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We present interferometric imaging at 33 GHz, with the new superextended configuration of the Very Small Array (VSA), of a very deep decrement in the cosmic microwave background (CMB) temperature. This decrement is located in the direction of the Corona Borealis supercluster, at a position with no known galaxy clusters, and was discovered by a previous VSA survey. A total area of 3 deg 2  has now been imaged, with an angular resolution of 7 arcmin and a flux sensitivity of 5 mJy beam −1 .  These observations confirm the presence of this strong and resolved negative spot at  −37 ± 5  mJy beam −1  (−229 ± 32 μK) . This structure is also present in the  Wilkinson Microwave Anisotropy Probe  5‐year data. The temperature of the  W ‐band (94 GHz) data at the position of the decrement agrees within  0.3σ n   with that observed by the VSA at 33 GHz and within  0.2σ n   with the Sunyaev–Zel'dovich (SZ) spectrum.  Our analyses show that it is a non‐Gaussian feature in the CMB at a level of 4.4σ, where sigma accounts for primordial CMB fluctuations, thermal noise and residual radio source contributions. The probability of finding such a deviation or larger in simulations including Gaussian CMB is only 0.63 per cent. Therefore, an explanation other than primordial Gaussian CMB is required. We have considered the possibility of an SZ effect generated in a diffuse, extended warm/hot gas distribution. This hypothesis is especially relevant, as the presence of such structures, if confirmed, could provide the location for a significant fraction of the missing baryons in the Local Universe. However, from the absence of X‐ray emission in this region we conclude that the whole decrement cannot be generated solely via the SZ effect in such structure. Therefore, the most plausible scenario is a combination between a negative CMB feature and a SZ effect, probably generated by a warm/hot gas distribution.

monthly notices of the royal astronomical society

Observations of the Corona Borealis supercluster with the superextended Very Small Array: further constraints on the nature of the non‐Gaussian cosmic microwave background cold spot