Swift Observations of GRB 190326A

M.H. Siegel (PSU) and B. Sbarufatti (PSU) for the Swift team

1. Introduction

At 07:35:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190326A (trigger=895006) (Siegel et al. GCN Circ. 24009). Due to an observing constraint, Swift did not slew to the burst. At the time of the trigger, the initial BAT position was 43° from the Sun (1.5 hours West) and 101° from the 68%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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. 24020), the BAT ground-calculated position is RA, Dec = 341.652, 39.914 deg which is RA(J2000) = 22h46m36.6s Dec(J2000) = +39°54'50.3" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 57%.

The mask-weighted light curve (Figure 1) shows a FRED-like pulse structure that starts and peaks at ~T0, and ends at ~T+0.1 s. T90 (15-350 keV) is 0.08 ± 0.03 s (estimated error including systematics).

The time-averaged spectrum from T+0.00 to T+0.10 s is best fit by a power law with an exponential cutoff. This fit gives a photon index -0.67 ± 0.92, and Epeak of 33.9 ± 4.9 keV (χ2 97.5 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.3 ± 0.8 x 10-8 erg cm-2 and the 1-s peak flux measured from T-0.45 s in the 15-150 keV band is 1.55 ± 0.30 ph cm-2 s-1. This fluence is larger than that of 47% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). All the quoted errors are at the 90% confidence level.

If this is a GRB, this is one of the softest short bursts detected by BAT (based on a sample with constrained spectral fits from the 3rd BAT GRB catalog; Lien & Sakamoto et al. 2016). Another BAT-detected short GRB with similar softness and duration is GRB140622A, which was classified to be a short GRB because the XRT light curve (Figure 1) is consistent with the normal behavior of a short burst (Sakamoto et al. 2014; Burrows et al. 2014). Unfortunately, due to the observing constraint, we do not have information from XRT/UVOT, and thus it is difficult to determine the burst origin at this point. Further X-ray observations may clarify the physical origin.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/895006/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 24129).

4. UVOT Observations and Analysis

UVOT results are not available.

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.

RA (J2000) Dec (J2000) Error Note Reference
22h46m36.6s +39°54'50.3" 1.6' BAT-refined Markwardt et al. GCN Circ. 24020

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Niwano et al. 24016 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Hu et al. 24023 2.2m CAHA telescope optical limit CAHA upper limits
Gamma-ray Luo et al. 24018 Insight-HXMT/HE detection Insight-HXMT T90=0.018 seconds
Other Biltzinger et al. 24010 Fermi Trigger 575278303 / GRB
190326314 / GRB190326B: BALROG
localization

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

Filter Tstart(ks) Tstop(ks) Exp(ks) Mag
uvw1 1633 1640 1.89 >20.47

Table 3. UVOT observation reported by Sbarufatti et al. (GCN Circ. 24129). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary 3-σ upper limit is given. No correction has been made for extinction in the Milky Way.

April 19, 2019