Extremely dark GRBs: the case of GRB 100614A and GRB 100615A

TL;DR

This paper investigates the extreme darkness of GRB 100614A and GRB 100615A, analyzing their spectral energy distributions to explore high extinction, redshift, or exotic origins as explanations for their lack of optical/NIR afterglows.

Contribution

The study models the SEDs of these dark GRBs with various extinction laws, providing constraints on their dust extinction and redshift, and discusses implications for their origins.

Findings

High extinction (AV > 50) is required under Milky Way or SMC laws, which is unlikely.

Starburst attenuation curves suggest AV > 10, still very high but less extreme.

High redshift or exotic origins (z > 17) could explain the darkness.

Abstract

Dark gamma-ray bursts (GRBs) are sources with a low optical-to-X-ray flux ratio. Proposed explanations for this darkness are: i) the GRB is at high redshift ii) dust in the GRB host galaxy absorbs the optical/NIR flux iii) GRBs have an intrinsically faint afterglow emission. Within this framework, GRB 100614A and GRB 100615A are extreme. In fact, they are bright in the X-rays, but no optical/NIR afterglow has been detected for either source, despite several follow-up campaigns began early after the triggers. We build optical-to-X-ray spectral energy distributions (SEDs) at the times at which the reddest upper limits are available, and we model our SEDs with the extinction curves of the Milky Way (MW), Small Magellanic Cloud (SMC), and the attenuation curve obtained for a sample of starburst galaxies. We find that to explain the deepest NIR upper limits assuming either a MW or SMC extinction law, a visual extinction of AV > 50 is required, which is extremely unlikely. Since both GRBs are bright in X-rays, explanation iii) also cannot explain their dark classification, unless optical radiation and X-rays are not part of the same synchrotron spectrum. An alternative, or complementary explanation of the previous possibility, involves greyer extinction laws. A starburst attenuation curve gives AV>10, which is less extreme, despite still very high. Assuming high redshift in addition to extinction, implies an AV>10 at z=2 and AV>4-5 at z=5, regardless of the adopted extinction recipe. A different, exotic possibility would be an extremely high redshift origin (z>17 given the missing K detections). Population III stars are expected to emerge at z ~ 20 and can produce GRBs with energies well above those inferred for our GRBs at these redshifts. Mid- and far-IR observations of these extreme class of GRBs can help us to differentiate between the proposed scenarios.