GRB 210515C

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), J. A. Kennea (PSU),
K. L. Page (U Leicester), B. Sbarufatti (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:02:01 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a GRB 210515C (trigger=1048618). 
The BAT on-board calculated location is RA, Dec 1.050, -15.011 which is 
  RA(J2000) = 00h 04m 12.00s,
  Dec(J2000) = -15d 00' 39.6"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). 
There is no lightcurve currently available to estimate the duration. 

Using promptly downlinked data we find an uncatalogued X-ray source
located at RA, Dec 1.09221, -14.95325 which is equivalent to:
   RA(J2000)  = 00h 04m 22.13s
   Dec(J2000) = -14d 57' 11.7"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at https://www.swift.ac.uk/sper. No spectrum from the
promptly downlinked event data is yet available to determine the column
density. 

No UVOT data are available at this time. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT inaf.it). 
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/)

D. M. Palmer (LANL), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), A. D'Ai (INAF-IASFPA),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
T. Sakamoto (AGU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL) (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 210515C (trigger #1048618)
(D'Ai et al., GCN Circ. 30020
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 ).  The BAT ground-calculated position is
RA, Dec = 1.106, -14.972 deg which is
   RA(J2000)  =  00h 04m 25.4s
   Dec(J2000) = -14d 58' 18.2"
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a weak pulse that starts at
~T-3 s, peaks at ~T+1 s, and ends at ~T+11 s, with some tail emission
lasting until ~T+60 s. T90 (15-350 keV) is 55.7 +- 26.7 sec (estimated
error including systematics).

The time-averaged spectrum from T-2.53 to T+56.90 sec is best fit by
a simple power-law model.  The power law index of the time-averaged
spectrum is 1.26 +- 0.28.  The fluence in the 15-150 keV band is
5.9 +- 1.1 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from
T+0.84 sec in the 15-150 keV band is 0.6 +- 0.2 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/1048618/BA/

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), A.P.
Beardmore (U. Leicester) and A. D'Ai report on behalf of the Swift-XRT
team:

We have analysed 7.0 ks of XRT data for GRB 210515C (D'Ai et al. GCN
Circ. 30020
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 ), from 76 s to 29.8 ks after the  BAT trigger. The data
comprise 98 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 1614 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 =
1.09417, -14.95334 which is equivalent to:

RA (J2000): 00h 04m 22.60s
Dec(J2000): -14d 57' 12.0"

with an uncertainty of 2.3 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.63 (+0.13, -0.12), followed by a break at T+853 s to
an alpha of 0.57 (+/-0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.06 (+0.24, -0.21). The
best-fitting absorption column is  1.7 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.11 (+0.26, -0.23)
and a best-fitting absorption column of 6.4 (+5.7, -3.8) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.1 x 10^-11 (3.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.4 (+5.7, -3.8) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.11 (+0.26, -0.23)

If the light curve continues to decay with a power-law decay index of
0.57, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.4 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/01048618.

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

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (U. of the Virgin Islands/College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB 210515C (D'Ai, et al., GCN 30020
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 ) with the LCO 1-m Sinistro instrument at the Cerro Tololo Interamerican Observatory, Chile site, on May 16, from 09:37 to 09:51 UT (corresponding to 13.58 to 13.82 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x130s exposures in R and I. We do not detect an uncatalogued optical source in the coadded images in the XRT enhanced error region (Osborne, et al., GCN 30023
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 ).  Using the USNO-B.1 catalog as reference, we calculate the following upper limits:

R>21.47

I>20.62

These magnitudes are not corrected for galactic extinction.

R.S. is funded by NSF AST grant #1831682

GRB 210515C: Swift/UVOT Upper Limits

S. J. LaPorte (PSU) and A. D'Ai (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210515C
40 s after the BAT trigger (A. D'Ai et al., GCN Circ. 30020
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al. GCN Circ. 30023
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 )
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

b                      486            851            58      >18.60
uvm2                758           5041         216       >18.99
u                      231           5285         311       >19.33
v                      561           4836         235       >18.60
uvw1                610           5246         235        >18.89
uvw2              4432           4631         196        >18.90
white                 72             942         271        >20.12


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.011 in the direction of the burst
(Schlegel et al. 1998).