GRB 150530A

S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 150530A (Swift-BAT trigger #642018:
Starling  et al., GCN 17888
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 ; Markwardt et al., GCN 17890
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 ;
Fermi GBM detection: Yu, Younes and Meegan, GCN 17891
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 )
triggered Konus-Wind at T0=42136.529 s UT (11:42:16.529).

The burst light curve shows a single pulse lasting from  ~T0-4 s to ~T0+6 s.
The emission is seen up to ~1.2 MeV.

As observed by Konus-Wind, the burst
had a fluence of 2.54(-0.26,+0.28)x10^-6 erg/cm2,
and a 64-ms peak energy flux, measured from T0+0.082 s,
of 1.06(-0.43,+0.43)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 1.2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.63(-0.38,+0.46),
and Ep = 114(-17,+23) keV (chi2 = 59/58 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -3.07 (chi2 = 59/57 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150530_T42136/

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB),
Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino
Cucchiara (ORAU/GSFC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jos�� A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Neil Gehrels (GSFC),
and Harvey Moseley (GSFC) report:

We observed the field of GRB 150530A (Starling et al., GCN 17888
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 ) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional on
Sierra San Pedro M��rtir from 2015/05 31.32 to 2015/05 31.45 UTC (20.08
to 23.21 hours after the BAT trigger), obtaining a total of 2.49 hours
exposure in the r, i, and z bands.

For a source within the Swift-XRT error circle, in comparison with 2MASS, we
obtain the following upper limits (3-sigma):

  r > 24.42
  i > 24.18
  z > 20.44

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), V.
D'Elia (ASDC) and R.L.C. Starling report on behalf of the Swift-XRT
team:

We have analysed 9.2 ks of XRT data for GRB 150530A (Starling et al.
GCN Circ. 17888
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 ), from 90 s to 22.6 ks after the  BAT trigger. The data
comprise 6 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Goad et al. (GCN Circ. 17889
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.98 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+/-0.5). The
best-fitting absorption column is  2.2 (+0.9, -0.8) x 10^22 cm^-2, in
excess of the Galactic value of 1.0 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.6 x 10^-11 (1.3 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+0.9, -0.8) x 10^22 cm^-2
Galactic foreground: 1.0 x 10^22 cm^-2
Excess significance: 2.7 sigma
Photon index:	     1.9 (+/-0.5)

If the light curve continues to decay with a power-law decay index of
0.98, the count rate at T+24 hours will be 1.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (2.2 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/00642018.

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

M. De Pasquale (UCL-MSSL) and R. L. C. Starling (U Leicester)report on 
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150530A 
106 s after the BAT trigger (Starling et al., GCN Circ. 17888
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 ).
No optical afterglow consistent with the XRT position (Goad et al.
GCN Circ. 17889
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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

white_FC           106          256          147 >20.1
u_FC               319          562          238 >19.2
white              106         5690          541 >21.0
v                 4465         6101          393 >19.3
b                 3849         5485          393 >20.4
u                  319         5280          435 >19.7
w1                4875         5075          197 >19.5
m2                4670         6306          393 >19.9
w2                4260         5896          393 >20.1

The magnitudes in the table are not corrected for the strong Galactic
extinction due to the reddening of E(B-V) = 1.81 in the direction of
the burst (Schlegel et al. 1998).

Yu H.-F. (MPE), Younes G. (USRA at GWU), and Meegan C. (UAH)
report on behalf of the Fermi GBM Team:

"At 11:42:15.61  UT, on 2015 May 30, the Fermi
Gamma-Ray Burst Monitor triggered and located GRB 150530A
(trigger 454678938 / 150530488) which was also detected
by Swift (Starling  et al. 2015, GCN 17888
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 ). The GBM on-ground
location is consistent with the Swift position. The angle from the
Fermi LAT boresight is about 46 degrees.

The GBM light curve consists of two peaks with a duration (T90) of
7 s (50-300 keV). The time-averaged spectrum from T0-4.6 s to
T0+6.7 s is well fit by a BAND function with Epeak = 126 +/- 14 keV,
alpha = -0.8 +/- 0.1, and beta = -2.4 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(4.1 +/- 0.2)E-06 erg/cm^2. The 1.024-sec peak photon
flux measured starting from T0+2.8 s in the 10-1000 keV
band is 6.7 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), R. L. C. Starling (U Leicester), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-61 to T+242 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 150530A (trigger #642018)
(Strling, et al., GCN Circ. 17888
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 ).  The BAT ground-calculated position is
RA, Dec = 327.511, 57.519 deg, which is 
   RA(J2000)  =  21h 50m 02.6s 
   Dec(J2000) = +57d 31' 07.4" 
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 17%.
 
The mask-weighted light curve shows a couple overlapping peaks starting
at ~T-10 sec, peaking at ~T+2 sec, and ending at ~T+40 sec.
T90 (15-350 keV) is 6.62 +- 0.84 sec (estimated error including systematics).
 
The time-averaged spectrum from T-4.21 to T+3.49 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.49 +- 0.11.  The fluence in the 15-150 keV band is 1.8 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+0.48 sec in the 15-150 keV band
is 4.1 +- 0.6 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/642018/BA/

M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 418 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 150530A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 327.51297, +57.51681 which is equivalent
to:

RA (J2000): 21h 50m 3.11s
Dec (J2000): +57d 31' 00.5"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

R. L. C. Starling (U Leicester), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 11:42:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150530A (trigger=642018).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 327.523, +57.490 which is 
   RA(J2000) = 21h 50m 06s
   Dec(J2000) = +57d 29' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak with
some structure with a duration of about 10 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV). 

The XRT began observing the field at 11:44:00.8 UT, 102.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 327.5109, 57.5164 which
is equivalent to:
   RA(J2000)  = 21h 50m 02.61s
   Dec(J2000) = +57d 30' 58.9"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 96 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 http://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.00
x 10^22 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 106 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain extinction expected. 

Burst Advocate for this burst is R. L. C. Starling (rlcs1 AT star.le.ac.uk). 
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/too.html.)