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X-ray and optical transient SRGe J195057.5+672122: a possible nearby GRB afterglow/hypernovaстатья
- Авторы: Sazonov S., Burenin R., Khorunzhev G., Lyapin A., Medvedev P., Uskov G., Zaznobin I., Afanasiev V., Dodonov S., Uklein R., Belinsky A., Tatarnikov A., Vozyakova O., Zheltoukhov S., Chugai N., Sunyaev R., Gilfanov M.
- Журнал: The Astronomer’s Telegram
- Номер: 13987
- Год издания: 2020
- Аннотация: On June 25-29, 2020, during its second sky survey, SRG/eROSITA discovered a new X-ray source SRGe J195057.5+672122 at RA=19:50:57.5 Dec=67:21:21.5 (J2000) with a 95% error radius of 4.1 arcsec. The spectrum of the source in the 0.2-8 keV energy band can be described by an absorbed power law with a photon index of 3.3+/-0.8 and NH=(6+/-2)x10^21 cm^-2. The Galactic absorption column of 9.8x10^20 cm^-2 is outside the error interval at 3 sigma confidence. The 0.3-2 keV flux gradually declined from ~7x10-13 erg/s/cm^2 on June 25 to ~3x10^-13 erg/s/cm^2 on June 29. The object was undetected by eROSITA during its previous scan of this region on Dec. 27-31, 2019, with a 3 sigma upper limit on the 0.3-2 keV flux of 1.2e-14 erg/s/cm^2. On July 20, 2020, we obtained g,r,i images of the SRGe J195057.5+672122 field at the Russian-Turkish 1.5-m telescope. There are two optical objects within the eROSITA localization region. The first one, at RA=19:50:56.6, DEC=+67:21:23.8 (5 arcsec from the X-ray position of SRGe J195057.5+672122), is extended, has magnitudes g=20.50+/-0.07, r=19.35+/-0.02, i=18.81+/-0.02 and is also present in archival DESI LIS and Pan-STARRS images. On July 20, we performed optical spectroscopy of this object using RTT-150. The spectrum is typical of an elliptical galaxy and exhibits a series of absorption lines at z=0.2205. Given the absence of any signs of transient optical activity, this galaxy is probably unrelated to SRGe J195057.5+672122. The second optical object within the eROSITA localization region has coordinates RA=19:50:57.6, DEC=+67:21:21 (1 arcsec from SRGe J195057.5+672122 and 6.6 arcsec from the center of the galaxy discussed above) and brightness g=20.74+/-0.09, r=20.65+/-0.07, i=20.68+/-0.09. This is a newly discovered object, not present in archival DESI LIS images (g>24, r>23.4, i>22.5). We repeated its photometry on August 17 using the SAI MSU 2.5-m telescope and found that it faded and reddened to g=21.48+/-0.07, r=21.36+/-0.07, i=21.14+/-0.07. We conclude that this optical transient is associated with SRGe J195057.5+672122. On August 17, 2020, we performed spectroscopy of the optical transient at the BTA 6-m telescope. The spectral energy distribution in the 4000-8500A range reveals a broad maximum at ~5300A and low-contrast broad features resemblant of hypernova spectra. The spectrum has some similarity with those of hypernovae SN1998dw and SN2003dh associated with GRB980425 and GRB030329, respectively (e.g., Deng et al., ApJ, 624, 898, 2005), if the object is located at z~0.06. However, the observed broad bumps are less distinct and there are some other differences as compared to hypernova spectra. We can also discern two absorption lines at 5159A and 5124A in the spectrum. Their association is unclear. Given the relatively hard (diskbb model best-fit temperature of ~0.5 keV with NH=(2.8+/-0.15)x10^21 cm^-2), persistent over 5 days, X-ray spectrum of SRGe J195057.5+672122, it seems unlikely that we are dealing with a tidal disruption event. Based on some similarities in the photometric and spectral properties to GRB980425/SN1998dw and GRB030329/SN2003dh we suggest that SRGe J195057.5+672122 might be a nearby (z~0.1) GRB afterglow/hypernova. Within this scenario, the GRB should have happened at most a few weeks before June 25 (the first detection by eROSITA). However, there were no reports of a GRB happening during this period in this sky region, probably because of the visibility constraints of GRB monitors. There is also an interesting possibility that it was a misaligned GRB and SRG/eROSITA has caught an "orphan" afterglow. Given the nondetection of the host galaxy prior to the discovery of SRGe J195057.5+672122 and assuming it is located at z<0.2, it must be a dwarf galaxy with a stellar mass less than a few times 10^8 solar masses. Further X-ray, radio and spectroscopic and photometric (including archival) observations are encouraged.
- Добавил в систему: Белинский Александр Александрович