt data are not available at this time.  We are waiting for
the full dataset to search for an UVOT counterpart. 

Due to a communications issue, no realtime alerts were received for this GRB. 
Therefore, results in this GCN are based on rapidly reported results, 
and will be refined in a future GCN Circular. 

Burst Advocate for this burst is S. Dichiara (sbd5667 AT psu.edu). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

Paul Kuin (MSSL/UCL) and Simone Dichiara(PSU) report on behalf
of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230506B
114 s after the BAT trigger (Dichiara et al., GCN Circ. 33727
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 ).
A candidate afterglow consistent with the XRT position is
detected in the initial UVOT exposures.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white              113          271          147          20.19 +/- 0.24
white              614          807           38         >19.8
u                  334          583          247         >19.9
u                10511        11899          738          20.66 +/- 0.30
b                  589          783           39         >19.1
uvw1               714          734           20         >17.6
uvm2               688          708           20         >17.2
uvw2               639          658           20         >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.293 in the direction of the burst
(Schlegel et al. 1998).

[GCN OPS NOTE: Corrected title from 230606B to 230506B at author's request.]

D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), V. D'Elia (SSDC & INAF-OAR), B.
Sbarufatti (INAF-OAB), A. D'Ai (INAF-IASFPA), J. D. Gropp (PSU), S.
Dichiara (PSU) report on behalf of the Swift-XRT team:

We have analysed 7.9 ks of XRT data for GRB 230506B, from 68 s to 50.9
ks after the  BAT trigger. The data comprise 124 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 335 s of PC mode data and
1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 326.98168, +44.51111 which is equivalent to:

RA (J2000): 21h 47m 55.60s
Dec(J2000): +44d 30' 40.0"

with an uncertainty of 2.6 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=1.83 (+0.16, -0.17), followed by a break at T+146 s to
an alpha of 4.87 (+0.29, -0.24).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.45 (+/-0.13). The
best-fitting absorption column is  5.8 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 3.8 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.6 x 10^-11 (7.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     5.8 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^21 cm^-2
Excess significance: 3.0 sigma
Photon index:	     1.45 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
4.87, the count rate at T+24 hours will be 5.0 x 10^-13 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-23 (3.6 x 10^-23) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01167266.

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

R. Strausbaugh (University of Minnesota), A. Cucchiara (NASA) report on
behalf of a larger collaboration:

We observed the GRB 230506B (Dichiara et al., GCN 33727
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 ) field with the
LCOGT 1-meter Sinistro instrument at the Teide Observatory, Tenerife site,
on May 7, from 04:09 to 04:41 UT (corresponding to 15.43 to 15.93 hours
from the GRB trigger time) with the SDSS r and i filters.

We performed a series of 3x300s exposures in each band.  We do not detect
an uncatalouged source within the Swift-XRT error region in either band.

The following upper limits are calculated using the PanSTARRS catalog as
reference:

r > 22.3
i > 21.8

These magnitudes are not corrected for galactic extinction.

S. D. Barthelmy (GSFC), S. Dichiara (PSU),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC),
T. Sakamoto (AGU), M. Stamatikos (OSU)
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 230506B (trigger #1167266)
(Dichiara et al., GCN Circ. 33727
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 ).  The BAT ground-calculated position is
RA, Dec = 326.986, 44.514 deg which is
   RA(J2000)  =  21h 47m 56.6s
   Dec(J2000) = +44d 30' 50.9"
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 73%.

The mask-weighted light curve shows a multi-peaked structure that
starts at ~T0 and ends at ~T+80 s. The main peak occurs at ~T+11 s.
T90 (15-350 keV) is 61.54 +- 10.89 sec (estimated error
including systematics).

The time-averaged spectrum from T+0.02 to T+76.30 sec is best fit
by a simple power-law model.  The power law index of the time-averaged
spectrum is 1.41 +- 0.11.  The fluence in the 15-150 keV band
is 2.9 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured
from T+10.60 sec in the 15-150 keV band is 4.6 +- 0.4 ph/cm2/sec.
All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/1167266/BA/

Paul Kuin (MSSL/UCL) reports on behalf of the Swift/UVOT team:

The XRT position of GRB230506B falls on a Gaia DR3 source of magnitude
BP = 19.89, Gmag = 19.34. We see from further analysis of the UVOT data
that within errors the detections are consistent with the Gaia source.
The UVOT data are therefore not the optical afterglow of the GRB.

O.J. Roberts (USRA/NASA-MSFC), S. Bala (USRA), C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 12:45:18.77 UT on 06 May 2023, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230506B (trigger 705069923 / 230506531), which was
also detected by the Swift/BAT (S. Dichiara et al. 2023, GCN 33729
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  and 
S.D. Barthelmy et al. 2023, GCN 33742
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 ). The GBM on-ground location is 
consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 47 degrees.

The GBM light curve consists of multiple short bursts
with a duration (T90) of about 38 s (50-300 keV).
The time-averaged spectrum from T0-0.8 s to T0+39.2 s is best fit by
a power law function with an exponential
high-energy cutoff. The power law index is -1.28 +/- 0.04 and
the cutoff energy, parameterized as Epeak, is 1767 +/- 576 keV

The event fluence (10-1000 keV) in this time interval is
(1.13 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+10.4 s in the 10-1000 keV band
is 8.1 +/- 0.3 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/"

B. O'Connor (GWU), S. Dichiara (PSU), E. Troja (UTV), S. B. Cenko (NASA-GSFC, UMD), C. Kouveliotou (GWU), T. Sakamoto (AGU), A. Kutyrev (UMD), G. Ryan (Perimeter), R. Ricci (INAF), G. Bruni (INAF):

We carried out two Chandra ToO observation of GRB 230506B beginning on May 11, 2023 at 06:59:55 UT and June 01, 2023 at 19:31:19 UT with ACIS-S as part of program 24400201 (PI: O'Connor) for a total of 19 ks and 55 ks, respectively. The initial observation began ~4.8 days after the GRB trigger (Dichiara et al. GCN 33727
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 ). 

Within the XRT localization (Burrows et al. GCN 33737
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 ), we detect a single point source at RA, DEC = 21:47:55.6, 44:30:41.6 with an uncertainty of 0.9" (90% confidence, statistical + systematic). The source displays rapid fading between the two Chandra epochs confirming that it is the X-ray afterglow of GRB 230506B.

We would like to thank Patrick Slane, Harvey Tananbaum, Dan Schwartz, Scott Wolk, 
and the CXO staff for rapidly approving and planning this observation.

K. Noonan (UVI), P. Gokuldass (ERAU), N. Orange (OrangeWave Innovative Science, LLC), D. Morris (NASA), K. Smith (UVI), R. Querrard (UVI), T. Lombardi (Eckerd College)

We observed the field of GRB230506B (Dichiara et al. GCN 33727
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 ) with the 0.5m Virgin Island Robotic Telescope (VIRT) at the University of the Virgin Islands' Etelman Observatory on 05-07-2023 starting at 05:20:40.065 (T+16.33 hrs). We performed a series of exposures in R filter with a total exposure of 3860 s. The weather conditions were clear during the hours of observation with an average airmass of ~2.6.

We do not detect an uncatalogued source within the Swift-XRT error region and report the following 3-sigma upper limit: 

T_mid     ||Exposure ||Filter ||Limit 
T+17 hrs ||3860s     ||R        ||>20.1 

The limit is estimated from comparison to nearby USNO B1 stars and is not corrected for Galactic extinction. The VIRT is still in the commissioning phase. 

We acknowledge financial support from NASA MUREP MIRO award 80NSSC21M0001, NASA EPSCoR award 80NSSC22M0063, and NSF EiR award 1901296. R.Q. and N.B.O. also acknowledge financial support from South Carolina Space Grant award 80NSSC20M0054. This message can be cited.