GRB 150301C

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),  W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (CPI), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), J. Tueller (GSFC), T. N. Ukwatta (LANL),
F. Marshall (NASA/GSFC), D. Malesani (DARK/NBI), L. Hagen (PSU),
M. de Pasquale (INAF-IASFPA), A. Maselli (INAF-IASFPA), D.N. Burrows (PSU),
K. Page (U. Leicester), N. P. M. Kuin (UCL-MSSL)

  
Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of possible GRB 150301C (trigger #633105)
(Lien, et al., GCN Circ. 17512
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 ).  The BAT ground-calculated position is
RA, Dec = 11.319, 41.861 deg which is
    RA(J2000)  =  00h 45m 16.6s
    Dec(J2000) = +41d 51' 40.0"
with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment).
The significance in the BAT image was 10.1, so we are sure that this was
a real astrophysical source.  The partial coding was 69%.

The location is well within the visible light of M31.  A more distant galaxy,
[JSD2012] 170, which has a size of 0.11 arcmin, is within the BAT error
circle.  In 1.9 ks of data, the Swift XRT found only a single very weak source
(2.8 sigma), consistent with the position of a known source within the BAT
error circle, [PFH2005] 622, with a count rate of (5.3+/-1.9) x 10^-03 c/s.  We
cannot state whether there is any variability of this source at this time.  The
XRT position of this source is:

RA (J2000)  00h 45m 14.4s
Dec (J2000) +41d 50' 37.7"

The BAT mask-weighted light curve shows a single weak FRED peak.  T90 (15-350 keV)
is 14 +- 3 sec (estimated error including systematics).
  
The time-averaged BAT spectrum from T-0.6 to T+15.0 sec is best fit by a power-
law model with an exponential cutoff.  The power law index of the time-averaged
spectrum is poorly defined but is nominally -0.8 +- 2.  The peak energy is
38 +- 7 keV.  The fluence in the 15-150 keV band is 2.1 +- 0.4 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+1.32 sec in the 15-150 keV band is
0.6 +- 0.2 ph/cm2/sec.  A simple power-law fit has a photon index of 1.96 +- 0.25.
All the quoted errors are at the 90% confidence level.

If this source was in M31, its isotropic energy output was 1.5 x 10^43 ergs
in 15.6 seconds in the range 15-150 keV.

It is very possible, given the softness of the source, that this is a non-GRB
in M31.  The color:duration is within the distribution of long GRBs.  No XRT
source was seen in the initial observations, whereas we might expect most
sources in M31 bright enough to be seen in BAT would be very bright in XRT.
Therefore the source is likely to be highly absorbed.  The results of the
batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/633105/BA/

The Swift/UVOT began settled observations of the field 102 s after
the BAT trigger (Lien et al., GCN Circ. 17512
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 ). No new source within
the BAT error circle 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
exposures are:

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

white              102         4151          369        >20.39
v                  592         4562          216        >18.55
b                  518         5241           96        >19.08
u                  261         5177          461        >19.69
uvw1               641         4973          216        >19.27
uvm2               617         4768          216        >19.20
uvw2               568         4357          216        >19.41

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.06 in the direction of the burst
(Schlegel et al. 1998).

Further Swift observations are planned.

[GCN OPS NOTE(01mar15): Per author's request, the affiliation was corrected for KP.]

Phil Evans (U. Leicester) , Alessandro Maselli (INAF-IASFPA), David Burrows (PSU), 
Sergio Campana (INAF-OAB), Neil Gehrels (GSFC), Jamie Kennea (PSU), 
Amy Lien (GSFC/UMBC), Lea Hagen (PSU), Daniele Malesani (DARK/NBI), 
Frank Marshall (GSFC), Julian Osborne (U. Leicester), Kim L. Page (U Leicester), 
Massimiliano De Pasquale (INAF-IASFPA) report on behalf of the Swift team:

We report further XRT observations of the highly unusual transient or GRB detected 
by BAT on March 1 ( trigger 633105 or GRB150301C; Lien et al., GCN Circ 17512
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 ; 
Stamatikos et al., GCN Circ 17516
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 ).  It lasted about 20 seconds in BAT and has a 
location in M31, but had very weak X-ray emission. The XRT found a single source 
within the BAT error circle, which is coincident with the known X-ray source [PFH2005] 622. 
This source was previously reported in an XMM-Newton survey of M31 (Pietsch et al., 
A&A, 434, 483, 2005), and was classified as a "hard" source based on the hardness ratio (see 
definition in Pietsch et al. 2005). The paper suggests that these "hard" sources  may be
X-ray binaries, Crab-like SNRs, or AGNs.

Using the faint-source-optimised detection system from the 1SXPS catalogue (Evans et al., 
ApJS 210, 8, 2014), the XRT source is detected with a likelihood of 18.3 in the first observation 
of the BAT error region. This corresponds to a "Good"-flagged object, i.e. a >3-sigma detection.

The refined XRT position is RA,Dec = 11.3107, +41.8431 degrees which corresponds to

RA (J2000):   00h 45m 14.57s
Dec (J2000): +41d 50' 35.1"

with an uncertainty of 5.7 arcsec (radius, 90% confidence). This is 4 arcsec from the catalogued 
position of [PFH2005] 622, which is at RA=00h 45m 14.87,Dec=+41d 50' 33'' with an uncertainty 
of 1.5 arcsec. Hence, the XRT position is consistent with the position of [PFH2005] 622.

During the first XRT observation, starting at T0+105s and lasting 1.9 ks, the source is detected with 
a count-rate of 6.0 (+2.3, -1.8) x10^-3 ct/sec. In the subsequent observations, from T0+44.9 ks and 
totalling 6ks exposure, the source is undetected, with an upper limit of 1.2 x10^-3 ct/sec, consistent 
with fading.

Swift-XRT has previously observed this location for a total of ~4ks, corresponding to "Stacked image 
7598" in the 1SXPS catalogue. The source was not detected in this dataset, with an upper limit of 
2.2x10^-3 ct/sec, significantly below the level at which we have detected it in this observation. 
Additionally, XMM-Newton observations (Pietsch et al., 2005) detected this X-ray source in 2005 
with a flux of 6.9x10^-15 erg/cm^2/s. The peak rate in our observations corresponds to 
~2x10^-13 erg/cm^2/s, i.e. ~35 times brighter than the XMM detection. At the distance of M31, 
these fluxes correspond to a luminosity of ~5x10^35 erg/s (0.2-4.5 keV) in the XMM-Newton observations, 
and ~1.4x10^37 erg/s (0.3-10 keV) in the XRT observations. For comparison, the average luminosity 
in BAT at this distance was 1.25x10^42 erg/s in 15-150 keV (Stamatikos et al., GCN Circ 17516
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 ).

We therefore conclude that XRT has detected a bursting source that is likely the counterpart to 
BAT trigger 633105.

L. M. Z. Hagen (PSU) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:


A point source is detected by UVOT that is consistent with the XRT
counterpart and 2.5 arcsec from the XMM-Newton source discussed in
Evans et al. (GCN Circ. 17536
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 ).  This source is also seen in archival
DSS imaging.  It is unclear if this is the same object detected by XRT
or a serendipitous source.  Given how crowded the field is, it is also
likely that the UVOT source is comprised of multiple individual sources.
The coordinates of the UVOT source are
   RA(J2000) = 00:45:14.76 = 11.31150
   DEC(J2000) = +41:50:35.2 = 41.84311

Preliminary detections and 3-sigma upper limits using the UVOT
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)
for the available exposures are:

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

white              102         4151          369         19.22+-0.13
v                  592         4562          216        >18.53
b                  518         5241           96        >19.08
u                  261         5177          461         18.60+-0.15
uvw1               641        79666         5117         18.64+-0.05
uvm2               617        45953         1237         18.94+-0.13
uvw2               568         4357          216         18.99+-0.27

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.06 in the direction
of the source (Schlegel et al. 1998).

This field has also been observed previously by UVOT as part of a
different observing campaign, primarily in 2013 and 2014. The
stacked images have the following detections:

Filter      Exp(s)        Mag

v            1406     19.04+-0.19
b            1445     19.77+-0.15
u           27521     18.65+-0.03
uvw1        52236     18.57+-0.02
uvm2        58908     18.65+-0.03
uvw2        65719     18.61+-0.02

These indicate possible variability of the source, but further
analysis will be required to confirm this.

Z. Cano (Univ. Iceland), D. Malesani, J. P. U. Fynbo (DARK/NBI),
A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), N. R. Tanvir (Univ.
Leicester),
P. Jakobsson (Univ. Iceland), M. Saajasto, T. Pursimo (NOT), report on
behalf of a larger collaboration:

We observed the field of BAT trigger 633105 (Lien et al., GCN Circ. 17512
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 )
with the
2.5-m Nordic Optical Telescope (NOT) equipped with ALFOSC on 01-Mar-2015 and
03-Mar-2015, obtaining 6x300 s of SDSS r-band exposures in each epoch.  The
mid
exposure times of our observations correspond to +0.60 and +2.59 days from
the
initial trigger, respectively.

Consistent with the X-ray position of the known source [PFH2005] 622
(thought to
be associated with the gamma-ray transient; Evans et al., GCN Circ. 17536
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 ),
we
detect one object at coordinates:

RA = 00:45:14.80
Dec = +41:50:32.35

Its magnitude is R =  22.7 +- 0.1 (using a USNO-A2.0 star in the field for
calibration). Two more objects are also detected in close proximity of the
XMM
position (Pietsch et al. 2005, A&A, 434, 483), and potentially consistent
with
it, including the source seen by UVOT by Hagen & Lien (GCN 17541
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 ), which
appears
to be quite blue (it has R = 22.3 +- 0.1 in our image).

We carried out digital image subtraction between our two images, where we
subtracted the first epoch from the second by using an adaptation of the
ISIS
software. In our difference image we do not detect any residual flux at the
combined XRT and XMM location, down to an upper limit of R > 23.2.

All objects seen in the NOT image are also apparent in the deep, archival
data
from the PAndAS survey (McConnachie et al. 2009, Nature, 461, 66), which
also reveals
a few more, fainter objects in the close proximity of the XMM position. No
object seems
to have varied significantly.

A gif animation of the two epochs of NOT imaging, and the difference image,
can
be retrieved from:
http://www.astro.ku.dk/~malesani/GRB/150301C/subtraction.gif

A comparison image of the NOT image from 01-Mar-2015 and the PAndAS image
can be found at:
http://www.astro.ku.dk/~malesani/GRB/150301C/comparison_NOT_PAndAS.png

Alessandro Maselli (INAF-IASFPA), Phil Evans (U. Leicester),
David Burrows (PSU), Sergio Campana (INAF-OAB),
Neil Gehrels (GSFC), Jamie Kennea (PSU), Amy Lien (GSFC/UMBC),
Daniele Malesani (DARK/NBI), Craig. B. Markwardt (GSFC),
Frank Marshall (GSFC), Julian Osborne (U. Leicester),
David. M. Palmer (LANL), Kim L. Page (U Leicester),
Rhaana L. C. Starling (U Leicester)
report on behalf of the Swift team:

Swift continues to observe the field of BAT trigger 633105
(aka GRB 150301C) with sky location in M31. To avoid further
naming confusion, this object will be referred as Swift J0045.2+4151
hereafter. The XRT continues to detect only a single source within
the BAT error circle at a position coincident with the previously
known XMM-Newton source, [PFH2005] 622 (Pietsch et al. 2005).
For the rest of this circular we assume that the X-rays detected by
XRT are from [PFH2005] 622.

The XRT has acquired four observations so far. The source
(which we presume is [PFH2005] 622) was detected in the first snapshot,
(T0+107s to T0+720s), in which we detect 8 events in 614 seconds,
with a mean background expectation of 0.34 counts.  This results in a rate
of 1.48 (+0.6, -0.5) e-2 counts/s (1 sigma errors).

In the 2nd epoch (T0+4 ks to T0+80 ks), we detect 3 events in 7.2 ks, with
an expected background of 1.25 events, giving a 3 sigma upper limit of
< 0.002 counts/s.

In the 3rd epoch (T0+171 ks to T0+206 ks), we find 5 events in 4.8 ks with a
mean background expectation of 0.64 events, giving a 3 sigma detection
of 1.2 (+0.8, -0.6) e-3 counts/s (1 sigma errors).

All measurements use a source radius of 9 pixels (21 arcseconds),
a background measurement from an annulus around the source region, and
the Bayesian method of Kraft, Burrows, and Nousek (1991, ApJ, 374, 344) to obtain
the 1-sigma confidence intervals (for detections) or 3-sigma upper limits (for non-detections).

Therefore, after a detection in the initial XRT observation, the source
faded below the XRT detection limit and may have brightened again
in the 3rd epoch.

We cannot conclude firmly at this point whether the source
[PFH2005] 622 is associated with the BAT trigger or not.
More observations at all wavelengths are encouraged.

A table and a light curve that summarize current XRT observations
of [PFH2005] 622 are available at:
http://swift.gsfc.nasa.gov/results/BATbursts/633105/xrt/atel_20150304.html

[GCN OPS NOTE(05mar15), Per author's request, the last line in the 5th paragraph
was changed from "the 3 sigma confidence regions and upper limits."
to "the 1-sigma confidence intervals (for detections) or 3-sigma upper limits (for non-detections).".
And the places in the 2nd & 4th paragraphs that said "(1 sigma errors)"
were changed to "(1 sigma errors)".]

T. Laskar, E. Berger (Harvard) and D. Fox (PSU) report on behalf of a larger
collaboration:

"We observed GRB 150301C (Lien et al; GCN 17512
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 ) with the VLA beginning on 2015
March 04.03 UT (2.77 days after the burst) at a mean frequency of 9.8 GHz.
No radio counterpart is detected in the 90-minute observation within the
XRT error circle (Evans et al; GCN 17536
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 ) to a 3 sigma upper limit of 18
uJy. The 90-minute observation covers the full BAT error circle and does not
contain any radio source brighter than the NVSS flux limit. Well-detected
sources fainter than the NVSS limit have been identified for possible
future monitoring. We thank the VLA staff for scheduling and obtaining
these observations."

Kari A. Frank (PSU), David N. Burrows (PSU), Alessandro Maselli (INAF-IASFPA), Phil Evans (U. Leicester), Chryssa Kouveliotou (GWU), Ersin Gogus (Sabanci Univ.), Daniele Malesani (DARK/NBI):


On Friday, March 6, 2014, beginning at 2015-03-06T16:29:31 (T0+468.4 ks relative to the Swift BAT trigger) we obtained a Chandra DDT observation at the position of the XMM-Newton source [PFH2005] 622 (Pietsch et al. 2005, ApJ 434, 483, ATel #7181), coincident with the BAT source SWIFT J0045.2+4151 (GCN #17512
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 , GCN #17516
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 , ATel #7166, ATel #7176).  Net exposure time was 9.94 ks. The source was detected at the > 10 sigma level, with a count rate of 0.0023 (-0.0011,+0.0012) counts/s in the 0.5-8 keV band (errors are 90% confidence).    

The Chandra source position is:

RA(J2000) = 00h 45m 14.78s
Dec(J2000) = +41d 50m 34.53s

with a 90% uncertainty of 0.8 arcsec.  A total of 23 source counts were detected. This position is consistent with the XMM position of [PFH2005] 622 and the XRT position (ATel #7166).

Assuming a powerlaw spectrum with photon index of 2, we find a flux 2.05e-14 (-0.97,+1.07) erg/cm^2/s (errors are 90% confidence).  This is about 10 times fainter than the flux measured in the first XRT observation (ATel #7166), and similar to the archival XMM fluxes for [PFH2005] 622 reported in ATel #7181, though it is brighter than the 6.9e-15 erg/cm2/s reported by Pietsch et al. (2005).

At the distance of M31, the observed flux corresponds to a luminosity of 9.5e35 erg/s.