Gamma-ray Burst 150101B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #17267
  J. R. Cummings reports on behalf of the Swift-BAT science team:
  A short burst occurred during a Swift slew at 15:23 on Jan 1, 2015. It triggered Fermi GBM, trigger number 441818617. A significant source was found in ground analysis of BAT slew data at RA, Dec 188.044, -10.956, which is

     RA (J2000) 12h 32m 10.5s
     Dec (J2000) -10d 57' 21"
  

  with an estimated 90% containment radius of 2.5 arcmin.
  The lightcurve shows a single peak with a T90 of 0.018 +- 0.006 seconds. There appears to be significant spectral lag.
  The spectrum seen in BAT is soft. The best fit function is a blackbody spectrum with kT = 9.6 +- 1.5 keV. A simple powerlaw fit has a photon index of 3.3 +- 0.5. The blackbody fit gives a fluence between 15-150 keV in 0.1 seconds of (2.3 +- 0.6) x 10^-8 ergs/cm^2. Errors are 90% confidence.
  A Swift TOO request has been submitted in order to determine the nature of the source.
• GCN Circular #17268
  J. R. Cummings, M. Gropp (PSU), K. L. Page (U. Leicester), M. Siegel (PSU)
  We report on Swift followup observations of the source of the ground-detected short, soft burst reported on Jan 1, 2015 by Cummings (GCN Circ. 17267).
  We have analysed 3.3 ks of XRT data from 139.2 ks to 155.5 ks after the burst. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the BAT error circle. Using 2057 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 = 188.02119, -10.93389 which is equivalent to:

  RA (J2000): 12h 32m 05.09s
  Dec(J2000): -10d 56' 02.0"
  

  with an uncertainty of 2.4 arcsec (radius, 90% confidence). This position is 111 arcsec from the BAT position. The source has a mean count rate of 1.4e-02 ct/sec; we cannot determine at the present time whether it is fading.
  The spectrum is not well-determined. However, the best fit results from the data collected so far are: Gamma = 2.5 ± 0.5, with no evidence for NH above the Galactic value of 3.5e20 cm^-2.
  The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020464.
  There is an optical source in the DSS consistent with the XRT source. The source is detected in UVOT with a white magnitude of 18.44 +- 03. It shows no significant variability over the 15 ks spanned by the observations.
  A followup Swift TOO has been requested with the target number 20464, in order to observe any fading and to refine the spectrum.
• ATEL #6871
  Swift J123205.1-105602: Swift observations of a short, soft burst source
  A short burst occurred during a Swift slew at 15:23 on Jan 1, 2015. It triggered Fermi GBM, trigger number 441818617. A significant source was found in ground analysis of BAT slew data at RA, Dec 188.044, -10.956, which is

  RA (J2000): 12h 32m 10.5s
  Dec (J2000): -10d 57' 21"
  

  with an estimated 90% containment radius of 2.5 arcmin. The lightcurve shows a single peak with a T90 of 0.018 +- 0.006 seconds. There appears to be significant spectral lag. The spectrum seen in BAT is soft. The best fit function is a blackbody spectrum with kT = 9.6 +- 1.5 keV. A simple powerlaw fit has a photon index of 3.3 +- 0.5. The blackbody fit gives a fluence between 15-150 keV in 0.1 seconds of (2.3 +- 0.6) x 10^-8 ergs/cm^2. Errors are 90% confidence.
  We have analysed 3.3 ks of XRT data from 139.2 ks to 155.5 ks after the burst. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the BAT error circle, which we designate Swift J123205.1-105602. Using 2057 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 = 188.02119, -10.93389 which is equivalent to:

  RA (J2000): 12h 32m 05.09s
  Dec(J2000): -10d 56' 02.0"
  

  with an uncertainty of 2.4 arcsec (radius, 90% confidence). This position is 111 arcsec from the BAT position. The source has a mean count rate of 1.4e-02 ct/sec; we cannot determine at the present time whether it is fading.
  The spectrum is not well-determined. However, the best fit results from the data collected so far are: Gamma = 2.5 +- 0.5, with no evidence for NH above the Galactic value of 3.5e20 cm^-2.
  There is an optical source in the DSS consistent with the XRT source. The source is detected in UVOT with a white magnitude of 18.44 +- 03. It shows no significant variability over the 15 ks spanned by the observations.
  A followup Swift TOO has been requested with the target number 20464, in order to observe any fading and to refine the spectrum.
  XRT automated analysis: http://www.swift.ac.uk/xrt_products/00020464
• GCN Circular #17271
  W. Fong (U. Arizona), B. J. Shappee (Carnegie) and E. Berger (Harvard) report:
  "We observed the location of the short-duration soft burst, Swift J123205.1-105602, detected by Fermi/GBM (Trigger no. 441818617) and Swift/BAT (Cummings; GCN 17267) with the Inamori Magellan Areal Camera and Spectrograph (IMACS) mounted on the Magellan/Baade 6.5-m telescope. We obtained 8x150-sec of r-band observations at a mid-time of 2015 Jan 03.297 UT (1.66 days post-burst) in 0.8" seeing at an airmass of 1.6. Within the XRT position (Cummings et al.; GCN 17268), we detect a faint point-like optical source located at

  RA(J2000) = 12:32:05.09
  Dec(J2000) = -10:56:03.0
   

  with an uncertainty of 1" in each coordinate (registered to 2MASS). We note that this candidate optical afterglow is situated ~3" southeast from the center of a bright catalogued galaxy 2MASX J12320498-1056010, also detected by Swift/UVOT (Cummings et al.; GCN 17268).
  Further observations are planned to determine the nature of this source."
• GCN Circular #17276
  Matthew Stanbro (UAH) reports on behalf of the Fermi GBM Team:
  "At 15:23:34.47 UT on 01 January 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150101B (trigger 441818617 / 150101641). which was also detected by the BAT during a slew (Cummings et al. 2015, GCN 17267). The GBM on-ground location is consistent with the Swift/BAT position.
  The angle from the Fermi LAT boresight is 44 degrees.
  The GBM light curve consists of one peak with a duration (T90) of about 0.08 s (50-300 keV). The time-averaged spectrum from T0-0.016 s to T0+0.064 s is best fit by a simple power law function with index -1.70 +/- 0.09.
  The event fluence (10-1000 keV) in this time interval is (1.09 +/- 0.14)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0+0.00 s in the 8-1000 keV band is 10.48 +/- 1.35 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #17278
  A. J. Castro-Tirado (IAA-CSIC, ISA-UMA), R. Sanchez-Ramirez, J. Gorosabel (IAA-CSIC, UPV/EHU) and R. Scarpa (GTC), on behalf of a larger collaboration, report:
  Following the detection of the short-duration GRB 150101B by Swift/BAT (Cummings et al. 2015, GCNC 17267) and Fermi (Stanbro et al. GCNC 17276), we have obtained an optical spectrum with the 10.4 m GTC (+OSIRIS) starting on Jan 4, 5:41 UT (i.e. 38.1 hr postburst), covering the 3700-10000 A wavelength range, under poor weather conditions (passing clouds) and bad (>2.5=E2=80=9D) seeing. The slit included the position of the proposed optical afterglow (Fong et al. GCNC 17271) as well as the putative host galaxy also detected by Swift/UVOT (Cummings et al. GCNC 17258). We detect a faint emission line consistent with [OII] at z = 0.093, which we propose to be that of the potential host galaxy. If the association is confirmed, this would make of GRB 150101B the nearest short-duration GRB found to date. Observations at all wavelengths are encouraged.
• GCN Circular #17281 / ATEL 6873
  A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), K. Wiersema, N.R. Tanvir (U. Leicester) report on behalf of a larger collaboration:
  "We obtained imaging and spectroscopy of GRB 150101B/Swift J123205.1-105602 (Cummings et al. ATel 6871) with the ESO VLT on 4 Jan 2015, beginning at approximately 06:40 UT. At this epoch 1280s of I-band and1080s of z-band observations were obtained with FORS2 in ~1" seeing. The known galaxy, 2MASX J12320498-1056010, is clearly detected in these images, and its light extends under the position of the candidate optical counterpart reported by Fong et al. (GCN 17271). We do not detect any point source at this location, although the combination of differing seeing, band and the underlying host galaxy mean that we cannot make any strong statements about the variability of the source. We additionally note that the 90% confidence of the refined XRT position (http://www.swift.ac.uk/xrt_live_cat/00020464/) currently includes the nucleus of 2MASX J12320498-1056010, but not the source identified by Fong et al.
  In addition we obtained spectroscopic observations of 2MASX J12320498-1056010, covering the wavelength range 5300-8500A. The continuum is well detected across this wavelength range, with several prominent absorption features, most notably Mg b, and Na D at a preliminary redshift of z=0.134. We do not see any emission lines, although H-beta, [OIII] (4959,5007) and H-alpha all fall within our wavelength range at this redshift. We are not sensitive to the line identified by Castro-Tirado et al. (GCN 17278) as [OII] at z=0.093 since it lies blueward of our spectrum, but note that z=0.093 is not consistent with the observed absorption features. The absence of emission lines implies that the galaxy is a passive system. With an absolute magnitude of M_K ~ -25.5 (Vega), it lies the bright end of the luminosity function.
  The properties of the galaxy are broadly similar to those of some short-GRBs (most notably GRB 050509B, Gehrels et al. 2005 Nature 437 851; Fong et al. 2013 ApJ 769 56). At z=0.134 the isotropic energy release, E_iso, would be approximately 1e48 erg, again in the energy range seen for other short-GRBs. However, the soft gamma-ray spectrum of the prompt emission is unusual, as is the long lived X-ray emission of the potential counterpart. The X-rays persist for several days post burst, at a luminosity of approximately 2e43 ergs/s. This is typical of an AGN, but such bright sources are unusual in the small BAT error box. The log(N)-log(S) of Manners et al. (2003 MNRAS 343 293), predicts <10 sources of this flux or brighter per square degree (<~0.05 in a region the size of the BAT error box). Furthermore, the optical spectrum does not show obvious signs of AGN activity. Hence it is likely, but not yet conclusive, that the X-ray source, and galaxy are associated with GRB 15010! 1B/Swift J123205.1-105602.
  If associated with a short-GRB we might expect the development of kilonova emission over the next few days which may peak at a H-band magnitude of around H(AB)=22-23, if similar to that seen in GRB 130603B (Tanvir et al. 2013 Nature 500 547, Berger et al. 2013 ApJ 774 23). Alternatively, the soft gamma-ray emission, prolonged X-rays, and location close to the nucleus of its host galaxy could be indicative of an unusual AGN outburst or tidal disruption flare.
  We thank Dimitri Mawet and the staff of Paranal for their excellent assistance with these observations."
• GCN Circular #17285 / ATEL 6881
  W. Fong (U. Arizona), E. Berger (Harvard) and B. J. Shappee (Carnegie) report:
  "We re-observed the location of the short soft GRB 150101B (Cummings; GCN 17267) with Magellan/IMACS starting on 2015 Jan 04.30 UT (2.66 days post-burst and 24.0 hr after our initial observations; Fong et al., GCN 17271). We obtained 10x120-sec of r-band observations in 0.82" seeing at an airmass of 1.5. Digital image subtraction with our first set of observations using the ISIS package reveals no variable sources within or near the location of the XRT source ( http://www.swift.ac.uk/xrt_positions/00020464/). This indicates that the unresolved optical source identified by Fong et al., (GCN 17271) is unrelated to the GRB.
  Using the redshift of z=0.134 (Levan et al. GCN 17281) for the galaxy 2MASX J12320498-1056010 which coincides with the XRT position, we find that the XRT source has an X-ray luminosity of LX ~ 3e43 erg/s, and remains steady from 1.7 to 4.0 days post-burst (http://www.swift.ac.uk/xrt_curves/00020464/). The galaxy also coincides with a bright radio source in the NRAO VLA Sky Survey (Condon et al., 1998, AJ, 115, 1693), with a luminosity at 1.4 GHz of nuLnu ~ 6e39 erg/s. If due to star formation, this requires SFR ~ 150 solar masses per year, which contradicts the lack of emission lines from the galaxy (Levan et al. GCN 17281). On the other hand, the radio and X-ray luminosities match the properties of low-luminosity AGN with Lbol/LEdd < 10^-3 (e.g., Ho 2008, ARA&A, 46, 475). This interpretation is also supported by the large NIR luminosity and very red UV-NIR color (from GALEX and 2MASS). We therefore conclude that 2MASX J12320498-1056010 is a low-luminosity AGN, and that the XRT source is unrelated to GRB 150101B."
• GCN Circular #17286 / ATEL 6883
  A.J. van der Horst (Univ. of Amsterdam), K. Wiersema (Univ. of Leicester), A.J. Levan (Univ. of Warwick), J. Hjorth (DARK/NBI), N.R. Tanvir (Univ. of Leicester) report on behalf of a large collaboration:
  "We observed the position of GRB 150101B/Swift J123205.1-105602 at 4.9 GHz with the Westerbork Synthesis Radio Telescope at January 7 00.30 to 09.15 UT, i.e. 5.38 - 5.74 days after the burst (GCN 17267). We detect two radio sources within the BAT error circle (GCN 17267), with flux densities of 7.21 +/- 0.07 and 0.99 +/- 0.18 mJy. The brightest source coincides with the XRT position (GCN 17268), the position of the galaxy 2MASX J12320498-1056010 (GCN 17271), and the NRAO VLA Sky Survey source at 1.4 GHz (GCN 17285). The spectral index from 1.4 to 4.9 GHz of this source is -0.3, consistent with the radio emission emanating from an AGN (GCN 17285).
  The second source is located at

  RA (J2000) = 12:32:08.2
  Dec (J2000) = -10:56:14
  

  with an uncertainty of 5", which is large because of its low declination. There is no counterpart of this source in a stacked XRT image, no optical counterpart in our VLT observation (GCN 17281), and also not at 1.4 GHz in the NRAO VLA Sky Survey.
  We would like to thank the WSRT staff for scheduling and obtaining these observations."
• GCN Circular #17288 / ATEL 6890
  W. Fong (U. Arizona) reports on behalf of a larger collaboration:
  "We observed the field of the short/soft GRB 150101B (Cummings; GCN 17267) with the Karl G. Jansky Very Large Array (VLA) beginning on 2015 Jan 07.378 UT (5.73 days post-burst) at a mean frequency of 9.8 GHz. These observations are contemporaneous with the WSRT observations at 4.9 GHz (van der Horst et al.; GCN 17286) and cover the BAT position (90% containment). In 1 hour of observations, we detect two sources within the BAT position also identified by WSRT.
  The first source coincides with the galaxy 2MASX J12320498-1056010, the XRT position (Cummings et al.; GCN 17268) and the bright source at 4.9 GHz (van der Horst et al.; GCN 17286). The 9.8 GHz flux density is 3.15 +/- 0.02 milliJy which translates to nuLnu ~ 1.5e40 erg/s (at z=0.134; Levan et al.; GCN 17281), consistent with the interpretation that this source is an AGN (Fong et al.; GCN 17285).
  The second, fainter source is located at:

  RA(J2000) = 12:32:08.19
  Dec(J2000) = -10:56:14.2
  

  with an uncertainty of 1" in each coordinate, and coincides with the WSRT position (van der Horst et al.; GCN 17286). This source has a 9.8 GHz flux density of 0.45 +/- 0.01 milliJy. Assuming a single power law between 4.9 and 9.8 GHz, we calculate a spectral index of -1.13 +/- 0.25 for this source. We note that there is no optical source at this position in our Magellan r-band observations (Fong et al.; GCN 17285) or in deeper r-band imaging we obtained with Gemini-South/GMOS (17x90-sec) at 5.67 days post-burst.
  We do not detect any other radio sources within the BAT position to a 3-sigma limit of 27 microJy.
  We thank the VLA staff for quickly executing these observations."
• GCN Circular #17289 / ATEL 6893
  E. Troja (NASA/GSFC/UMCP), T. Sakamoto (AGU), A. Lien (NASA/GSFC/UMBC), S. B. Cenko (NASA/GSFC), and N. Gehrels (NASA/GSFC) report:
  We observed the field of the transient Swift J123205.1-105602 (Cummings; GCN 17267) with the Chandra X-ray Observatory beginning on 2015 Jan 09.48 UT (7.83 days post-burst) for a total exposure of 14.9 ks. At the transient position we detect two X-ray sources within the Swift/XRT point spread function:
  SRCX #1 The first source is located at:

  RA(J2000) =3D 12:32:04.985
  Dec(J2000) =3D -10:56:00.21
  

  with a 90% uncertainty of 0.6 arcsec. This position is consistent with the XRT position (http://www.swift.ac.uk/xrt_positions/00020464/), the bright radio source reported by van der Horst et al. (GCN 17286), and the nucleus of 2MASX J12320498-1056010. The source is well described by an absorbed power-law with photon index Gamma=3D2.3 =C2=B1 0.2, and absorption column consistent with the Galactic value of 3E20 cm^-2 (Kalberla et al. 2005). The observed flux in the 0.5-8.0 keV energy band is (4.0 =C2=B1 0.4)E-13 erg cm^-2 s^-1.
  SRCX #2 The second, fainter source is located at:

  RA(J2000) =3D 12:32:05.104
  Dec(J2000) =3D -10:56:02.78
  

  with a 90% uncertainty of 0.6 arcsec. This position is consistent with the optical source reported by Fong et al. (GCN 17271). The source is well described by an absorbed power-law with photon index Gamma=3D1.8 =C2=B1 0.3, and absorption column consistent with the Galactic value of 3E20 cm^-2 (Kalberla et al. 2005). The observed flux in the 0.5-8.0 keV energy band is (1.2 =C2=B1 0.2)E-13 erg cm^-2 s^-1.
  Both sources do not display significant temporal variability during our observation.
  We do not detect any X-ray counterpart at the position of the second radio source reported by van der Horst et al. (GCN 17286) down to a limit of 3E-4 cts s^-1 (95% confidence level).
  We thank Belinda Wilkes and the entire Chandra staff for approving and rapidly executing this observation.
• GCN Circular #17309 / ATEL 6955
  A.J. van der Horst (George Washington Univ.), A.J. Levan (Univ. of Warwick), K. Wiersema, N.R. Tanvir (Univ. of Leicester), J. Hjorth (DARK/NBI) report on behalf of a large collaboration:
  "We reobserved the field of GRB 150101B/Swift J123205.1-105602 at 4.9 GHz with the Westerbork Synthesis Radio Telescope (WSRT) at January 9 00.25 to 09.00 UT, and January 17 23.75 to January 18 08.50 UT, i.e. 7.37 - 7.73 and 16.35 - 16.71 days after the burst (GCN 17267), respectively. The two radio sources within the BAT error circle (GCN 17267) that were detected in the first WSRT observation (GCN 17286) do not display significant variability. The flux density of the brightest source is 7.21 +/- 0.07, 7.56 +/- 0.07, and 7.40 +/- 0.07, in the three epochs, which is consistent with a steady source when a 5% calibration uncertainty is taken into account. The second source has a flux density of 0.99 +/- 0.18, 0.77 +/- 0.19, and 0.78 +/- 0.18 mJy, in the three epochs, again consistent with a steady source.
  In addition, we obtained IR observations with VLT/HAWK-I on January 16, deeper than the ones reported in GCN 17281. There is still no evidence for any source at the location of the second, fainter radio source to a preliminary limiting magnitude of H(AB)>23.5.
  We would like to thank the WSRT staff for scheduling and obtaining these observations."
• GCN Circular #17318 / ATEL 6961
  S. Campana (INAF-Osservatorio astronomico di Brera) reports
  XMM-Newton observed GRB 150101B/Swift J123205.1-10560 (Cummings 2015, GCN 17267) on Jan 07, 2015 10:13:31 UT (5.79 d after the burst discovery). The last ~16 ks of the observation were affected by a mildly-enhanced background and were filtered out. The resulting EPIC/pn exposure times is 33.2 ks.
  A source is well detected at a position consistent with the Swift's (Cummings et al. 2015, GCN 17268), Chandra's (Troja et al. 2015, GCN 17289), and radio (Fong 2015, GCN 17288) ones. No X-ray source is detected at the position of a second radio source (Fong 2015), whereas the two closeby sources detected by Chandra are too close to be separated by XMM-Newton. The source is relatively bright with a pn count rate of (2.8+/-0.1)x10^{-1} cts/s. We extracted 8620 source photons from the pn and fitted the X-ray spectrum with a power law model including a non-negligible absorption component at the galaxy redshift (z=0.134, Levan et al 2015, GCN 17281) in addition to the Galactic one (3.5x10^{20} cm^{-2}). The best fit power law (chi2=1.03 with 244 degrees of freedom) implies a photon index of Gamma=2.29+/-0.06 (90% c.l. for one parameter of interest) and an additional column density marginally not consistent with zero NH_z=(1.3+/-0.9)x10^{20} cm^{-2} (90% c.l.). Any unresolved iron emission line (6.4-6.9 keV interval) should have an equivalent width <230 eV. The 0.5-8 keV unabsorbed flux is (4.3+/-0.1)x10^{-13} erg/cm^2/s, fully consistent with the with the Chandra observation (Troja et al 2015).
  This testifies that we are observing emission from a low luminosity Active Galactic Nucleus (2x10^{43} erg/s) and not from the GRB afterglow.
• GCN Circular #17321
  A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), N.R. Tanvir, K. Wiersema (U. Leicester), A.J. van der Horst (George Washington University) report on behalf of a larger collaboration:
  "We obtained further deep optical observations of the field of GRB 150101B/Swift J123205.1-105602 (Cummings; GCN17267, 17268) using the VLT and FORS2, on the 19 & 20 Jan 2015. At this epoch 1200s of observations were taken in both g, R and I, in dark time with seeing between 0.6-0.8".
  Within these deep images we locate a faint counterpart to the radio source identified by van der Horst et al. (GCN 17286) and Fong et al. (GCN 17288). It appears to be a marginally extended source with R~25 (calibrated against USNO), and to be relatively blue given its non-detection in the IR. However, its persistent radio emission implies it is likely not related to the outburst.
  We do not see any object at the location of the putative optical counterpart identified in Magellan imaging by Fong et al. (GCN 17271), whose location is consistent with the presence of an X-ray source with L_X ~ 1e43 ergs/s in Chandra observations (Troja et al. GCN 17289). Given our detection of the possible radio counterpart above, we conclude that these images are deeper than those reported previously by Fong et al. (GCN 17271, GCN 17288). Although the filters are slightly different, this may suggest optical fading of this source, which would strengthen its association with GRB 150101B.
  We further note that this position clearly lies on the stellar field of the galaxy 2MASX J12320498-1056010. Such luminous, non-nuclear X-ray sources are extremely rare, and while a background AGN is possible, the small impact parameter makes this unlikely within the limited BAT error circle. The luminosity is much higher than for most short-GRB afterglows at this epoch, and there is apparently little evidence for X-ray variability (although this is complicated by the presence of the nearby AGN which is a factor ~4 brighter, Troja et al. GCN 17289). Hence, while we can not yet make firm claims about the relationship of the Magellan and Chandra source to GRB 150101B/ Swift J123205.1-105602, it remains a plausible candidate. "
• GCN Circular #17326
  P. D'Avanzo (INAF-OAB), V. D'Elia (INAF/OAR & ASI/ASDC), V. Lorenzi, G. Mainella, W. Boschin, A. Garcia de Gurtubai Escudero (INAF-TNG) report on behalf of a larger collaboration:
  We observed the field of GRB 150101B/Swift J123205.1-105602 (Cummings GCN 17267; Stanbro GCN 17276) using the NICS near-infrared camera on the 3.6m TNG Telescope (La Palma, Canary Islands).
  We obtained two epochs of imaging observations in the J band on Jan 11 and Jan 16 at mean times of 9.62 and 14.56 days after the trigger, respectively.
  In both epochs we covered about 50% of the the BAT error circle (Cummings GCN 17267) including the positions of the X-ray, optical and radio sources reported by Cummings et al. (GCN 17268), Fong et al. (GCN 17271), van der Horst et al. (GCN 17286), Fong (GCN 17288), Troja et al (GCN 17289), Campana (GCN 17318) and Levan et al. (GCN 17321). Digital image subtraction between these two epochs carried out with the ISIS package (Alard 2000, A&AS, 144, 363) reveals no variable source within the field of view. The 3sigma limiting magnitude is J > 20.8 for epoch #1 and J > 21.5 for epoch #2 (Vega system, calibrated against the 2MASS catalogue).
• GCN Circular #17333 / ATEL 6972
  W. Fong (U. Arizona), E. Berger (Harvard), D. Fox (PSU) and B.J. Shappee (Carnegie) report:
  "We re-observed the field of the short/soft GRB 150101B (Cummings, GCN 17267) with the Gemini Multi-Object Spectrograph (GMOS) mounted on the Gemini-South 8-m telescope starting on 2015 Jan 12.326 UT, 10.7 days after the BAT trigger and 9.0 days after our initial Magellan observations (Fong et al., GCN 17271). We obtained a total of 1710-sec of r-band imaging in 0.58" seeing. To assess fading of the candidate optical afterglow (Fong et al., GCN 17271), we performed digital image subtraction using the ISIS package between the Gemini and Magellan observations. We find a residual point source with a refined position of

  RA(J2000) = 12:32:05.08
  Dec(J2000) = -10:56:03.16
  

  with an uncertainty of 0.5" in each coordinate.
  This indicates that the optical source has faded between 1.7 and 10.7 days post-burst. Thus far, this is the only evidence for a fading source within the BAT position. Fading is also supported by the lack of optical counterpart in other filters from late-time VLT observations (Levan et al., GCN 17321). We note that digital image subtraction between our two Magellan epochs at 1.7 days and 2.7 days post-burst revealed no residuals (Fong et al., GCN 17285), suggesting that the source had an initial period of shallow decay.
  The optical source is ~3.1" offset from the center of the galaxy 2MASX J12320498-1056010. Given the separation and optical brightness (Cummings et al., GCN 17268), the galaxy has a low probability of chance coincidence of ~2e-3. At the redshift of the galaxy, z=0.134 (Levan et al., GCN 17281), the projected physical offset is ~7.4 kpc. In addition, the location of the optical source is consistent with the position of the faint Chandra source (SRCX #2 in Troja et al., GCN 17289).
  We conclude that the fading of the optical source, spatial coincidence with an X-ray counterpart, and proximity to a galaxy with low probability of chance coincidence confirms the source as the optical afterglow, and 2MASX J12320498-1056010 as the host galaxy of GRB 150101B. At z=0.134, this is the among the lowest confirmed redshifts for a short GRB to date."
• GCN Circular #17431
  A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), N.R. Tanvir (U. Leicester), A.J. van der Horst (GWU) report on behalf of a larger collaboration:
  "We obtained a second epoch of observations of the very short GRB 150101B/ Swift J123205.1-105602 (Cummings et al. GCN 17267) with Chandra. Observations began on 10 Feb 2015, 39 days after the burst, and 32 days after the first epoch of observations (Troja et al. GCN 17289). As in the first epoch of observations a total of ~15 ks of observations were obtained with the target placed on the ACIS-S3 chip.
  In these observations we clearly identify the two sources identified in previous observations. In a 1.5" aperture to avoid contamination of the sources with light from each other, we finds count rates for source SRCX1 and SRCX2 as identified by Troja et al. (GCN 17289) to be SRCX1=(35.8 +/- 1.6)e-3 and SRCX2=(1.5 +/- 0.3)e-3 respectively. This compares to SRCX1= (38.3 +/- 1.6)e-3 cps and SRCX2=(9.4 +/- 0.9)e-3 cps in the first epoch. This implies that the AGN has remained constant, and that the second source, coincident with the optical afterglow (Fong et al. GCN 17333, Levan et al. 17321) has faded by a factor of ~6.
  The X-ray variability observed from source SRCX2 confirms it as the afterglow of GRB 150101B/Swift J123205.1-105602. The corresponding decay index is approximately t^-1.1, this is relatively slow for a short GRB afterglow so late after burst, and at z=0.134 (Levan et al. GCN 17281) places it in a region of parameter space much more luminous than most short bursts of comparable prompt fluence at similar times.
  We thank Belinda Wilkes, Scott Wolk at the team at the Chandra X-ray Observatory for their help with these observations."
• 1606.00140 from 2 Jun 16
  Chen Xie et al.: On the Host Galaxy of GRB 150101B and the Associated Active Galactic Nucleus
  We present a multi-wavelength analysis of the host galaxy of short-duration gamma-ray burst (GRB) 150101B. Follow-up optical and X-ray observations suggested that the host galaxy, 2MASX J12320498-1056010, likely harbors a low-luminosity active galactic nuclei (AGN). Our modeling of the spectral energy distribution (SED) has confirmed the nature of the AGN, making it the first reported GRB host that contains an AGN. We have also found the host galaxy is a massive elliptical galaxy with stellar population of $\sim 5.7\ Gyr$, one of the oldest among the short-duration GRB hosts. Our analysis suggests that the host galaxy can be classified as an X-ray bright, optically normal galaxy (XBONG), and the central AGN is likely dominated by a radiatively inefficient accretion flow (RIAF). Our work explores interesting connection that may exist between GRB and AGN activities of the host galaxy, which can help understand the host environment of the GRB events and the roles of AGN feedback.
• 1608.08626 from 1 Sep 16
  Wen-fai Fong et al.: The Afterglow and Early-Type Host Galaxy of the Short GRB 150101B at z=0.1343
  We present the discovery of the X-ray and optical afterglows of the short-duration GRB 150101B, pinpointing the event to an early-type host galaxy at z=0.1343 +/- 0.0030. This makes GRB 150101B the most nearby short GRB with an early-type host galaxy discovered to date. Fitting the spectral energy distribution of the host galaxy results in an inferred stellar mass of ~7x10^10 M_sol, stellar population age of ~2-2.5 Gyr, and star formation rate of <0.4 M_sol yr^-1. The host of GRB 150101B is one of the largest and most luminous short GRB host galaxies, with a B-band luminosity of ~4.3L* and half-light radius of ~8 kpc. GRB 150101B is located at a projected distance of 7.35 +/- 0.07 kpc from its host center, and lies on a faint region of its host rest-frame optical light. Its location, combined with the lack of associated supernova, is consistent with a NS-NS/NS-BH merger progenitor. From modeling the evolution of the broad-band afterglow, we calculate isotropic-equivalent gamma-ray and kinetic energies of ~1.3x10^49 erg and ~(6-14)x10^51 erg, respectively, a circumburst density of ~(0.8-4)x10^-5 cm^-3, and a jet opening angle of >9 deg. Using observations extending to ~30 days, we place upper limits of <(2-4)x10^41 erg s^-1 on associated kilonova emission. We compare searches following previous short GRBs to existing kilonova models, and demonstrate the difficulty of performing effective kilonova searches from cosmological short GRBs using current ground-based facilities. We show that at the Advanced LIGO/VIRGO horizon distance of 200 Mpc, searches reaching depths of ~23-24 AB mag are necessary to probe a meaningful range of kilonova models.
• GCN Circular #22889
  FROM: Carlo Ferrigno at IAAT/ISDC
  J. Rodi, A. Bazzano, P. Ubertini (IAPS-Roma) E. Bozzo, C. Ferrigno, V. Savchenko (ISDC, University of Geneva, CH) E. Kuulkers (ESTEC/ESA, The Netherlands) D. Gotz (DRF/Irfu/DAp Saclay/CEA) L. Hanlon, A. Martin-Carrillo (UCD, Ireland) J. M. Mas-Hesse (CSIC-INTA, Spain) S. Mereghetti (INAF IASF-Milano, Italy) L. Natalucci (IAPS-Roma) A. Lutovinov, R. Sunyaev (IKI, Russia) report on behalf of the INTEGRAL Gravitational Wave Team
  We have analyzed INTEGRAL archival data of the SPI-ACS and IBIS/PICsIT in coincidence with GRB150101B (GCN 17267, GCN 17276). Troja et al. (2018, arXiv:1806.10624) suggest that GRB150101B is a kilonova event comparable to GW170817/GRB170817A, but at cosmological distance and without the observations of a gravitational-wave trigger. The INTEGRAL orientation was 24.9 degrees from the GRB location and implies a somewhat suppressed response for SPI-ACS, but an improved response for IBIS, especially IBIS/PICsIT (Savchenko et al. 2017, A&A 603, A46).
  We confirm the independent detection by SPI-ACS and IBIS/PICsIT of a short duration (~0.012 sec) event, consistent with that reported in Troja et al. The signal in SPI-ACS (75 keV-10 MeV) and in IBIS/PICsIT (200 keV-1.2 MeV) has a S/N of 5.1, and 4.2, respectively.
  From the SPI-ACS observation, we estimate a 75 keV-2 MeV fluence of GRB150101B in the time interval T_0-0.05s - T_0+0.1s of (1.3 =B1 0.3)e-7 erg/cm2, assuming a simple power-law spectrum with a slope of 1.2 (as measured by Swift/BAT and Fermi/GBM).
  Analysis is on-going to constrain any possible soft gamma-ray afterglow with contemporaneous INTEGRAL observations.
  A plot of the light curve can be found at https://zenodo.org/record/1304812#.WzyLLa14VGw