Swift Observations of GRB 200125A

A.Y. Lien (GSFC/UMBC), A. Tohuvavohu (U. Toronto) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 11:31:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200125A (trigger=952164) (Lien et al. GCN Circ. 26876). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (5.3 hours West) and 84° from the 0%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

Melandri et al. (GCN Circ. 26887) reported the position from TNG for the optical afterglow of this GRB. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Markwardt et al. (GCN Circ. 26897), the BAT ground-calculated position is RA, Dec = 228.069, -8.142 deg which is RA(J2000) = 15^h12^m16.6^s Dec(J2000) = -08°08'31.3" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 42%.

The mask-weighted light curve (Figure 1) shows a multi-pulse structure that starts at ~T-40 s and ends at ~T+430 s. The three main pulses occurs at ~T₀, ~T+200 s, and ~T+300 s, respectively. The highest peak is at ~T+287 s. The burst went out of the BAT FOV at T+501 s. T₉₀ (15-350 keV) is 350.83 ± 34.03 s (estimated error including systematics).

The time-averaged spectrum from T-36.22 to T+427.74 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.95 ± 0.08. The fluence in the 15-150 keV band is 8.7 ± 0.4 x 10^-6 erg cm^-2. This fluence is larger than that of 91.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+286.67 s in the 15-150 keV band is 4.0 ± 0.2 ph cm^-2 s^-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/952164/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 26884). We have analysed 16 ks of XRT data for GRB 200125A, from 127 s to 383.9 ks after the BAT trigger. The data comprise 286 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 26878).

The late-time light curve (Figure 2) (from T0+3.8 ks) can be modelled with an initial rise, with a power-law index of α=-0.71 (+0.19, -0.23), followed by a break at T+15.8 ks to an α of 0.84 ± 0.07.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.523 ± 0.029. The best-fitting absorption column is 3.05 (+0.17, -0.16) x 10²¹ cm^-2, in excess of the Galactic value of 7.7 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.17 (+0.19, -0.18) and a best-fitting absorption column of 1.3 (+0.5, -0.4) x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 (+0.5, -0.4) x 10²¹ cm^-2
Galactic foreground: 7.7 x 10²⁰ cm^-2
Excess significance: 2.0 σ
Photon index: 2.17 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00952164.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200125A 179 s after the BAT trigger (Simpson and Lien GCN Circ. 26892). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 26878) is detected in the initial UVOT exposures. Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of E_B-V of 0.08 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s^-1 illuminated-detector^-1. The vertical green dash-dotted lines show the T₅₀ interval, the vertical black dashed lines show the T₉₀ interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference

15^h12^m11.30^s -08°08'38.9" 1.5" XRT-final UKSSDC

15^h12^m11.52^s -08°08'40.3" 2.5" XRT-enhanced Beardmore et al. GCN Circ. 26878

15^h12^m16.6^s -08°08'31.3" 1.8' BAT-refined Markwardt et al. GCN Circ. 26897

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 26881 Swift GRB 200125A: Global MASTER-Net
observations report MASTER

Optical Watson et al. 26882 RATIR Optical Observations RATIR

Optical Liu et al. 26883 Nanshan-0.6m optical upper limit Xinjiang Astro. Obs. upper limits

Optical Melandri et al. 26887 TNG detection of a NIR counterpart
candidate TNG detection

Optical Belkin et al. 26889 AbAO optical upper limit Abastumani Astro. Obs. upper limits

Optical Watson et al. 26891 Further RATIR Optical Observations and
Confirmation of Fading RATIR detection

Gamma-ray Ridnaia et al. 26935 Konus-Wind observation Konus-Wind Fluence=2.04(-0.16,+0.22)x10^-5erg cm^-2

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

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

white_FC 179 329 147 >20.4

white 179 4176 344 >21.1

v 4387 4586 197 >18.4

b 3771 3971 197 >19.9

u 337 414 75 >18.9

w1 4796 4835 38 >18.6

m2 4591 4791 197 >19.3

w2 4182 4382 197 >20.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
15^h12^m11.30^s	-08°08'38.9"	1.5"	XRT-final	UKSSDC
15^h12^m11.52^s	-08°08'40.3"	2.5"	XRT-enhanced	Beardmore et al. GCN Circ. 26878
15^h12^m16.6^s	-08°08'31.3"	1.8'	BAT-refined	Markwardt et al. GCN Circ. 26897

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	26881	Swift GRB 200125A: Global MASTER-Net observations report	MASTER
Optical	Watson et al.	26882	RATIR Optical Observations	RATIR
Optical	Liu et al.	26883	Nanshan-0.6m optical upper limit	Xinjiang Astro. Obs.	upper limits
Optical	Melandri et al.	26887	TNG detection of a NIR counterpart candidate	TNG	detection
Optical	Belkin et al.	26889	AbAO optical upper limit	Abastumani Astro. Obs.	upper limits
Optical	Watson et al.	26891	Further RATIR Optical Observations and Confirmation of Fading	RATIR	detection
Gamma-ray	Ridnaia et al.	26935	Konus-Wind observation	Konus-Wind	Fluence=2.04(-0.16,+0.22)x10^-5erg cm^-2

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	179	329	147	>20.4
white	179	4176	344	>21.1
v	4387	4586	197	>18.4
b	3771	3971	197	>19.9
u	337	414	75	>18.9
w1	4796	4835	38	>18.6
m2	4591	4791	197	>19.3
w2	4182	4382	197	>20.2

Table 3. UVOT observations reported by Simpson and Lien (GCN Circ. 26892). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

January 30, 2020