Recent increased Activity of Mrk 421 measured by HAWC

ATel #13993; Jose Andres Garcia-Gonzalez (IF-UNAM, jagarcia@fisica. unam. mx, ITESM-EIC, anteus79@tec. mx), Israel Martinez (UMD, imc@umd. edu) on behalf of the HAWC Collaboration
on 8 Sep 2020; 00:19 UT
Credential Certification: José Andrés García-González (jagarcia@fisica.unam.mx)

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In the past four days the HAWC gamma-ray observatory has observed an increased TeV gamma-ray emission from Markarian 421, with a highest flux above 1 TeV of (10.87 +/- 1.15) X 10^-11 cm^-2s^-1 ( 6.15 equivalent Crab units) between 2020-09-06 15:21:36 and 2020-09-06 21:37:34 UTC. This new increase in the integrated flux has been preceded for an extended period of high activity that seems to be ongoing since Jun 14th , and has continued with short interruptions since then (see for ref. #Atel 13808, #Atel 13831, #Atel 13875 and #Atel 13917).

The summary of this new activity period is as follows:

Duration of each full transit is 6.2 hr.

MJD 59096 (2020-09-04 15:29:28 - 2020-09-04 21:45:26 UTC) Preliminary flux level: 1.78 +/- 0.90 X 10^-11 cm^-2s^-1 [>1 TeV] (1 CU)

MJD 59097 (2020-09-05 15:25:32 - 2020-09-05 21:41:30 UTC) Preliminary flux level: 6.72 +/- 1.00 X 10^-11 cm^-2s^-1 [>1 TeV] (3.8 CU)

MJD 59098 (2020-09-06 15:21:36 - 2020-09-06 21:37:34 UTC) Preliminary flux level: 10.87 +/- 1.15 X 10^-11 cm^-2s^-1 [>1 TeV] (6.15 CU)

MJD 59099 (2020-09-07 15:17:40 - 2020-09-07 21:33:38 UTC) Preliminary flux level: 9.98 +/- 1.13 X 10^-11 cm^-2s^-1 [>1 TeV] (5.65 CU)

HAWC continuously monitors this source and will keep reporting in case that the enhanced activity follows. Follow-up observations are encouraged.

All flux values are obtained from a maximum likelihood fit under the assumption of a fixed spectral shape with power law index of 2.2 and exponential cut-off at 5 TeV. A preliminary plot showing the source activity from the beginning of the high activity to this ATel may be found at https://data.hawc-observatory.org/atels/2020-09-04to09-07_HAWC_Markarian421_halfTransits.png. The HAWC contact people for this analysis are Jose Andres Garcia-Gonzalez (IF-UNAM, jagarcia@fisica.unam.mx, ITESM-EIC, anteus79@tec.mx) and Israel Martinez (UMD, imc@umd.edu). HAWC is a TeV gamma-ray water Cherenkov array located in the state of Puebla, Mexico, that monitors two thirds of the sky every day with an instantaneous field of view of ~2 sr